Chapter 3 Flashcards
What is snow?
Crystallised form of water in the atmosphere.
How much of Earth does snow cover?
Snow covers 23% of Earth temporarily or permanently. NB: 71% oceans and their marginal seas, so 29% land.
What are snow needles?
They form when the temperature is above minus 12 Celsius. Below that, water vapour forms into hex crystals i.e. snowflakes.
What are snowflakes, what is their structure and how do they form?
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.
Types of snow crystals and particles.
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).
Evolution of snow on the ground.
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.
Where does snow fall?
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).
What are the effects and benefits of snow?
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.
Composition of air.
78% nitrogen 21% oxygen 1% argon; very small traces of other gases, including carbon dioxide and water vapour.
Carbon dioxide in atmosphere.
CO2 atmospheric concentration is rapidly increasing owing to the burning of fossil fuels - primary driver of the ongoing global warming.
Water vapour in atmosphere.
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.
Air molecules/m3 (and) air pressure at various altitudes.
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
Temp decrease with altitude. Lowest temp and atmosphere layer where this is recorded.
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.
Thermal conduction
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.
Convection
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.
Thermal Radiation
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.
Thermals and heat haze.
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.
Atmosphere temperature structure.
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.
Endothermic process.
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.
How does water get into the atmosphere?
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.
But how does evaporation remove energy from the Earth’s surface?
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). )
What are clouds?
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.
Types of clouds
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.
1.2.1 Investigating clouds - work it out!
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.
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.
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.
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).
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.
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.
Why does snow crunch underfoot?
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.
