Clouds are formed when the air reaches its saturation or dew point, when it contains as much water vapour as it can physically hold. This can occur when water vapour gradually accumulates into the air by evaporation over the sea or, as is more usual, when the temperature of moisture laden air gradually falls. When saturation occurs, some of the moisture vapour condenses into tiny mist-like water droplets that form fog and clouds.
[img:]http://www.squ1.com/archive/climate/images/satelite-image.jpg[/img]
Clouds can form anywhere in the troposphere, even at ground level (at which point it is commonly called fog). Although composed of condensed liquid, they are light enough to float in the air and be carried along by winds. In addition to occasionally raining on the areas below them, clouds can also affect temperatures. They not only block incoming solar radiation during the day, but also block long-wave radiation from escaping from the Earth's surface at night which can keep temperatures warm.
Measuring Cloudiness
The term cloudiness refers to the amount of the overall sky dome covered by cloud. Estimating this amount of coverage is usually done by eye - the meteorological observer pops outside the office every hour and has a quick look. This means that the readings are not particularly precise, mainly because they don't need to be. To make it easy on the observer, cloud cover is typically given in octa which ranges in value from 0 to 8, 0 being a clear blue sky and 8 being a fully overcast sky. It can also be given in tenths or even as a percentage when a computer is used to analyse digital photographs of the sky dome.
[img:caption=Figure 1 - A 360° fisheye view of the sky dome, from which computer software can discern the percentage cloud cover.]http://www.squ1.com/archive/climate/images/fish-eye.jpg[/img]
Discerning cloud cover from clear sky is a simple task for most humans, but requires a relatively sophisticated computer program. An early experiment with this in Western Australia some 10 years ago took a standard PC more than 30 minutes to process an image. Naturally the project required images taken every 15 minutes so the backlog soon mounted up. Both computer speed and processing techniques have progressed since then, however very few stations around the world have automated cloud cover measurements.
The Effect of Clouds
As noted earlier, apart from dumping the odd bit of rain, the main effect of clouds is to moderate the availability of solar radiation. When a cloud passes in front of the sun, it shades the ground below from the direct sun. Depending on the shape and density of the cloud, it can either increase or decrease the available diffuse radiation. A bright fluffy cloud may actually increase the diffuse component as the sunlight passes through and reflects off all the tiny water droplets. A dark black cloud heavy with rain will likely reduce overall radiation. Figure 2 below illustrates this effect, taken from actual weather data in Perth, Western Australia.
[image:caption=Figure 2 - The effect of cloud cover on hourly solar radiation.
Image taken from ECOTECT v4.]http://www.squ1.com/archive/climate/images/cloud-effect-daily.gif[/img]
To more clearly illustrate the effect, Figure 3 below shows the annual solar radiation in Perth falling on an unobstructed horizontal surface. The X axis represents days of the year (with a dividing line between each month) whereas the Y axis shows hours of the day. The top graph shows the incident radiation if there were absolutely no clouds in the sky at any time throughout the year. The bottom graph shows the actual data with clouds. Perth is not actually that cloudy but, even in summer, their effect is clearly visible.
[img:caption=Figure 3 - The effect of cloud cover on solar radiation incident on an unobstructed horizontal surface.
Images taken from ECOTECT v4.]http://www.squ1.com/archive/climate/images/cloud-effect-annual.gif[/img]
Lighting Effects
The amount of solar radiation directly affects sunlight and daylight levels. Given that clouds affect direct and diffuse radiation differently, it turns out that a clear blue sky is not always the brightest sky, especially when the sun is directly overhead. If there are bright white fluffy clouds in the sky, especially close to the horizon, they can actually reflect a significant amount of daylight into a building through vertical windows. In design studies and computer lighting analysis, this effect is taken into account by the mathematical model used to distribute luminous intensity over the sky dome. Figure 4 below shows the most commonly used models developed by the Commission International de l’Eclairage (CIE). For more information on the use of these sky models, see the daylighting topics.
[img:caption=Figure 4 - CIE standard sky types and their luminous distribution.
Rendered in RADIANCE.]http://www.squ1.com/archive/climate/images/cie-sky-types.jpg[/img]
Cloud Types
Clouds are classified according to their appearance and height. Based on appearance there are two major types: Clouds of vertical development formed by the condensation of rising air, and clouds that are layered formed by condensation in the air without vertical movement. The following table details the different types of cloud and their appearance.
Table 1 - Cloud name prefixes and their meaning.
| NAME | DESCRIPTION |
|---|---|
| Stratus, strato | Layered or sheet-like |
| Cumulus, cumulo | Puffy, heaped (vertical) |
| Nimbus, nimbo | Dark and rain bearing |
| Cirrus, Cirro | Curly, featherlike (high cloud) |
| Alto | High (but used to describe a middle cloud) |
| Fracto | Broken or fractured |
After appearance, classification is based on the height of the cloud above ground level, as shown in the following table.
Table 2 - Cloud types classified by height above ground level.
| CLASS | NAME | COMPOSITION | BASE HT |
|---|---|---|---|
| High | Cirrus, Cirrocumulus, Cirrostratus | Frozen water droplets or ice crystals | > 6km |
| Middle | Altostratus, Altocumulus | Ice crystals and/or water droplets | 2 - 6km |
| Low | Nimbostratus, Stratus, Stratocumulus, Fractostratus, Fractocumulus | Water droplets (ice crystals in winter) | 15m - 2km |
| Vertical | Cumulus, Cumulonimbus | Water droplets at lower levels and ice crystals at upper levels | < 15m |
Cirrus - Ci
[img:]http://www.squ1.com/archive/climate/images/cloud-cirrus.jpg[/img]
Cirrus are thin, wispy clouds that usually form above 5km. These clouds are blown by strong westerly winds aloft into streamers known as mares' tails. Cirrus clouds generally move from west to east across the sky and usually 'point' to fair weather. Cirrus form when water vapour undergoes deposition and forms into ice crystals. They are thin because they form in the higher levels of the atmosphere where very little water vapour is present.
Cirrocumulus - Cc
[img:]http://www.squ1.com/archive/climate/images/cloud-cirrocumulus.jpg[/img]
These clouds form above 18,000 feet and appear as small, rounded white puffs that are isolated or in long rows. When the white puffs are in rows, they give the cloud a rippling appearance that distinguishes it from a cirrus or a cirrostratus cloud. Cirrocumulus clouds rarely cover the entire sky. The individual cloud elements that reflect the red or yellow light of a setting sun make this one of the most beautiful of all clouds. The small ripples in a cirrocumulus cloud resemble the scales of a fish, which earned the nickname "mackerel sky".
Cirrostratus - Cs
[img:]http://www.squ1.com/archive/climate/images/cloud-cirrostratus.jpg[/img]
These sheet-like, thin clouds form above 18,000 feet. Cirrostratus clouds are so thin that the sun and moon can be seen clearly through the cloud. Sometimes these clouds are so thin that the only clue to their presence is a halo around the sun or moon. Ice crystals in cirrostratus clouds bend the light rays from the sun or moon as they pass through the cloud, which forms the halo around the sun or moon. Thick cirrostratus clouds give the sky a bright white appearance and often signal rain or snow within 24 hours, especially if they are followed by mid-level clouds.
Altostratus - As
[img:]http://www.squ1.com/archive/climate/images/cloud-altostratus.jpg[/img]
These grey or bluish-grey clouds form between 6,000 and 20,000 feet. Altostratus clouds cover the entire sky over an area that usually extends over hundreds of square kilometres. The sun may be visible under thinner sections of the cloud as a dim, round disk, known as a 'watery sun'. Altostratus clouds usually form ahead of a storm producing widespread and mostly continuous precipitation. Altostratus clouds do not allow enough sunlight to break through the cloud to form any shadows on the ground.
Altocumulus - Ac
[img:]http://www.squ1.com/archive/climate/images/cloud-altocumulus.jpg[/img]
These clouds form between 6,000 and 20,000 feet and appear as grey, puffy blobs, sometimes rolled out in parallel waves or bands. One part of the cloud is usually darker than the rest, which helps distinguish this cloud from the higher cirrocumulus clouds. Altocumulus clouds that look like "little castles" in the sky indicate rising air at the cloud level. The appearance of these clouds on a warm, humid summer morning are often followed by afternoon thunderstorms.
Nimbostratus - Ns
[img:]http://www.squ1.com/archive/climate/images/cloud-nimbostratus.jpg[/img]
These dark grey clouds usually form below 6,000 feet and are almost always associated with continuous light to moderate precipitation. The prefix "nimbo" implies a precipitation-producing cloud. Precipitation with a nimbostratus cloud is never the heavy, showery type and often can last for several hours to more than a day. The sun and moon are not visible through a layer of nimbostratus clouds. These clouds usually form in a stable atmosphere where warm, moist air is overrunning colder air at the surface over a relatively large area. Visibility is often very poor below nimbostratus clouds as fog and fast-moving, ragged clouds, known as scud or stratus fractus clouds, form in the precipitation-cooled air below the cloud.
Stratus - St
[img:]http://www.squ1.com/archive/climate/images/cloud-stratus.jpg[/img]
Stratus clouds are uniform grey clouds that usually cover the entire sky. They can form when weak, upward vertical air currents lift a thin layer of air high enough to initiate condensation. Stratus clouds can also form when a layer of air is cooled from below. Stratus clouds look like a layer of fog that never reaches the ground. In fact, fog that "lifts" off the ground forms a layer of low stratus clouds. Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes occur.
Stratocumulus - Sc
[img:]http://www.squ1.com/archive/climate/images/cloud-stratocumulus.jpg[/img]
These low, lumpy cloud layers form below 6,000 feet and can appear in rows, patches or as rounded masses with blue sky in between the cloud elements. The colour of stratocumulus clouds can range from white to dark grey Precipitation rarely falls from stratocumulus clouds. The individual cloud elements form where the air is rising while the blue sky in between the cloud elements indicates sinking air.
Fractostratus - Fs
[img:]http://www.squ1.com/archive/climate/images/cloud-fractostratus.jpg[/img]
The prefix 'Fracto' basically means broken or fractured. Thus, Fractostratus clouds are relatively low-level uniform grey clouds that are beginning to break up with increased low level winds.
Fractocumulus - Fc
[img:]http://www.squ1.com/archive/climate/images/cloud-fractocumulus.jpg[/img]
Cumulo-form cloud material often gets shredded into ragged chunks in high-shear surroundings like that of a super-cell. 'Scud' is a popular term used by storm observers for shredded cloud material (fractus) of all types.
Cumulus - Cu
[img:]http://www.squ1.com/archive/climate/images/cloud-cumulus.jpg[/img]
Cumulus clouds form as water vapour condenses in strong, upward air currents above the earth's surface. These clouds usually have flat bases and lumpy tops. Cumulus clouds are usually very isolated with large areas of blue sky in between the clouds. Most cumulus clouds form below 2km and are relatively thin and associated with fair weather. However, when the atmosphere becomes unstable and very strong, upward air currents form, cumulus clouds can grow into cumulus congestus, or towering cumulus. If the atmosphere is unstable enough, cumulonimbus clouds (better known as thunderstorms) form. Cumulus congestus and cumulonimbus clouds can tower from below 2km to greater than 15km.
Cumulonimbus - Cb
[img:]http://www.squ1.com/archive/climate/images/cloud-cumulonimbus.jpg[/img]
These clouds are more commonly known as thunderstorms. The suffix "nimbus" means rain or precipitation producing clouds. They are characterised by their dark under-surface, due to their rain-bearing nature.
Related Links
Robert Nowland's Cloud Types Page
http://www.users.bigpond.com/robertnowland/cloud.html
A page on cloud forms
http://www.gurney.co.uk/NorfolkScouts/res/ledow/cloud1.html