Chapter 3 Flashcards

1
Q

What is snow?

A

Crystallised form of water in the atmosphere.

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2
Q

How much of Earth does snow cover?

A

Snow covers 23% of Earth temporarily or permanently. NB: 71% oceans and their marginal seas, so 29% land.

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3
Q

What are snow needles?

A

They form when the temperature is above minus 12 Celsius. Below that, water vapour forms into hex crystals i.e. snowflakes.

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4
Q

What are snowflakes, what is their structure and how do they form?

A

Snowflakes are formed by crystals of ice that generally have a hexagonal pattern, often beautifully intricate.

The size and shape of the crystals depend mainly on the temperature and the amount of water vapour available as they develop.

At temperatures above about −40 °C (−40 °F), ice crystals form around minute particles of dust or chemical substances that float in the air.

At lower temperatures, crystals form directly from water vapour. If the air is humid, the crystals tend to grow rapidly, develop branches, and clump together to form snowflakes. In colder and drier air, the particles remain small and compact.

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5
Q

Why does snow crunch underfoot?

A

Because it’s so fragile. Snow crystal branches break up by the thousands creating the sound. Even when the snow is compacted, it still crunches because the broken crystals rub against each other. Snow only crunches when it’s cold. When it warm up, crystals bend instead of breaking or rubbing against each other.

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6
Q

Types of snow crystals and particles.

A

Frozen precipitation has been classified into seven forms of snow crystals and three types of particles—graupel (granular snow pellets, also called soft hail), sleet (partly frozen ice pellets), and hail (hard spheres of ice).

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7
Q

Evolution of snow on the ground.

A

The texture and density of fallen snow undergo constant change. Snow on the ground tends to become increasingly dense, and, where it survives spring and summer melting for years, it may turn into ice and form a glacier. On hillsides when temperature changes reduce the coherence of snow particles in the snow cover, gravity and viscosity may overcome friction, causing snow slides and avalanches.

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8
Q

Where does snow fall?

A

Snow falls at sea level poleward 35° N / 35° S, though on the west coast of continents it generally falls only at higher latitudes. Close to the equator, snowfall occurs exclusively in mountain regions (4,900 m + elevation).

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9
Q

What are the effects and benefits of snow?

A

By increasing the reflection of solar radiation and interfering with the conduction of heat from the ground, it induces a cold climate. The low heat conduction protects small plants from the effects of the lowest winter temperatures. Snow melt runoff feeds rivers and supplies water for irrigation etc.

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10
Q

Composition of air.

A

78% nitrogen 21% oxygen 1% argon; very small traces of other gases, including carbon dioxide and water vapour.

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11
Q

Carbon dioxide in atmosphere.

A

CO2 atmospheric concentration is rapidly increasing owing to the burning of fossil fuels - primary driver of the ongoing global warming.

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12
Q

Water vapour in atmosphere.

A

Close to the surface of a large desert such as the Sahara in Africa, or over a very cold polar region, there is almost no water vapour in the air, but in hot tropical rainforests the air may contain up to 4% vapour by volume. The behaviour of water vapour in the air is a vitally important factor in weather systems.

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13
Q

Air molecules/m3 (and) air pressure at various altitudes.

A

Table 1.1 The number of air molecules per unit volume, and air pressure, at different altitudes.
Altitude/km Number of air molecules per metre cubed Air pressure/Pa Number of air molecules per metre cubed as a percentage of the value at sea level
0 (sea level) 2.6 × 1025 1.0 × 105 100
1 2.2 × 1025 8.9 × 104 85
10 8.7 × 1024 2.8 × 104 33
40 8.9 × 1022 3.2 × 102 0.34
80 5.2 × 1020 1.3 2.0 × 10−3
120 1.2 × 1018 4.6 × 10−3 4.6 × 10−6
160 3.8 × 1016 2.7 × 10−4 1.5 × 10−9

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14
Q

Temp decrease with altitude. Lowest temp and atmosphere layer where this is recorded.

A

The temperature decreases by about 6 °C for every 1000 m of increase in elevation above sea level, though the values vary considerably with location, season and time of day as well. This steady reduction in average temperature with increasing altitude continues up through the atmosphere long after the top of the highest mountain has been left behind, until a temperature of about −57 °C is reached. The ‘layer’ of the atmosphere in which this steady reduction in temperature with increasing altitude occurs is called the troposphere.

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15
Q

Thermal conduction

A

Thermal conduction is the transfer of energy (heat) arising from temperature differences between adjacent parts of a body. Thermal conductivity is attributed to the exchange of energy between adjacent molecules and electrons in the conducting medium.

A substance of large thermal conductivity k is a good heat conductor, whereas one with small thermal conductivity is a poor heat conductor or good thermal insulator. Typical values are 0.093 kilocalories/second-metre-°C for copper (a good thermal conductor) and 0.00003 kilocalories/second-metre°C for wood (poor thermal conductor).

Air in contrast is a poor heat conductor, but heat is nevertheless transferred by conduction from the ground to the lowest layer of the atmosphere when the ground is hotter and from the lowest layer of the atmosphere to the ground when the ground is cooler.

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16
Q

Convection

A

Water at the bottom of the pan initially gets hotter than water higher up. The hot liquid rises. Cooler liquid sinks to replace it, and the cooler liquid then gets warmed in its turn. The process of convection is heat transfer from one part of a liquid or a gas to another by movement of the liquid or gas itself. Convection is an important mechanism for heating gases as well as liquids and plays a crucial role in the behaviour of the atmosphere and in the formation and development of clouds.

convection, process by which heat is transferred by movement of a heated fluid such as air or water.

Natural convection results from the tendency of most fluids to expand when heated—i.e., to become less dense and to rise as a result of the increased buoyancy. Circulation caused by this effect accounts for the uniform heating of water in a kettle or air in a heated room: the heated molecules expand the space they move in through increased speed against one another, rise, and then cool and come closer together again, with increase in density and a resultant sinking.

Forced convection involves the transport of fluid by methods other than that resulting from variation of density with temperature. Movement of air by a fan or of water by a pump are examples of forced convection.

Atmospheric convection currents can be set up by local heating effects such as solar radiation (heating and rising) or contact with cold surface masses (cooling and sinking). Such convection currents primarily move vertically and account for many atmospheric phenomena, such as clouds and thunderstorms.

Visible heat haze above a hot surface caused by heated air rising rapidly by convection.

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17
Q

Thermal Radiation

A

In radiation the heat from the hotplate, the pan and the warm liquid can be felt if you put your hand near them, with the heat being transferred through the air by radiation. All warm objects emit radiation, some of which we feel as ‘heat’. If the hotplate in Figure 1.3 were glowing, that would mean it was emitting visible radiation in the form of red light. But even if the plate were not hot enough to be glowing, as soon your hand got close to it you would still be able to feel the heating effect of ‘invisible’ infrared (IR) radiation that was being emitted. The warm surface of the Earth also emits IR radiation. The heat from the Sun is transferred to the top of the atmosphere by both visible radiation (‘sunlight’) and invisible radiation, including IR and the ultraviolet (UV) radiation that causes sunburn.

thermal radiation, process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light; thermal radiation does not require an intervening medium to carry it.

Thermal radiation ranges in wavelength from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays. The intensity and distribution of radiant energy within this range is governed by the temperature of the emitting surface. The total radiant heat energy emitted by a surface is proportional to the fourth power of its absolute temperature (the Stefan–Boltzmann law).

The rate at which a body radiates (or absorbs) thermal radiation depends upon the nature of the surface as well. Objects that are good emitters are also good absorbers (Kirchhoff’s radiation law). A blackened surface is an excellent emitter as well as an excellent absorber. If the same surface is silvered, it becomes a poor emitter and a poor absorber. A blackbody is one that absorbs all the radiant energy that falls on it. Such a perfect absorber would also be a perfect emitter.

The heating of the Earth by the Sun is an example of transfer of energy by radiation. The heating of a room by an open-hearth fireplace is another example. The flames, coals, and hot bricks radiate heat directly to the objects in the room with little of this heat being absorbed by the intervening air. Most of the air that is drawn from the room and heated in the fireplace does not re-enter the room in a current of convection but is carried up the chimney together with the products of combustion.

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18
Q

Thermals and heat haze.

A

When convection happens the pattern at any instant at a particular place is typically as in Figure 1.5. There are columns of rising air, with much larger regions of descending air between them. This pattern can exist on scales that, at ground level, range from a few centimetres to many metres. The rising air loses energy to its surroundings, and is subsequently displaced downwards. Columns of convection are so gentle that they usually go unnoticed, although some birds (and glider pilots) use these so-called thermals for lift.

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19
Q

Atmosphere temperature structure.

A

Google atmo temp graph!

As the air rises it plays a critical role in the energy loss from the Earth’s surface.

The ways in which radiation from the Sun interacts with the atmosphere are numerous and complex, but one basic principle is that anything that absorbs the radiation heats up. So the atmosphere can acquire heat directly, by absorbing some of the solar radiation.

The radiation absorbed directly by the atmosphere never reaches the Earth’s surface, nor does the substantial proportion of the incoming radiation that is scattered in all directions as it travels through the atmosphere.

The lower part of the atmosphere, the troposphere, is mainly heated from below, and heat is mostly redistributed to higher levels by convection as shown in Figure 1.5. The result is that the temperature of the troposphere decreases with altitude. Altitude in this context is equivalent to height above sea level.

Above the troposphere is the tropopause, and here the temperature remains roughly constant with increasing altitude. In this layer convection stops. Above the tropopause comes the stratosphere, where the temperature increases with increasing altitude.

This increase in temperature in the stratosphere is mainly due to the radiation from the Sun entering the atmosphere from above. Figure 1.6 shows a schematic of the variation of temperature in the atmosphere with altitude. The successive ‘layers’ or ‘spheres’ (i.e. concentric shells around the Earth) are separated by ‘pauses’ where the change in temperature with altitude switches from increasing to decreasing or vice versa.

The outermost reaches of the atmosphere extend up to about 100 km. The height of the top of the troposphere varies. It is thinner at the poles, and thicker at the Equator, but it is about 13 km over the UK or Australia.

The temperature range in the troposphere is −60 °C to 20 °C and most of the processes that make up our weather occur in this relatively small layer of air. The next section discusses how water gets into the atmosphere.

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20
Q

Endothermic process.

A

An endothermic process is a chemical reaction or physical change that absorbs energy from its surroundings, typically in the form of heat. In an endothermic reaction, the energy required to break bonds in the reactants is greater than the energy released when new bonds are formed in the products, resulting in a net absorption of energy. This is in contrast to exothermic processes, where energy is released. An example of an endothermic process is the decomposition of limestone (CaCO₃) into lime (CaO) and carbon dioxide (CO₂), which requires heating the limestone to a high temperature.

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21
Q

How does water get into the atmosphere?

A

To start to answer this question we need to think about the energy requirement for the change of state of water. Water on Earth exists as liquid, solid as ice and snow, and gas as water vapour. You will remember that by evaporation liquid water becomes a vapour, and through sublimation ice can become vapour as well. Both processes require an input of energy to occur; this is called an endothermic process.

The main heating process for the lower atmosphere is convection and, as well as transferring heat from rising warm air to colder air further up, convection also moves water vapour, in the form of moist air, around in the atmosphere. Water vapour exists in the atmosphere because of the processes of evaporation from oceans, lakes and rivers and sublimation from snow and icefields.

In fact the movement of water from Earth to the atmosphere and back to Earth in the form of rain (or snow) plays a crucial role in transferring heat between the Earth and the atmosphere.

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22
Q

How does evaporation remove energy from the Earth’s surface?

A

The energy required for evaporation or sublimation is called latent heat; the word latent means ‘hidden’ and effectively the energy input is a sort of ‘hidden heat’ in that the temperature of the water changing state does not increase as it changes state, but the water (or ice) left behind at the surface will decrease in temperature.

In the same way, evaporation of water from the Earth’s surface will tend to cool the surface.

Air heated by the ground rises up through the atmosphere. As it rises, it expands and cools. If the temperature falls low enough, some of the water vapour in the air condenses to form a large number of liquid droplets or icy particles – clouds.

If it takes an input of heat to produce vapour from liquid or solid, you could expect heat to be given out in the condensation of vapour to produce liquid and solid. And this is exactly the case. The latent heat energy given out during the change of state by condensation heats the surrounding atmosphere. This means the evaporation and condensation of water transfers energy from the Earth’s surface to the Earth’s atmosphere. Ultimately, precipitation returns the condensed water to the ground, mostly as rain or snow, where it is susceptible again to evaporation or sublimation, as shown in Figure 1.7. (Clouds forming in rising columns of air as water vapour is carried upwards by convection. This water vapour is evaporated from the Earth’s surface, which consequently loses energy via latent heat. When the water vapour condenses, the latent heat is given out and raises the temperature of the surrounding atmosphere. Overall energy is transferred from the Earth’s surface to the atmosphere. Precipitation from the atmosphere returns water to the ground (i.e. as rain or snow). )

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23
Q

What are clouds?

A

When you look up into the sky, particularly if you live at the mid latitudes, it’s unusual not to spot clouds. What we see as a white or grey fluffy cloud is in fact a collection of tiny water droplets or (at very high altitude) minute ice crystals suspended in the air. For this collection of droplets or crystals to qualify as a cloud, its base must be above the ground. A similar collection of droplets in contact with the ground is called mist or fog. Cloud droplets are extremely tiny: the largest ones are no more than a tenth of a millimetre (0.1 mm) across and the smallest are a hundred times smaller than that.

Clouds are formed when rising air that contains water vapour cools to a point that the vapour condenses into liquid. This temperature is called the dew point (in some locations on cold mornings you can see the droplets of water vapour or dew on the ground). In the atmosphere the water drops that form as the air cools are clouds. They are so small and light, that even very gently moving air is enough to keep them aloft against the pull of gravity and they remain suspended.

You will learn in Topic 4 about how light is made up of a spectrum of colours. Clouds scatter incoming sunlight in all directions and because no particular colour of light is preferentially scattered or absorbed by cloud droplets, they generally appear to be white.

Scattered light can penetrate all the way through thin clouds and thin clouds therefore look white all over. As a cloud grows vertically, less scattered light penetrates all the way to its base, so less light emerges through the bottom of the cloud. In fact, the taller the cloud from top to bottom, the greater the proportion of the incoming light that is scattered back towards space, so the brighter the cloud appears from above and the darker it appears from below. ‘Threatening black clouds’ are often the tallest and these are indeed very likely to produce heavy rain.

Every cloud is unique, but they do fall into recognisable groups on the basis of their appearance from ground level. Figure 1.8 shows two contrasting cloud examples: wispy high clouds and puffy clouds.

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24
Q

Types of clouds

A

The generally accepted classification of names of cloud types comes from the work of an English naturalist called Luke Howard in 1803. Howard’s scheme involved just four basic cloud forms:

cirrus (‘a curl of hair’) for wispy clouds – these generally form above 5 km altitude.
stratus (‘layer’) for horizontal sheet-like clouds – these can be found from sea level to above 5000 m.
cumulus (‘heap’) for puffy clouds – these can generally be found from below 2000 m.
nimbus (‘rain’) for rain-bearing clouds – which are generally below 2000 m altitude.
Other cloud types can be described by combining these terms: for example, ‘stratocumulus’ is a puffy cloud that shows horizontal layering. The word nimbus is used in combination with two other component names, giving for example cumulonimbus and nimbostratus. Clouds can also be classified on the basis of the altitude of their base above ground level. Mid-level clouds (2000–5000 m high) are given the prefix ‘alto’. A full list of clouds with associated images is shown on the Cloud identification chart.

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25
1.2.1 Investigating clouds - work it out!
26
You can see in the video clip that the cloud has powerful updrafts and turbulence. As the parachutist falls, he is measuring temperature, and as you would expect the air is warming as he gets closer to the ground. The rate at which atmospheric temperature decreases with an increase in altitude is called the ‘lapse rate’. One important thing to note from the temperature data collected by the parachutist, is that the rate of cooling from ground level to 2000m altitude slows, passing through the cloud.
27
The cloud base is at 3000 feet, which is ~1000 m. The temperature at this height is approximately 21 °C. What would you expect the temperature on the ground to be? Hide answer It is 1000 m to the ground so we would expect the temperature to be 6 °C warmer than the base of the cloud. The temperature on the ground would therefore be 21 °C + 6 °C = 27 °C.
28
Big Cloud Energy.
The data reveals that the atmosphere cools at a predictable rate, called the lapse rate, from ground level to the cloud base. But then the rate of cooling slows. So the cloud is clearly generating heat. FELICITY And why that’s really lovely to see that is because what we know happens is that when water condenses out of air, it releases a huge amount of energy. And that energy warms the air around it. And that creates big bursts of energy inside the cloud. And that’s why clouds have big, uneven, fluffy tops. So this is exactly what helps to keep the cloud afloat. NARRATOR This energy released by water vapour as it condenses is called latent heat. And it is possible to work out how much energy is delivered to a cloud by this process. A typical cumulus cloud similar to the one Dane and Andy measured generates enough heat energy to power the average home for 17 years, or about 300 tonnes of TNT. Scale that up to a million-ton cumulonimbus, and you’re looking at the heat energy equivalent to a nuclear warhead. There is a lot of heat energy within a cloud! Personally I am not very comfortable when I hear the energy in natural processes being described as equivalent to nuclear bombs or TNT exploding, but the point is clear: clouds are not the peaceful, calm looking things you could imagine from the ground, they are extremely energetic features that move vast quantities of water around the planet.
29
Precipitation.
Precipitation is the name given to water in any solid or liquid form that falls from a cloud and reaches the ground. Precipitation is important: we need rain for crops to grow and the water for domestic use. Rain, drizzle, hail and snow are precipitation – but dew, frost, mist and fog are not – even though they may result in objects at the surface getting very wet. Many people would say ‘it’s raining’ if any liquid water were falling from the sky, but we classify rain based on the size of the droplets. If they are at least 0.5 mm in diameter they are rain, and most raindrops are between 1 mm and 2 mm across. Drops smaller than 0.5 mm in diameter are known as drizzle. So the amount of water coming down is irrelevant; it is drop size that matters, not intensity. This means you can get much wetter in ‘heavy drizzle’ than in ‘light rain’. The small droplets of drizzle will affect the visibility too. You saw in 1.2.1 Investigating clouds, that the centre of a cloud is dark because of the light scattered by the water droplets. These droplets are produced by condensation, but condensation is not enough to produce rain or drizzle. As you saw, the droplets mostly remain suspended or if they do fall when tiny they evaporate in the warmer air below. If the forces are unbalanced and the drops rise they can merge together (we call this coalescence) and grow bigger. If the cloud is warm (that is above 0 °C), then the heavier raindrops can then fall as rain (Figure 1.10).
30
Condensation v freezing nuclei.
Condensation of water vapour takes place within the air itself. Essential to this process are tiny solid particles of ash, dust and salt which are normally present in the atmosphere. When air is warm the water vapour molecules are moving quickly and they just bounce off these solid particles. When the air is cooled the molecules of water vapour move more slowly on average and the most slowly moving among them may stick to the surface of the solid particles. Further water vapour molecules can then condense onto these surfaces, until eventually droplets of liquid water form. For this reason the solid particles are called condensation nuclei. In a similar way, minute solid particles can promote the formation of ice crystals from water droplets and these are called freezing nuclei.
31
Cold clouds.
At high altitudes, atmospheric temperature falls below zero. Clouds where the atmospheric temperature is well below 0 °C are called ‘cold clouds’, and here the raindrops turn to ice.
32
What is the Bergeron process?
Mid- and high-latitude clouds extend into regions in the atmosphere where the air temperature is well below 0 °C. Such clouds are called ‘cold clouds’. Scientists noted that although the simple coalescence idea worked for warm clouds it could not apply universally. The mechanism for the formation of precipitation in cold clouds is usually called the Bergeron process, after the Swedish meteorologist who first suggested in 1933 that raindrops derived from ice crystals.
33
Water trivia
Most rain starts as ice crystals which get fatter and then drop out of the cloud and fall to earth as rain. Water freezes more easily around some substances than others. Can bacteria make its own rain? Pure water freezes at well below 0* C, it needs impurities to freeze at higher temp. Tap water full of particles, minerals, dust, some bacteria so it freezes quite readily. In the experiment, bacteria water (-8) followed by mineral (-11) followed by pure water froze. The addition of bacteria helps the ice crystals to nucleate.
34
What is supercooling?
A substance that is liquid at a temperature below its normal freezing temperature is said to be supercooled, and supercooled water is common in the atmosphere. Supercooling is a particularly important process within clouds, where in the absence of freezing nuclei, liquid water droplets may exist at temperatures down to −40 °C.
35
Structure of mid latitude cumulonimbus cloud
A typical mid-latitude cumulonimbus cloud is shown in Figure 1.13. A large section of this cloud is colder than 0 °C, and it contains supercooled liquid water droplets as well as ice crystals. Only at the very top of the cloud are there ice crystals but no liquid droplets. The reason for this lies in the conditions necessary for ice to form. The water droplets form on condensation nuclei, and freezing nuclei are required for ice crystals to form. Freezing nuclei work best if their geometry is hexagonal, like that of an ice crystal, but such particles are rare. A cold cloud thus contains many more condensation nuclei than freezing nuclei, and therefore there will be more liquid droplets than ice crystals, even at temperatures well below 0 °C. In the portion of the cloud that contains both water droplets and ice crystals, water molecules can escape more easily from the liquid droplets than from the solid crystals.
36
Phase changes in the cloud.
The air in the cloud is saturated, so if water vapour is to be added to the air by evaporation from the droplets, an equal amount of water vapour must be removed from the air by condensation or deposition. The water vapour condenses or freezes most readily onto the ice crystals and the crystals therefore grow at the expense of the droplets. Once the crystals are large enough that they can no longer be held aloft by the updraughts, they start to fall through the cloud of supercooled droplets. Various things can then happen to them. If the cloud contains many ice crystals, some may break up as they fall through the cloud, producing tiny ‘secondary’ crystals. Small falling crystals may collide with and stick to one another and this process of aggregation forms snowflakes. Nimbostratus clouds often have the high number of ice crystals required to produce snow. If the snowflakes melt in the lower, warmer parts of the cloud they will emerge from the base of the cloud as rain. Figure 1.14a and b show these processes schematically.
37
In locations where we only see and experience snow in winter and temperatures are close to 0 °C, snow just tends to feel wet and cold, and it is melting as it is falling. In the UK snowfall is usually associated with air temperatures between 0 °C and 2 °C. Snow is the most common type of solid precipitation (Figure 1.14a demonstrates this), but there are other types. At slightly higher temperatures a mixture of rain and melting snow, known as sleet, may fall. You saw in Activity 1.4 that the inside of a cloud is a dynamic and energetic place. Ice crystals can drift through the cloud several times with crystals partially melting, and moisture freezing onto the crystals to form ‘rimed’ or ‘graupel’ ice (Figure 1.14). When this leaves the cloud it can be as hail; a solid precipitation in the form of lumps of ice that are at least 5 mm in diameter, although they can be bigger and cause damage and injury. The up and down motion through the cloud means that the ice will be deposited on the growing hail pellet under a number of different temperature condition and hailstones are often ‘layered’, with some transparent ice (glaze) and some opaque ice (rime/graupel). The precipitation finally seen at ground level depends on the conditions at the base of the cloud and below it. Hail usually reaches the ground as hail because there isn’t time for it to melt completely even if it falls in warm air. Snow falls more slowly so it may have turned to rain by the time it reaches the ground.
38
Why no snow Southern Hemisphere?
While the Southern Hemisphere does experience snow, especially in mountainous regions and Antarctica, it's less common and less extensive than in the Northern Hemisphere due to the dominance of oceans and a smaller landmass south of 40°S. Here's a more detailed explanation: Dominance of Oceans: The Southern Hemisphere is characterized by a large expanse of ocean, which moderates temperatures and prevents extreme cold, making snow less frequent and widespread. Smaller Landmass: Compared to the Northern Hemisphere, the Southern Hemisphere has a smaller amount of landmass, particularly at higher latitudes, where snow is more common. Maritime Climate: The Southern Hemisphere's climate is largely influenced by its maritime environment, which means it experiences milder winters and summers compared to continental climates in the Northern Hemisphere. Snow in Mountainous Areas: Snow is still possible in the Southern Hemisphere, particularly in mountainous regions like the Andes in South America and the Australian Alps, as well as in Antarctica. Seasons: The Southern Hemisphere experiences all four seasons, just like the Northern Hemisphere, but the seasons are opposite, with winter occurring when the Northern Hemisphere is experiencing summer. Global Climate Change: Global climate change may lead to earlier melting and less coverage area of snow in the Southern Hemisphere
39
Part 1 summary.
the physical structure of the atmosphere and how clouds form and make snow. The Earth’s atmosphere is made up of a number of gases including water vapour. Air pressure decreases with height. The temperature of the atmosphere decreases by about 6 °C for every 1000 m height above the Earth’s surface. Convection, radiation and conduction change the density of air causing it to move. Water vapour condenses into tiny drops through latent heat transfer to form clouds. When it is cold enough and where freezing nuclei exist, water vapour can form ice, and then through aggregation and further crystal growth, snow. All snowflakes have a hexagonal (i.e. six-fold) symmetry. Snow generally falls over much of the planet as an ephemeral feature.
40
What is gravity?
More precisely, gravity is the force that attracts one object with mass to another object with mass.
41
What is friction?
friction, which can be defined as the force resisting motion or the force that is created when two surfaces move, or try to move, across each other.
42
Unbalanced force and undisturbed motion.
A force that changes the motion of an object either at rest or moving in a straight line at constant speed is known as an ‘unbalanced force’. In all situations undisturbed motion is either being at rest, or moving in a straight line at a constant speed.
43
Newton's first law.
An object remains at rest or moves in a straight line at constant speed unless it is acted on by an unbalanced force. an object does not accelerate unless it is acted on by an unbalanced force equivalently, if an object is acted on by an unbalanced force it will accelerate.
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Speed v velocity. Car going round the bend: what does the change in direction tell us in relation to Newton's First Law? Are the forces acting on the car balanced?
Now the motion of an object actually has two attributes: speed and direction; together, these define an object’s velocity. In everyday speech velocity and speed tend to be used interchangeably, but they are different. Speed is just one attribute – the magnitude or size – of velocity. The other attribute of velocity is direction. In Figure 2.7 the car is driving at a constant speed, but the velocity is changing because the direction is changing. This means that, according to Newton’s first law of motion there must be an unbalanced force acting on the car. So for example, it is correct to say that the speed of a car is 22 m s−1, and that the velocity of a car is 22 m s−1 in the northwest direction. However, it is not correct to say that the velocity of a car is 22 m s−1, or that the speed of a car is 22 m s−1 in the northwest direction. If the car starts reversing it will be moving but with a negative velocity. Velocity is speed and direction – the car is moving with a particular speed, however the direction of travel is reversed.
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Describe another instance of an unbalanced force acting on an object to effect a change in direction?
Swinging a ball around on a string.
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Describe unbalanced forces.
Unbalanced forces An object moves with constant velocity unless it is acted on by an unbalanced force. When an object is at rest it has zero velocity, which is of course, a constant velocity. An unbalanced force will cause a change in velocity; that is a change in speed, a change in direction, or a change in both.
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Why do we use the word magnitude for acceleration?
The use of the word ‘magnitude’ implies that acceleration also has direction.
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When do we have acceleration?
Whenever there is a change in speed, a change in direction, or a change in both, there is an acceleration. To write it more simply an acceleration is a change in velocity, and just like velocity, acceleration has both magnitude, and a direction. For example, you would say ‘the car accelerated at 2 m s−2 in a southerly direction.’ So, to follow the circular path the direction of motion has to continuously change, and the object is continuously accelerating.
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Example
Consider a car going around a bend at constant speed. a.Is the car showing a change in velocity? b.Is there an acceleration? c.Is an unbalanced force acting? Justify your answer. a.Yes, there is a change in velocity, in this case, a change in direction. b.As there is a change in velocity, there is, by definition, an acceleration. c.From Newton’s first law of motion, an unbalanced force must be acting.
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Newton's Second Law of Motion.
Newton's second law of motion states that the magnitude of the unbalanced force on an object equals the mass of that object multiplied by the magnitude of its acceleration (the acceleration being in the same direction as the action of the resultant of the individual forces on the object, i.e. the action of those forces combined). force = mass × acceleration In symbolic notation this is: F = m a where F is the magnitude of the unbalanced force, m is the mass of the object, and a is its acceleration. And we can summarise the second law as: The magnitude of the unbalanced force on an object equals the mass of that object multiplied by the magnitude of its acceleration.
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Inertia.
This means that mass is a property of an object that expresses its ‘reluctance’ to accelerate: the greater the mass, the smaller the acceleration for a given unbalanced force. The scientific name for this ‘reluctance’ to accelerate is inertia. Inertia is a noun which means a tendency to do nothing and remain unchanged and the definition in physics is as follows: Inertia is a property of matter by which it continues in its existing state of rest or uniform motion in a straight line unless that state is changed by an external force.
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Newton's Third Law of Motion.
For every action there is an equal and opposite reaction.
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Newton's Law of Gravity.
Newton’s law of gravity states the gravitational force between two objects increases when either of their masses increases or when they are brought closer together.
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Gravitational interactions: mass and distance
Grav force is proportional to the masses of the objects and inversely proportional to the squared distance between the two objects.
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Constant of proportionality
When two things are proportional to one another, we can replace the proportionality sign , with an equals sign plus a constant, and so turn it into a precise equation. We can also replace an equals sign with a proportional sign. For example, you know that C = 2πr, but you could also write that C r. When we replace with = we are defining an equation and we have to include what is called a constant of proportionality – in this case the constant of proportionality is 2π.
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Acceleration due to gravity.
Fg = m g and F = m a therefore a = g, meaning that: The acceleration due to gravity experienced by any object that is falling freely close to the Earth’s surface is a constant: it has the same value, irrespective of the mass of the object.
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What is the scientific explanation for the fact that the bowling ball and feather drop at different rates in a normal atmosphere and at the same rate in a vacuum?
In the atmosphere, the bowling ball and the feather fall at different rates; we can see that the bowling ball falls at a faster rate because it hits the ground first. In normal atmosphere there are air particles that provide some resistance to the falling objects. This resistance acts as an upward force and is more effective on the feathers – which have less mass – than the bowling ball. When both objects are dropped in a vacuum there are no (or very few) air particles and so there is no air resistance. This means that both objects fall at exactly the same rate, irrespective of the difference in their mass.
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Weight.
The gravitational force Fg experienced by an object is called the weight of the object. We can rewrite Equation 2.7 as: weight = m g Weight, that is, the force due to gravity, is measured in newtons; mass is a measure of how much matter an object contains, measured in kilograms. The symbol g was introduced as a constant of proportionality. The value of g on the Earth’s surface is only approximately constant and it varies slightly depending on both altitude and latitude. It is actually slightly larger at the poles than at the Equator because the Earth is not exactly a sphere; it bulges around the Equator and it also rotates. The impact of this means that if you want to weigh less on Earth, you should go to the Equator. The variation in g means that, whereas the mass m of an object is constant (provided it does not lose or gain matter), its weight Fg depends on where on the Earth or in the Universe the object is located. In this module the acceleration due to gravity near to the Earth’s surface will be taken to be 9.8 m s−2. gMoon = 1.6 m s−2
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Part 2 summary.
Forces, Newton’s three laws and how they lead to motion Newton’s first law of motion states that an object remains at rest or moves in a straight line at constant speed unless it is acted on by an unbalanced force. Newton’s second law of motion states that an unbalanced force on an object equals the mass of that object multiplied by the magnitude of its acceleration, i.e. F = ma. Newton’s third law of motion states that for every action there is an equal and opposite reaction. Newton’s law of gravity at wide ranging spatial scales. Newton’s law of gravity states the gravitational force between two objects increases when either of their masses increases or when they are brought closer together. The acceleration due to gravity g experienced by any object that is falling freely close to the Earth’s surface is a constant. Unbalanced forces cause snow to fall to the ground under the influence of gravity.
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What would happen if the Earth's orbital speed were reduced to zero? What about if gravity were switched off?
If the Earth’s orbital speed was suddenly reduced to zero, as shown in Figure 3.3, it would still be acted upon by the same gravitational force of the Sun and so would continue to accelerate towards the Sun at the same rate. But because there is no sideways motion, the Earth would fall directly towards the Sun and collide with it. The maths is complicated but it can be calculated that the Earth would fall into the Sun approximately 65 days after the orbit ceased. At the moment that gravity is switched off, the Earth continues in a straight line in the direction it was moving at that time.
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Is Earth's orbit around the Sun a perfect circle?
In the Solar System, all the planets have an orbit that is very slightly non-circular, in a shape called an ellipse. In addition, the Sun is not quite at the centre of the orbit.
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What's the big deal with Newton then?
Newton’s laws of motion and his law of gravity explain the forces that drive our weather systems. They also give an excellent explanation of motions in the Solar System. Astronomers can calculate planetary orbits and the motion of spacecraft, comets and other bodies very accurately. And when we can accurately calculate positions of planetary objects, we can fly and land spacecraft on them! Newton’s laws also explain motion beyond our Solar System; our entire Solar System is in orbit around the centre of our galaxy.
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What do star-planet system orbit?
The planet and star are orbiting the combined centre of mass of both objects – rather than the planet orbiting the centre of a stationary star. All orbits trace out an ellipse, and the amount by which the orbit deviates from a perfect circle – known as the orbital eccentricity – depends on the masses of the two objects.
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Perihelion. Aphelion.
You can see that the distance between the two objects varies throughout the elliptical orbit, unlike if the planet was orbiting the star in a circle. The point where the planet is closest to the star is known as the perihelion, and the point where it is furthest away is known as the aphelion. If the masses of the object change, the properties of the orbit will change. The Earth is approximately 147 million kilometres from the Sun at perihelion in early January, and approximately 152 million kilometres at aphelion in early July. So the Earth is closest to the Sun in January and furthest from the Sun in July, with about 5 million kilometres difference between the two.
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What happens to the orbit if you double the mass of the star? CHECK THIS.
When you double the mass of the star, the orbit is smaller, faster, and its shape is more squashed.
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How much bigger is the Sun than the Earth and what are the implications in terms of centre of mass.
In the Sun–Earth system, the Sun is 334 000 times more massive than the Earth. Even though the Sun and the Earth are orbiting the centre of mass of the system, the Sun appears stationary because the centre of mass is so close to the centre of the Sun.
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How many planets and moons in our Solar System?
In our Solar System there are eight planets and 181 known moons orbiting around those planets (there may be more).
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How long to Pluto?
In 2015, using relatively basic physics, the National Air and Space Agency (NASA) successfully sent a spaceship to Pluto, a journey of 3 billion miles, in only nine and a half years.
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Distances from the Sun.
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune 57 million km 108 million km 150 million km 228 million km 779 million km 1.43 billion km 2.88 billion km 4.5 billion km
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Average temperature for Mercury, Neptune, Earth. Highest and lowest temps on Earth.
Mercury is closer to the Sun than Neptune, so it receives more energy from the Sun. Overall the energy received by each planet naturally affects its temperature – the average surface temperature of Mercury is ~160 °C, whilst the average surface temperature of Neptune is ~ −218 °C. On Earth the average surface temperature is approximately 14 °C.
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Why is it colder at higher latitudes? Explain also var of temp during day.
Part of the explanation for the large variations in temperature seen across Earth is due to the way the Earth orbits the Sun. Before we progress here, to recap, the Earth orbits the Sun in an ellipse, and so its distance from the Sun varies. This means the amount of energy the Earth receives during an entire orbit will vary. The variation is not great because the ellipse is not far from circular. Overall the effect is minor and only makes about 1% difference on the amount of energy that reaches our planet. So the Earth as a whole intercepts more or less the same amount of solar radiation as it orbits the Sun from day to day. The Earth is not a flat disc. It is a three-dimensional sphere. This means the energy is smeared over a wider area through distortion. The temperature is colder at higher latitudes because the energy from the Sun is spread over a larger surface area of the Earth at higher latitudes. Also the radiation grazes at an angle and therefore has to travel through a thicker layer of atmosphere which has the effect of scattering, absorbing and reflecting some of the radiation. The Earth rotates on its axis once every 24 hours to give day and night. This means that the amount of energy received at any single point on the Earth’s surface varies dramatically through 24 hours.
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What if axis of rotation were perpendicular to orbit?
If Earth’s axis of rotation were at right-angles to the plane of its orbit, day and night would be 12 hours everywhere except at the poles it would be perpetual twilight for 24 hours/day.
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Why do we have seasons?
The axis of rotation of the Earth is currently at an angle of approximately 23.4° to the vertical and, over the course of a year the direction the axis points to changes relative to the Sun. (We say approximately because this angle varies with time). It makes our planet habitable by providing a relatively benign season shift.
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Barringer Crater
Arizona. 50k years ago struck by meteorite only 50m in diameter caused a crater 1km across. 4.5bn years ago Solar System was forming. Collision with other proto-planet called Theia which obliterated. The protoplanet Theia smashed into Earth and knocked it off its 90 degree rotation giving it a tilt. Earth survived the collision, but Theia disintegrated. Earth nearly went too. Lots of debris produced by collision was blasted into space and coalesced to form the Moon which stabilised the Earth's spin. Nowadays, the gravity of Sun and Moon together act as a source of counterweight stabilising our tilt. The protoplanet Theia smashed into Earth and knocked it off its 90 degree rotation giving it a tilt. Earth survived the collision, but Theia disintegrated. The collision of the protoplanet Theia into Earth resulted in a huge amount of debris being blasted into space. Gradually, this debris coalesced, captured by the Earth’s gravity and it formed the Moon.
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Sun v Earth size comp
Sun is moto helmet, Earth is pea.
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Why do we have seasons?
The axis of rotation of the Earth is not vertical but inclined at an angle – this means that in the northern winter the North Pole is pointing away from the Sun, and in the northern summer the North Pole is pointing towards the Sun. People at the Equator don't know seasons!!
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What are aerosols?
The Earth’s atmosphere significantly affects incoming solar radiation, partly because of the gases that constitute the atmosphere and partly because of atmospheric aerosols. An aerosol is a collection of tiny liquid or solid particles (typically micrometre-sized) dispersed in a gas, such as water droplets in the atmosphere (inside or outside clouds). You can see these tiny droplets in aerosol spray from, for example, a deodorant can. Sometimes these aerosols are atmospheric dust – an extreme example being sandstorms (Figure 3.12).
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What happens to solar radiation before it reaches Earth's crust?
As solar radiation passes through the Earth’s atmospheric gases and aerosols, it is subject to two processes that each reduce the amount reaching the Earth’s surface. The first of these is absorption of solar radiation by the atmospheric gases and aerosols. When this happens the solar radiation is ultimately converted into heat, which causes a rise in the temperature of the atmospheric gases and aerosols (Figure 3.13a). The other atmospheric process is scattering, where the atmospheric gases and aerosols don’t absorb solar radiation but redirect and scatter it (Figure 3.13b). This scattered radiation travels in all directions and some of it escapes back to space, while the rest reaches the Earth’s surface through an indirect route. Clouds and some other aerosols are particularly very good at scattering. Both absorption and scattering occur throughout the whole atmosphere, although they occur particularly in the lower levels where most of the air is, and the atmosphere is densest.
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What happens with the radiation which reaches Earth's surface?
The solar radiation that escapes absorption or scattering back into space finally reaches the Earth’s surface where some of it is scattered (Figure 3.13c). At this point because the Earth is so dense, the radiation is scattered back into the atmosphere. We usually call this scattering surface reflection. The radiation reaching the Earth’s surface that is not reflected is absorbed (Figure 3.13d). In the oceans this happens throughout the top few tens of metres of water, whereas on the land it is confined to a much thinner surface layer. Just as in the atmosphere, the absorbed solar radiation gives rise to an increase in the surface temperature. In other words, the Earth’s surface is radiantly heated by the Sun.
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What is albedo?
The mean rate at which solar radiation is returned to space by the combined effects of scattering and reflection, expressed as a proportion of the total initially intercepted by the Earth, is called the albedo (derived from the Latin word albus, meaning white). a sea surface absorbs virtually all of the incident energy; (b) a typical mixed, snow-covered, sea-ice surface reflects most of the incident energy.
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What is specific heat capacity?
Specific heat capacity is a measure of how much energy it takes to raise the temperature of 1 kg of a particular substance by 1 °C. A lower specific heat capacity means that it takes less energy to heat up something, and vice versa.
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Specific heat capacity for iron, cork, water.
Material Specific heat capacity/J kg−1 °C−1 iron 450−490 cork 2000 fresh water 4190 You can see from Table 3.4 that fresh water has an extremely high specific heat capacity – it takes a lot of energy to increase its temperature. This physical property is often used to our advantage, for example virtually all car engines use water in their cooling systems to stop the engine overheating. The water in the cooling system takes heat away from the engine and then cools it in the radiator.
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Part 3 summary
The key concept and principles you have studied in this part are: the Earth’s orbit around the Sun and how it determines the overall climate The Earth orbits the Sun in the shape of an ellipse. The further a planet is from the Sun the less energy it receives. The energy from the Sun varies on a daily basis because the Earth rotates on its axis. how the axial tilt and rotation of the Earth affects climatic zones, temperature and seasons The tilt of the Earth’s axis from the vertical is responsible for the seasons the physical properties of materials determine the heat absorbed. Different surfaces on the Earth reflect different amounts of energy back out into space. Materials have a property called specific heat capacity which means they heat up by different amounts for the same input of heat.
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Air movement in room with radiator and freezer.
A room with a radiator on one wall and an open freezer on the other will cause air to rise and sink at opposite ends. Horizontal draughts are set up to replace ascending and descending air. The pattern on Earth is more complicated than the simple single circulation cell that you saw previously in Figure 1.5 in Part 1 (i.e. of convection patterns in the Earth’s atmosphere with warm air rising, transferring energy away from the Earth’s surface to the atmosphere and cooler air being displaced downwards), and it is on a planetary scale.
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How does a horizontal circulation cell come about?
Inside the freezer the air is cooled, the molecules are closer together and this increases the density compared with the air outside. When you open the door the air sinks and spreads across the floor. Where the air has risen by the radiator, because the molecules are widely spaced the air pressure has reduced. By the freezer, because the molecules are packed more closely, the air pressure has increased. Across the room there is now an imbalance in the air pressure with one side of the room lower and the other side higher. This imbalance means that Newton’s laws come into play. The unbalanced force leads to an acceleration following Newton’s second law. Because the upwards force from the radiator and the downwards force from the freezer are constant the two vertical motions set up by the vertical forces generate a horizontal circulation cell across the room as shown in Figure 4.1.
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What are the unbalanced forces acting within the atmosphere?
There is a force produced at the Equator as the atmosphere at the Earth’s surface becomes less dense, and a force at the poles where the air cools. But these forces are so strong that the flow becomes unstable and the simple circulation pattern breaks down. An analogy would be smoke rising from a bonfire or a cigarette. Close to the fire the smoke rises in smooth flow – this is called laminar flow. But as the smoke rises it is accelerating because the fire is continually supplying heat which leads to a force. At a certain height the smoke has accelerated so much that it suddenly becomes turbulent and forms rising billowing patterns.
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What structure do the unbalanced forces create and what are its components, including the region around the Equator?
On the Earth the forcing does eventually generate a stable north–south overturning pattern; one with six circulation cells – three in each hemisphere. From pole to Equator these are the polar cell, the Ferrel cell, and the Hadley cell (Figure 4.2). The region where the Hadley cells either side of the Equator meet is called the Inter-Tropical Convergence Zone.
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Describe the air circulation pattern and its components.
The image is based on a globe. The 60 degree and 30 degree lines of latitude are marked, north and south, as is the Equator. The globe is surrounded by a covering, rather similar to an insulated jacket, in that it is made up of a set of cells, or air pockets. The cells wrap around the globe following lines of latitude. The top cell is called the polar cell, and it covers the North Pole, as far south as the 60 degree line of latitude. The cell below is called the Ferrel cell and it circles the globe between 60 and 30 degrees north. The cell between 30 degrees north and the Equator is called the Hadley cell. South of the Equator, the cells are a mirror image, so there is a Hadley cell between 0 and 30 degrees south; a Ferrel cell between 30 and 60 degrees south, and a polar cell over the south pole. The cells are generally slightly thicker where they are closest to the Equator. A section of the globe and its cover has been cut away so that you can see a cross section through these cells. Inside the cells, arrows have been drawn to indicate the direction of air movement. In the northern polar cell, cold air (indicated by blue arrows) flows south along the surface of the Earth, then rises away from the surface at the southern boundary of the cell, and heads north again over the top of the south-bound flow, to create a continuous loop of air. In the northern Ferrel cell, warm air (indicated by a red arrow) flows south along the surface of the Earth, then rises away from the surface at the southern boundary of the cell, cools, and heads south again (indicated by a blue arrow) over the top of the north-bound flow, to create a continuous loop of air. In the northern Hadley cell, warm air (indicated by a red arrow) flows south along the surface of the Earth, then rises away from the surface at the Equator, cools, and heads north again (indicated by a blue arrow) over the top of the south-bound flow, to create a continuous loop of air. The air movement within the cells below the Equator is also a mirror image of that of the northern hemisphere
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Describe the air circulation from East to West and why it comes about.
The forcing provided by the Sun’s energy can generate horizontal wind (i.e. along the Earth’s surface) from pole to Equator. But this forcing can also generate a horizontal wind in an east–west direction. Sites in the same hemisphere and at an equal distance from the Equator receive the same amount of solar energy, but because of different specific heat capacities and albedos the land and sea heat up to different extents. The result is that the air above the land is warmed more than air above the ocean. The land surface of North America will heat up more rapidly than the Pacific Ocean – one of the main reasons is the lower specific heat capacity. Just like in Figure 4.1, this differential heating leads to differential forcing, and this leads to horizontal winds in an east–west direction. There is a force driving the Equator to pole winds, and a force driving east to west winds. Combined together, these winds move the moisture that the air picks up from the Earth around the planet. When moist air moves into cooler regions you can see how these effects contribute to our question of ‘Why does it snow in winter?’
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Coriolis effect
Once the forces have started the air moving, other forces take effect and the moving air is deflected from its path by the Coriolis effect. This force arises due to the rotation of the planet to the east, and when there is an unbalanced force, Newton’s laws come into play. The unbalanced force leads to an acceleration, and the result is that winds in the Northern Hemisphere are deflected to the right as the Earth rotates. In the Southern Hemisphere the winds are deflected to the left (Figure 4.3). Described image Figure 4.3 The Earth rotates about the north–south axis and as a result the winds are deflected to the right in the Northern Hemisphere and the left in the Southern Hemisphere, following example paths shown by the arrows. Note that the apparent movement increases with increasing latitude.
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Prevailing winds
The combined effect of the heat balance, the north–south overturning circulation in Figure 4.3, and the east–west forces that arise from the distribution of land gets the winds moving on a large scale. Once moving, the Coriolis force takes effect and a prevailing wind system is set up that means that the air pressure at the surface of the Earth is not constant: a system of less dense and more dense regions forms. And as the air rushes from high pressure to low pressure, horizontal winds form and they are in turn deflected, to give a mean pattern of surface winds on Earth. When you walk around in wide open spaces it is usually the weather rather than the prevailing wind that you experience. The difference is that winds from the weather can come from any direction. The prevailing winds are the winds that come from a preferential direction, when considered over the course of a season or longer time period. The effects of the prevailing wind can be seen in nature (Figure 4.4).
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Describe prevailing winds in January and July.
This figure is in two parts, (a) and b. Both are based on a map of the world, showing the outline of the continents, centred on the Atlantic. Part (a) is labelled July and Part b is labelled January. Both parts illustrate three things for the particular time of year: first, areas of relatively high and low air pressure using a red to blue colour scale; second, the mean position of the Inter-Tropical Convergence Zone, as a single green line; and third, the most common or prevailing wind direction. In Part (a), July, a band of generally high pressure air runs parallel to the Equator, between about 0 and 20 degrees south. There is also an air pressure high over the North Pacific, centred at about 160 degrees W and 40 degrees N, and over the north-central Atlantic (close to the Azores). A deep low pressure is centred over Tibet and the Gulf of Arabia. In Part (b), January, the areas of high pressure are more scattered. The band south of the Equator in Part (a) has broken here into three discrete and weaker zones, and there are now three areas of higher pressure north of the Equator. The largest area of high pressure is over the centre of the Asian continental mass. A weaker high crosses the Atlantic from the Caribbean to North Africa and Spain. In Part (a), July, the mean position of the Inter-Tropical Convergence Zone is shown as a wiggly line generally between the Equator and 20 degrees North. It appears to be deflected slightly northwards over land-masses, and slightly south over oceans, although there is not an absolute correlation. In Part (b), January, the mean position of the Inter-Tropical Convergence Zone is generally between the Equator and 20 degrees South. The correlation between position and land or sea is even less clear. The wind arrows are of two types. Faint arrows represent the most frequent wind direction. Bold arrows represent prevailing wind directions, when 50% or more of the wind observations are from the same direction. In both Part (a), and Part (b), the prevailing winds, indicated by the thicker arrows, are concentrated along the Equator. Winds circle anticlockwise round areas of high pressure in the southern hemisphere, and clockwise round area of high pressure in the northern hemisphere.
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What affects the winds.
At each time period, because of the inclination of the Earth’s axis, the energy balance is different. With the addition of the global distribution of land and the different heat capacities of the regions, the forces that drive the winds are constantly changing. Figure 4.5 shows the winds at the two seasonal extremes. If you need reminding of the difference in seasonal forcing you can look at the temperature records in Figure 3.7.
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Figure 4.5 also shows the mean summer and winter location of the ITCZ. Why does the ITCZ not lie directly along the 0° latitude line as in the schematic in Figure 4.2?
The distribution of land and the specific heat capacities of the water affect the forcing; overall this means that the join between the two Hadley cells does not run along the Equator.
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What do winds transport.
The global winds also transport one other thing around the planet: moisture. You saw in Activity 1.4 how much water is in a small cloud. The prevailing winds in Figure 4.5 move about vast amounts of water on the planet within these clouds. A photograph of our planet from space generally shows a mixture of blue from the ocean and white from the clouds rather than green and brown of land (Figure 4.6).
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Describe temperature animation.
This figure is an animation showing the monthly annual surface temperature on the Earth. It loops continually through the months of the year from January 1951 to December 1990. The map is centred on Europe and Africa. The changing temperatures are displayed as changing colours. Violet is coldest, then blue (approx. 0 degrees Celsius), then turquoise, green (approx. 10 degrees Celsius), then yellow, orange and red for the hottest temperatures shown. All year round, both poles are at the blue end of the scale, and the equatorial region is at the red end of the scale. The other colours form bands, in order, of variable thickness, between the poles and the Equator, creating a back-to-back rainbow effect. All year round, the Andes can be picked out on the west coast of South America as a finger of much cooler temperatures. Likewise, Tibet is almost always identifiable as a separate zone of cooler temperatures. The changes in temperature can be summarised broadly as follows: From September to December the northern blue zone expands from a covering over just the pole, to cover almost all of North America and Eurasia. The blue zone in the far south shrinks a little, and the rainbow bands of colours in between are pushed southwards. From January to July, the northern blue zone shrinks away again, to almost nothing and the southern violet zone expands. It never expands as far as the northern one did. Areas at middle latitudes in the southern hemisphere see less annual variation than areas of equal latitude in the northern hemisphere. For example, southern Australia varies only between orange and green, while New York goes from orange to almost violet. Generally speaking, coastal locations are also less extreme than locations in the middle of continents.
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Can you remember one other factor that controls the temperature of the surface of the land?
In Section 1.1.4 you saw that as altitude increases, the temperature falls. Mountains close to the Equator can and do have snow cover. In Africa Mount Kilimanjaro is ~300 km south of the Equator and is ~5900 m high. It has a snow cover which has been retreating over recent decades as our planet warms.
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The snow cover and frozen ocean for the Northern Hemisphere during January 1995.
This image shows a view of the Earth, as if looking down from directly above the North Pole. North America is clearly visible, as is Eurasia, and all countries north of the Equator can be seen, though greatly distorted relative to the more usual Mercator projection. Canada, Greenland, Russia and the Scandinavian countries all encircle the Arctic Ocean. Areas where sea ice covers more than 15% of the sea are shown as mid-blue. In a winter month, as shown here, the sea ice fills Baffin Bay and extends as far south as Newfoundland on the eastern American seaboard. It is further south on the eastern coast of Greenland than on the west, though it does not touch Iceland, and it surrounds Svalbard but does not touch the northern Scandinavian coasts. All of the northern coasts of Russia, Canada and Alaska are blue. Again, sea ice extends a good way south along the eastern seaboard of Russia and Kamchatka. Areas of snow cover on land are also shown. All of Canada, Alaska, Greenland, Iceland, Scandinavia, as well as most of eastern continental Europe and Russia, and the northern parts of the US have winter snow cover. The Himalayan chain of mountains can also be picked out by the snow cover..
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How does Norway keep pavements from icing over!
By heating them.
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Part 4 summary.
In this final part you have seen how the climate creates forces that, following Newton’s laws, drive global weather patterns. These weather patterns drive the water vapour and clouds to give us snow. Snow falls not only in winter, but at high altitudes as well. The key concept and principles you have studied in this part are: atmospheric circulation and how it impacts the weather Horizontal winds can be generated by vertical motions of air of different densities. The warming of air at the mid-latitudes and cooling at the high latitudes leads to an overturning circulation carrying moisture to colder regions. Large scale planetary winds are deflected by the Coriolis force which arises from the rotation of the Earth about its axis. In the Northern Hemisphere winds are deflected to the right, in the Southern Hemisphere they are deflected to the left. Our planet has a pattern of prevailing winds that changes with season as each pole successively points towards the Sun. The location of snow falls is determined by the regional climate. It is generally restricted to higher latitudes and it is ephemeral.
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Chapter 3 summary.
The topic has addressed the question ‘Why does it snow in winter?’ In Part 1 you saw how water moves in our atmosphere and in Part 2 you learned about Newton’s laws and about gravity. These laws lead to forces which move the clouds, and make the snow fall. In Part 3 you learned about the energy balance of the Earth and how our orbit leads to seasons and our climate, and in Part 4 you saw how the climate creates forces that, following Newton’s laws, drive global weather patterns. These weather patterns drive the water vapour and clouds to give us snow. Snow falls not only in winter, but at high altitudes as well.