clouds

clouds In 1803 Luke Howard developed a classification system for naming different cloud types. Many of the names that he gave to clouds are still in use by surface observers today. There are ten distinguishable types, called genera, which have a Latin name and a recognized abbreviation. The identification of a cloud as of a particular type is based on a subtle blend of its appearance and altitude. The basic cloud forms are cumulus, which are heaped clouds; stratus, which are layer clouds; and cirrus, which are wispy. If the cloud is raining then the term ‘nimbus’ is used, as in cumulonimbus and nimbostratus. Further extensions to the genera names, called species, provide a fuller description. Apart from some notable exceptions in the stratosphere, clouds form in the lowest layer of the atmosphere, that is, in the troposphere. Although clouds can form at any altitude in the troposphere, they are usually assigned to one of three levels, low, middle, or high, which further aids classification. The exact height of these groups varies with latitude and season. Some clouds, such as cumulonimbus, develop vertically beyond one level. This often leads to a fourth group being defined: those clouds with vertical extent. Table 1 lists the ten genera together with their abbreviation and height of the cloud bases above sea level.

The appearance that clouds take is determined by the mechanism which forms them. Clouds form when water vapour, which is always present in the atmosphere, condenses out. Condensation occurs when the air is cooled to such a point that the air is saturated with respect to water vapour. Further cooling results in liquid water or ice. Condensation in clean air is very difficult to achieve, and air can easily become supersaturated under such conditions. Most condensation takes place on aerosols which are hygroscopic (water-absorbing). Naturally occurring aerosols are abundant in the Earth's atmosphere, for example in the form of dust and salt from sea spray. Pollution from human activity also contributes suitable aerosols for cloud formation. Cooling of the air to produce clouds can occur in a variety of ways. The most common is when air rises; the pressure decreases, and so the air expands, doing work and expending energy and therefore cooling. Small parcels of air rising because the atmosphere is unstable result in cumulus clouds. Clouds can result when air is forced to rise over mountains. Large-scale uplift, as seen at a front (the intersection of two air masses with different characteristics) creates predominantly stratus clouds. Other cooling mechanisms, such as warm moist air being cooled when it comes into contact with a cold surface, can result in fog. Lower clouds are predominantly composed of liquid water droplets, but high clouds such as cirrus are composed entirely of ice crystals.

1. Cloud genera

Low clouds

Height of cloud bases: all regions below 2 km

Cloud types: stratus (St), stratocumulus (Sc)

Cloud types with vertical extent; all have bases in the low-level group:

nimbostratus (Ns), cumulus (Cu), cumulonimbus (Cb)

Middle clouds

Height of cloud bases: tropical region, 2–8 km; temperate region,

2–7 km; polar region, 2–4 km

Cloud types: altostratus (As), altocumulus (Ac)

High clouds

Height of cloud bases: tropical region, 6–18 km; temperate region,

5–13 km; polar region, 3–8 km

Cloud types: cirrus (Ci), cirrostratus (Cs), cirrocumulus (Cc)

Clouds, being white, are highly reflective and they reflect the incoming short-wave radiation from the Sun. They thus have a cooling effect on the Earth, the cloud albedo effect. However, because clouds are composed of water, which is highly efficient at absorbing long-wave radiation, clouds also have a greenhouse effect; that is, they cause the Earth's surface to become warmer by stopping outgoing long-wave radiation from escaping into space. Clouds therefore have a strong control on the radiation budget of the Earth; and as it is this budget which ultimately determines the Earth's climate, the radiative effect of clouds is of immense importance to climate studies. Low- and middle-level clouds are thought to cool the Earth's surface—that is, their albedo effect dominates—but for high cloud it is believed that the greenhouse effect dominates. The effect of clouds on the radiation budget is determined not only by their gross properties, such as their amount and height, but also by their microphysical properties, such as the size of the droplets in the clouds and their water content.

Surface observers throughout the world record the type, height, and amount of cloud covering the sky. The amount of sky covered is typically measured in oktas, eighths of sky cover, a non-linear scale. Although height can be measured by instrumental methods, it is often an estimate. All measures of cloud parameters observed at the surface are subjective and subject to human error. Moreover, human surface observers are unable to provide complete Earth coverage. Satellite observations of clouds, which began in the 1960s, potentially offer an objective measure of cloud parameters over the entire globe. Although trained meteorologists can, most successfully, interpret satellite cloud images, the volume of satellite data and the need for objectivity require computer analysis. The interpretation of digital satellite data through computer algorithms is, however, complex and not without its own problems. Total cloud amount, as a percentage of ground cover, is relatively easy to assess, but estimation of the cloud type and the amount of each cloud type has proved more difficult. The whole idea of cloud types as discussed earlier is in fact rather misleading when referring to satellite data: satellite-retrieved cloud parameters are essentially a measure of the cloud effects on the radiation reaching the satellite sensor. Satellite-retrieved cloud parameters do not therefore readily equate to cloud parameters observed at the surface. We thus have two incompatible sets of data. Satellite data nevertheless hold the promise of being able to provide far more information than merely the gross physical properties of clouds.

Frances Drake

Bibliography

Bohren, C. F. (1987) Clouds in a glass of beer: simple experiments in atmospheric physics. John Wiley and Sons, New York.
Scorer, R. (1986) Cloud investigation by satellite. Ellis Horwood, Chichester.