Part 2. Air Flashcards
1
Q
What is Meteorology?
A
Meteorologists study the physical and chemical properties of the atmosphere, large-scale circulations within it, and the ways in which it interacts with the Earth’s land and ocean surfaces. Both climate and weather are manifestations of the properties and behaviour of the atmosphere, so meteorology is also concerned with short-range weather forecasting and the study of long-term changes in climate.
2
Q
Imagine that you have a quantity of water in a sealed container, but not enough to fill it. Briefly describe, in everyday terms, how the water molecules (regard each H2O molecule as an individual particle) will behave when the water is in solid?
A
In solid form, as ice, the water molecules maintain a fixed position relative to each other. A lump of ice would retain its shape without the need for the container.
3
Q
Imagine that you have a quantity of water in a sealed container, but not enough to fill it. Briefly describe, in everyday terms, how the water molecules (regard each H2O molecule as an individual particle) will behave when the water is liquid?
A
As liquid water, the molecules can move relative to each other. However, they will remain close together, so the liquid water would flow and settle at the bottom of the container.
4
Q
Imagine that you have a quantity of water in a sealed container, but not enough to fill it. Briefly describe, in everyday terms, how the water molecules (regard each H2O molecule as an individual particle) will behave when the water is gas?
A
In the gas phase, the water molecules would move freely. They would fill the container, however big it was.
5
Q
What are the main constituents of dry air?
A
Nitrogen. Oxygen. Argon, Carbon Dioxide and Ozone
6
Q
What is a trace gas?
A
A trace gas is one that makes up only a small proportion of a sample.
7
Q
What are the trace gases in air?
A
Argon, Carbon Dioxide and Ozone
8
Q
What is a mixing ratio?
A
The mixing ratio is the number of molecules (or atoms of monatomic species, such as argon) of the gas divided by the total number of molecules of all gases present. For trace gases these are given as either parts per million by volume (ppmv, where 1 ppmv is a mixing ratio of 10−6) or parts per billion by volume (ppbv, where 1 ppbv is a mixing ratio of 10−9).
9
Q
What is the mixing ratio of Nitrogen in air?
A
0.781
10
Q
What is the mixing ratio of Oxygen in air?
A
0.209
11
Q
What is the mixing ratio of Argon in air?
A
0.0093
12
Q
What is the mixing ratio of Carbon dioxide in air?
A
400 ppmv
13
Q
What is the mixing ratio of Ozone in air?
A
0–200 ppbv (troposphere)
0.1–8 ppmv (stratosphere)
14
Q
What does the atmosphere do?
A
The atmosphere transports heat, water and gases around the Earth. It shapes the solid surface and drives ocean currents. Both the ocean and the land are affected mechanically through winds, by evaporation and the removal of fresh water from some regions and by the precipitation of water as rain and snow in other regions.
The atmosphere also protects the Earth’s surface from shortwave, ultraviolet radiation from the Sun, which would otherwise damage living cells. This tenuous layer of gas surrounding the Earth is essential to all complex forms of life for these reasons as well as for the supply of gases for respiration and photosynthesis. In turn, the Earth’s atmosphere has been modified over the planet’s history by the presence of life. One result is the supply of oxygen to the present-day atmosphere by the development of photosynthesis, without which humans would not be here.
15
Q
How does the atmosphere affect the temperature of the planet and what would happen if it wasn’t there?
A
The atmosphere is, in turn, supplied with gases and small particles (known as aerosols when suspended in a gas) from the interior of the planet by volcanic eruptions, which modify the climate and so the surface temperature of the Earth. A natural ‘greenhouse effect’, trapping thermal radiation emitted by the Earth, keeps the planet’s surface about 35 °C warmer than it would otherwise be.
16
Q
What is the troposphere?
A
the lowest 10–15 km of the atmosphere, where most weather phenomena occur and about three-quarters of the mass of the atmosphere is located
17
Q
What is the coldest part of the atmosphere?
A
The tropopause
18
Q
What is a lapse rate?
A
This change of temperature with height is known as the lapse rate
19
Q
What is the typical lapse rate for the lower troposphere?
A
A typical lapse rate for the lower troposphere is about 6 °C km−1 but this value varies with location and the time of yea
20
Q
In what layer of the atmosphere does the weather mainly occur?
A
The troposphere
21
Q
In what layer of the atmosphere does the temperature stop decreasing?
A
The temperature stops decreasing at the tropopause and then begins to increase in the stratosphere
22
Q
Why does the temperature increase in the stratosphere?
A
Largely, this change is caused by internal heating of the stratosphere where a layer of ozone absorbs incoming solar ultraviolet radiation. The stratosphere is so named because it is a very stable, or stratified, region with little convection.
23
Q
What parts of the atmosphere are known as the lower, middle and upper atmosphere?
A
The troposphere is known as the lower atmosphere and the stratosphere and mesosphere can be referred to together as the middle atmosphere. The expanse above the mesopause is called the upper atmosphere.
24
Q
What happens to the temperature at the stratopause?
A
the temperature again reverses at the stratopause, decreasing with height again throughout the mesosphere.
25
What is latitude?
Latitude is the angle that measures a place’s location with respect to the poles and the Equator
26
What is the latitude of the equator, north pole and south pole?
The Equator has a latitude of 0°, the North Pole is 90° N and the South Pole is 90° S.
27
How does the location on Earth affect the amount of solar radiation and why?
Locations on Earth that are closer to either of the poles generally receive less solar radiation than those closer to the Equator. The explanation for this is based on the angle between the direction of the solar radiation and the Earth’s surface, or, in everyday language, the height of the Sun above the horizon. In equatorial regions, the Sun is almost directly above an observer around noon. However, in polar regions the Sun is never high in the sky, even in summer, although it can remain above the horizon (i.e. there is continual daylight) for up to six months of the year. In winter, the Sun can be below the horizon for up to six months.
28
How will a low lying high latitude sun affect the solar radiation that reaches the Earth?
A low-lying, high-latitude Sun means that the incident (incoming) radiation is spread over a greater surface area than in equatorial latitudes, as shown in Figure 2.1.5. This ‘dilution’ of radiation per unit area is so important that it outweighs the effect of the almost continuous day in the polar summer, meaning that the poles do not become warmer than low latitudes. Also, as you can see in Figure 2.1.5, solar radiation has to traverse a greater depth of the atmosphere before arriving at locations that are further from the Equator. This means that atmospheric absorption of the solar radiation is increased, and less energy reaches the surface.
29
What causes the Earth to have seasons?
The tilt of the Earths axis
30
What is a hemisphere?
each half of the Earth, either north or south of the Equator
31
How can energy present itself in our atmosphere?
Energy is present in the atmosphere in a variety of forms. It can present as thermal energy of warm air, from the slightest breeze to the most violent thunderstorm, as kinetic energy in the weather as well as in radiation.
32
Where does almost all of the Earths energy come from? and in what form?
Almost all of this energy is ultimately supplied from our local star – the Sun – in the form of solar radiation.
33
Where does thermal energy that originates from within the Earth come from?
Through various cycles, some energy does enter the atmosphere from thermal energy within the Earth. This energy originates mainly from radioactive decay and some residual energy from the Earth’s formation (e.g. volcanoes, hot springs and geysers), but it represents an insignificant contribution to the atmosphere’s total energy compared with the energy from the Sun.
34
What is the SI unit of power?
The watt is the SI unit of power
35
What is the total energy output per second from the sun?
The total energy output per second from the Sun, or radiative power, amounts to a colossal 3.85 × 10^26 watts, where 1 watt (W) equals one joule per second (J s−1)
36
What is a commonly used term for solar radiation that reaches the Earth.
A commonly used term when considering the solar radiation that reaches Earth is insolation. This is the energy arriving per unit area of the planet’s surface.
37
What is electromagnetic radiation?
You can think of electromagnetic radiation as waves travelling through space, carrying energy in the form of oscillating electric and magnetic fields
38
describe the term wavelength?
The distance from one crest in each field to the next is the wavelength.
39
What you see electromagnetic fields?
You cannot see these fields themselves, but your eyes are designed to detect the waves at one specific range of wavelengths (known as the visible part of the spectrum or, in everyday language, ‘light’) and relay the information to the brain. Different wavelengths in the visible range are interpreted as colours, and white light is a mixture of all visible wavelengths.
40
How does a radio detect radio waves?
A radio performs a similar function to the eye in a different part of the electromagnetic spectrum, detecting radio waves and relaying them to a speaker to make sound.
41
Electromagnetic waves with wavelengths shorter than 4 × 10−7 m are described as ultraviolet radiation; this continues down to about 1 × 10−8 m, at which point they start to become known as X-rays. Electromagnetic waves with wavelengths longer than 7 × 10−7 m are described as infrared radiation; this continues up to about 1 × 10−3 m, at which point they start to become known as microwaves. Ultraviolet, visible and infrared radiation are the most important regions to be aware of for understanding the atmospheric energy balance.
Order ultraviolet, visible and infrared radiation by increasing frequency?
Infrared radiation has the lowest frequency, then visible radiation, and ultraviolet radiation has the highest frequency.
42
What is a black body in regards to electromagnetic waves?
A black body is an ideal, theoretical object which both emits and absorbs radiation perfectly at all wavelengths.
43
State how the peak wavelength of an object’s spectrum depends on temperature, and whether higher or lower wavelength radiation would be emitted by a hotter object. λT = 2.9 × 10−3 m K
So, peak wavelength is inversely proportional to temperature. A hotter object would emit lower wavelength radiation.
λ= (2.9 × 10−3 m K)/T
44
What is back scattering?
Scattering in which the path of radiation is changed by more than 90°, so that the radiation which was moving downwards is now moving upwards and vice versa, is known as back-scattering. Back-scattering is sometimes referred to as reflection (more accurately ‘diffuse reflection’). However, that term will be reserved for mirror-like reflection from a surface, where all the light is bent through the same angle, rather than the process here, where light is bent through a varied range of angles by particles or rough surfaces.
45
Why is the sky blue?
In fact, the reason the sky is blue is because incoming light radiation with a short wavelength (i.e. blue) is scattered through much larger angles than longer wavelengths (i.e. red) when both interact with particles (atoms and molecules, in this case) that are very much smaller than the wavelength of the incoming radiation. This phenomenon is called Rayleigh scattering after the British physicist Lord Rayleigh (1842–1919). When an observer is looking away from the Sun, a light ray from it would have to divert through a large angle to reach the person’s eye and only blue light (and shorter wavelengths) is scattered to such an extent. Hence, most of the sky appears blue.
46
What components of the atmosphere help to keep the Earth cool by limiting the amount of solar radiation?
Light is also scattered by particles that are larger than its wavelength. A good example is scattering by clouds, which appear white or various shades of grey. Clouds scatter solar radiation in all directions, including back into space (back-scattering). For this reason, clouds have a significant effect on the Earth’s atmosphere.
In addition to back-scattering by clouds, the surface of the Earth can also scatter light by different amounts. For example, snow and ice back-scatter much more light than forests or grassland. All of these factors contribute to how much of the incoming solar radiation is absorbed by the Earth. Clouds also scatter the outgoing terrestrial radiation, which can ‘close’ the atmospheric infrared window and keep the surface warmer; their net effect is complex, and is a subject of much ongoing research.
47
What is steady state?
The atmosphere is dynamic and constantly changing. However, if conditions in the atmosphere were averaged over long enough time periods (which might be many weeks to allow for weather variations, and possibly one or more years to allow for seasonal effects), you might expect to see a broadly steady picture in which the amount of energy in different forms was constant. This is known as a steady state
48
What is a dry adiabatic lapse rate?
In the troposphere, if a parcel of dry air is lifted rapidly (perhaps through being heated slightly more than its surroundings, or pushed up by a front or by wind passing over a mountain), it expands and is cooled by about 10 °C per 1 km of height gained. In a similar way, a lowered parcel of dry air is compressed and is warmed by about 10 °C per 1 km of height lost. This is the dry adiabatic lapse rate; ‘adiabatic’ means without energy or mass exchange with its surroundings, which is why the parcel was said to be lifted rapidly.
49
Is the dry adiabatic lapse rate the same as the observed background lapse rate?
The dry adiabatic lapse rate is not the same as the observed background lapse rate, which is generally around 6 °C km−1 in the lower troposphere. This background rate exists because of a combination of air movement, mixing and heating in the atmosphere (i.e. non-adiabatic processes as well as adiabatic lifting).
50
Imagine that an air parcel from Figure 2.1.17 at 5 km altitude is rapidly lifted up to 7 km. How would its temperature change? (Assume that the parcel remains unsaturated and use the dry adiabatic lapse rate for it.) What would be its final temperature?
The air parcel’s temperature would fall by 20 °C, since it has risen by 2 km (using the dry adiabatic lapse rate of 10 °C per 1 km). Its initial temperature was about −20 °C (reading the blue line) so, at 7 km, it would have a temperature of −40 °C.
51
What would happen to a rising air parcel at 5 km in Figure 2.1.17 if the background temperature profile instead had a slope of −12 °C km−1?
The parcel would always be warmer than its surroundings as it rose, since its temperature would only fall at −10 °C km−1, so it would continue to rise.
52
What is convection?
In unstable regions, air rearranges itself by rapid overturning motions and modifies the background temperature profile back towards a stable one. This process is called convection.
53
What does the amount of solar radiation that a location on Earth receive strongly depend on?
The amount of solar radiation that a location on Earth receives depends strongly on its latitude and the season of the year.
54
Which part of the Earth receives more solar radiation, how is this energy transferred or balanced?
The Earth receives an excess of solar radiation at the Equator but radiates more than it receives at the poles.
55
What is the Hadley circulation?
In 1735, an English scientist called George Hadley proposed a simple mechanism for the Earth’s atmospheric circulation. His idea is illustrated in Figure 2.1.20. Warm air rises from the hottest zone near the Equator and flows high up, towards the poles, where it then sinks. The air near the surface flows in the opposite direction, towards the Equator.
56
What does Hadleys circulation model not take in to account?
The rotation of the Earth
57
What is thermally indirect?
Circulation such as this, in which warm air sinks and cold air rises, is called ‘thermally indirect’.
58
Where does the force of atmospheric pressure come from?
the atmosphere is subject to attraction by the Earth’s gravity, as all masses are. The atmospheric pressure which you experience at the surface of the Earth is simply the force of gravity acting on the total mass of air per unit area above you. It is the pressure resulting from a force of 1 N exerted over an area of 1 m2.
59
What is the SI unit of pressure?
The SI unit of pressure is called the pascal (Pa).
60
Define pressure within the atmosphere?
Pressure is a fundamental variable which describes the atmosphere and is related to the total mass of air above any point.
61
What happens to pressure with increasing height?
Pressure falls exponentially with increasing height above the surface.
62
What are horizontal pressure gradients closely related to?
Horizontal pressure gradients are closely related to winds; large pressure gradients imply large winds.
63
What three quantities can describe any kind of gas?
A gas of any kind can be described in terms of three key quantities:
temperature
pressure
density.
64
what symbol is pressure denoted by and what is it measured in?
Pressure is denoted by p, and is measured in pascals (Pa)
65
What symbol is temperature denoted by and what is it measured in?
Temperature is denoted by T, and should be converted to absolute values in kelvin (K)
66
What symbol is density denoted by and what is it measured in?
Density is denoted by ρ (the Greek letter ‘rho’ – be careful not to confuse this with p in some fonts), and is measured in kg m−3. Density is simply a measure of the mass per unit volume of something.
67
What makes an ideal gas?
An ideal gas is one which can be modelled as a group of small particles which move randomly and do not interact.
68
What is the usual form of the ideal gas equation of state?
pV = nRT
69
What is the key driver of most weather systems?
The heating and cooling mechanisms, which are key drivers of weather systems, are generally manifested as a change in air temperature.
70
On the kelvin scale what is the freezing temperature of water?
On the kelvin scale, the freezing temperature of pure water is 273.15 K.
71
Convert 15 °C to a temperature measured in kelvin, rounding to the nearest whole degree, following the significant figure rules for adding numbers by only retaining the precision of the least precise.
15 °C = 15 + 273.15 K = 288 K
72
Convert a hot summer air temperature of 95 °F to degrees Celsius and then to kelvin, giving your answer to the nearest whole number in each case.
95 °F is equal to
| 35 °C = 35 + 273.15 K = 308 K
73
How does the coriolis effect winds in the northern and southern hemisphere?
Remember that the Coriolis effect tends to deflect winds to the right of their direction of travel in the Northern Hemisphere and to the left in the Southern Hemisphere. This means that winds in the Northern Hemisphere will tend to circle anticlockwise around low pressure systems and clockwise around high pressure systems, as would be seen from above. The opposite is true in the Southern Hemisphere.
74
What does a wind velocity comprise information about?
Wind velocity comprises information about both a wind’s speed and its direction.
75
What is the SI unit for speed?
The SI unit for speed is metres per second, m s−1
76
Convert 1mph to km and m s−1?
1 mph = 1.61 km h−1 = 0.447 m s−1
77
Covert knots to km and m s−1?
A knot is one nautical mile (one 3600th of the distance around a meridian of the Earth, now internationally defined as 1852 m) per hour, so the relevant conversions are:
1 kn = 1.852 km h−1 = 0.5144 m s−1
78
What does the beaufort scale do?
Wind speed can be estimated by considering how different wind strengths affect either the sea surface or features of the land surface. This is the basis of the well-known Beaufort scale, which was developed in 1805 by a British Royal Navy officer, Francis Beaufort. His efforts were used very widely, to relate how disturbed the sea surface is to the speed of the wind blowing across it.
79
What does the ideal gas equation of state link?
The ideal gas equation of state links the pressure, temperature and density of air. If any two of these variables are known, the third can be determined.
80
What is specific humidity?
The specific humidity (s.h.) is simply the mass of water vapour per kilogram of air: a mass mixing ratio. Since this quantity is a ‘mass per mass’, specific humidity is actually a number without any units. However, for convenience (there are usually several grams of water per kilogram of air), specific humidity is often given in units of grams per kilogram, g kg−1, which are specified to avoid any ambiguity.
81
What is relative humidity?
It is much more common to see relative humidity (r.h.) quoted in meteorological reports. The relative humidity is defined as the ratio of the amount of water vapour actually in the air at a given temperature to the amount that would correspond to saturation at that temperature (saturation means that the air would be in equilibrium with liquid water with which it was in contact, exchanging equal numbers of water molecules back and forth between gas and liquid). The ‘amount’ here means the number of molecules. This is directly proportional to the partial pressure of the water vapour. Each gas in a mixture of gases contributes a proportion of the total pressure in proportion to its number mixing ratio.
82
How are meteorological measurements of temperature made?
Meteorological measurements of temperature are made with drybulb and wetbulb thermometers.
83
What is a drybulb temperature?
The drybulb temperature is simply a conventional measurement of air temperature, and is the temperature that has been plotted so far in this part
84
How does a drybulb temperature differ from a wetbulb?
The wetbulb thermometer differs from the drybulb one in that its bulb is covered by a muslin wick. The wick leads to a bottle of distilled water to ensure that the thermometer bulb is always wet.
85
How is a wetbulb temperature gained?
The wetbulb temperature depends on the rate of evaporation of water from the soaked muslin surrounding its bulb. As water evaporates from the muslin, energy is converted into latent heat, cooling the cloth down (in the same way that evaporation of sweat from skin cools the body). The bulb loses heat to the cooler cloth, so the wetbulb thermometer registers a lower temperature.
86
What does a wetbulb temperature measure?
If the air is very damp, the evaporation rate is low and the consequent cooling of the bulb is small. In contrast, if the air is dry, there is a lot of evaporative cooling, so the temperature drops substantially. The wetbulb temperature for a given point in time is thus a measure of the air’s humidity and is cooler than the drybulb temperature at that same moment, except when the air is saturated and both are equal.
87
What is a psychrometric chart used to convert?
A psychrometric chart is used to convert between drybulb temperature, wetbulb temperature, partial pressure of water vapour and relative humidity
88
What is a psychrometer?
A psychrometer is just the scientific name for a dry-and-wet-bulb thermometer used to measure humidity.
89
What is a dewpoint temperature?
when water will condense is the dewpoint temperature. This is the temperature to which air would have to be cooled (without changing its total moisture content or any other conditions) for it to reach 100% saturation. For a given sample of air, the difference between the actual air temperature and the dewpoint temperature is therefore an indication of how close the air is to being saturated. At saturation, the drybulb, wetbulb and dewpoint temperatures are all the same.
90
How do you find the dewpoint temperature on a psychrometric chart?
To find the dewpoint temperature on the psychrometric chart, locate the point where the drybulb and wetbulb temperatures intersect, then follow that point horizontally (at constant water vapour pressure) to the left, and back to the curve of the edge of the graph (i.e. the 100% humidity line where the drybulb temperature equals the wetbulb temperature). This is the dewpoint temperature.
91
How does dry air cool compared to saturated air when it rises?
As you should remember, dry air cools by about 10 °C km−1 when it rises. Saturated air cools less quickly, by about 6 °C km−1 (close to sea level) when it rises. This is because, as saturated air cools, water condenses to liquid droplets or ice particles to maintain the saturation at about 100%, releasing latent heat of condensation which slightly warms the parcel compared with dry air.
92
How do unstable conditions arise with parcels of air?
Completely unstable conditions arise when a lifted parcel of air is always warmer, and therefore always less dense, than its surroundings.
Instability occurs when the air above is relatively cold and lower air is heated, perhaps by contact with a warming surface during the day. Completely unstable conditions are relatively rare, except sometimes close to the surface if it warms quickly during the day.
93
In what two ways can water content in the air be measured? explain what both of these measurements are?
Water content can be measured as specific humidity or relative humidity. The specific humidity is a measure of the total amount of water in the air. The relative humidity indicates how close the air is to the condensation temperature and so how dry the air will feel.
94
what is fog/mist?
A fog or mist is essentially a cloud that forms at the surface, with a cloud base (the lowest altitude of visible cloud) of 0 km.
95
At what temperature can supercooled water droplets exist at?
Supercooled water droplets can exist in clouds between 0 °C and −40 °C
96
How can the height of the base of a cloud be estimated?
The height of the cloud base can be estimated by finding the lifted condensation level (LCL)
97
How is the lifted condensation level of a cloud defined?
The LCL is defined as the height at which a parcel of air from the ground will reach its dewpoint if lifted rapidly with no exchange of energy or mass with its surroundings.
98
What would be considered a low cloud base?
low <2000 m
99
What could be considered a medium cloud base?
medium 2000–5000 m
100
What could be considered a high cloud base?
high >5000 m
101
What are the 4 main classifications of clouds?
stratus, cumulus, cirrus and ‘nimbus’.
102
What is the altitude of the base of a cirrus cloud?
High: above 5000 m (16 000 ft)
103
Describe the shape of a cirrus cloud?
Delicate and fibrous clouds, in patches or bands. Popularly called ‘mares’ tails’ because of their appearance.
104
What colour is a cirrus cloud?
Mainly white
105
What is the altitude of the base of a cirrocumulus cloud?
High: above 5000 m (16 000 ft) in mid-latitudes
106
What is the altitude of the base of a cirrostratus cloud?
High: above 5000 m (16 000 ft) in mid-latitudes
107
Describe the shape and visible features of a cirrostratus cloud?
Thin veil of cloud with some fibrous structure. The Sun is visible through the cloud and casts shadows. There are often halo effects round the Sun or a full Moon.
108
What colour is a cirrostratus cloud?
Transparent white
109
Describe the shape and visible features of a cirrocumulus cloud?
Thin patch or sheet of more or less separate, rounded cloudlets in ripples.Extensive cirrocumulus is popularly described as a ‘mackerel sky’.
110
What colour is a cirrocumulus cloud?
Mainly white
111
What is the altitude of the base of a Altostratus cloud?
Medium: 2000–7000 m (6500–23 000 ft) in mid-latitudes
112
What is the altitude of the base of a Altocumulus cloud?
Medium: 2000–7000 m (6500–23 000 ft) in mid-latitudes
113
Describe the shape and visible features of a Altostratus cloud?
Sheet cloud with a fairly uniform appearance. Often cover the entire sky. The Sun may be visible as if through ground glass, but no shadows are cast. No halo effects.
114
Describe the shape and visible features of a Altocumulus cloud?
Broken, patchy clumps of cloud, often in ripples or bands. Thicker altocumulus can have a ‘quilted’ appearance. With the arm extended at least 30° above the horizontal, altocumulus cloud elements appear to be two or three fingers’ width across.
115
What colour is a Altostratus cloud?
Greyish
116
What colour is a Altocumulus cloud?
White or grey
117
What is the altitude of the base of a cumulus cloud?
Low: below 2000 m (6500 ft)
118
What is the altitude of the base of a stratocumulus cloud?
Low: below 2000 m (6500 ft)
119
What is the altitude of the base of a stratus cloud?
Low: below 2000 m (6500 ft)
120
What is the altitude of the base of a Cumulonimbus cloud?
Low: below 2000 m (6500 ft)
121
What is the altitude of the base of a Nimbostratus cloud?
Low: below 2000 m (6500 ft)
122
Describe the shape and visible features of a Cumulus cloud?
Puffy, cauliflower-shaped, detached cloud, with a flat base and sharp outlines. Usually well separated, with a lot of blue sky between individual clouds.
123
Describe the shape and visible features of a stratocumulus cloud?
Sheet-like, but composed of rounded, lumpy, individual cloud elements that may form in rolls. With the arm extended, stratocumulus cloud elements appear roughly fist-sized.
124
Describe the shape and visible features of a stratus cloud?
Uniform, featureless cloud with a level base, often covering the entire sky. No accompanying precipitation, or only a light drizzle.
125
Describe the shape and visible features of a Cumulonimbus cloud?
Very tall, giant cloud, often with an anvil-shaped top. Associated with heavy showers, hail and thunderstorms.
126
Describe the shape and visible features of a nimbostratus cloud?
Featureless thick layer of cloud. Associated with continuous steady rain or snow, and poor visibility.
127
How is the amount of cloud reported by observers?
The amount of cloud is reported by observers numerically as eighths (or oktas, from the Greek for ‘eight’) of the sky covered both by individual types of cloud and as the total cover of all cloud types. Eighths are used because the transmitted report of cloud amount uses just one digit from 0 to 9. A value of 0 oktas clearly means no cloud at all, 4 oktas means half the sky covered with cloud, 8 oktas means fully overcast, but 9 oktas (i.e. nine-eighths) may seem strange. It means that the sky is obscured by fog or blowing snow, for example, so that nothing can be reported.
