A cloud is formed when atmospheric water vapour is cooled by vertical air motions (or in the polar regions by heat loss by radiation), condensing on microscopic airborne particles – dust, sea salt, bits of organic matter, or chemical aerosol particles, the most common being composed of sulphuric acid and other sulfate compounds. Between the evaporation of water from the surface and its condensation in a cloud, water vapour is carried along by winds from warmer, moister regions to cooler, drier ones.
Because the atmosphere, except for clouds, is nearly transparent to solar radiation, the surface absorbs 70 per cent of the total solar heat taken up by the earth-atmosphere system, making the air warmer near the surface than it is at high altitudes. Because sunlight strikes the planet most directly near the equator, the tropics are warmer than the polar-regions.
Both temperature gradients – the temperature variations from low to high altitudes and from low to high latitudes – are intensified by the effects of water vapour on radiative heating and cooling and by the transformations of water from liquid or solid into vapour and back. This happens because water vapour is nearly transparent at the wavelengths of sunlight (between 200 and 3,000 nm, nm = nanometer, one billionth of a meter), so it lets virtually all the sunlight reach the surface.
However, water vapour is nearly opaque at the wavelengths at which the sunlight-warmed surface radiates away its absorbed energy (thermal radiation with wavelengths between 3,000 and 100,000 nm). The absorption of most of the outgoing thermal radiation by water vapour creates most of Earth’s natural greenhouse effect – an effect that is now being increased by human pollution. Without the atmospheric water vapour Earth’s surface would be, on average, about 31°C (55°F) colder than it is now and the differences in temperature between high and low altitudes and between the poles and equator would be smaller.
Since cold air is denser than warm air, temperature differences give rise to atmospheric motions that work to eliminate the density differences. Winds generally move warmer, moister air upward and pole-ward from the tropical surface and move colder, drier air downward and toward the equator from higher altitudes and latitudes.
Although some water is transported to higher latitudes at upper levels, the winds near the equator actually transport water vapour towards the equator, concentrating it into a narrow, heavy rainfall zone there. The contrasts in heating, together with the winds, also drive ocean currents, which help reduce the temperature differences between the equator and the poles even more.
Some of the water evaporated from the surface (primarily from the oceans) condenses into clouds and eventually falls as rain or snow. These transformations not only redistribute water but also play an important role in global heat transport.
When surface water evaporates, the heat required to change liquid water into vapour is absorbed from the surface and carried along with the vapour into the air. When water vapour condenses into a cloud and falls as rain, it releases that heat, known as latent heat, into the air.
The processes that control the conversion of water vapour into cloud and precipitation particles are called cloud microphysical processes. The interaction of these processes determines the properties of clouds that, in turn, determine the effect of clouds on the radiative energy exchanges, whether the cloud will produce precipitation, how much and what type of precipitation it will produce, and how long the cloud will last.
At temperatures above freezing (0°C), the weak vertical air motions (slow cooling) associated turbulence near the surface or with large-scale circulations lead only to the formation of rather small cloud droplets (about 5-10 um in radius, 1 micron = 1 millionth meter) covering very large areas. For typical concentrations of small aerosols on which the droplets form (about 50-200 cm–3 over oceans to about 500-2000 cm–3 over land), the total amount of vapour converted to droplets is small, equivalent to about a layer of water about 0.01-0.03 mm deep.
Such clouds, ranging from scattered fair-weather cumulus to extensive sheets of stratocumulus, produce no precipitation and last only as long as the upward motions continue (usually about 10-20 min for cumulus but days for stratocumulus) because such small droplets fall very slowly (about 3 mm s–1) and evaporate within a few minutes. Stronger vertical air motions (rapid cooling as in stormy weather) tends to produce somewhat more numerous and much larger droplets, about 15-30 µm in radius. These larger droplets fall more rapidly (still only about 10 cm s–1) and collide.
Colliding droplets merge into even larger, more rapidly falling droplets, so the collision process quickly produces very large droplets, more than 300 µm. Such clouds, ranging from stratus and altostratus to nimbostratus produce drizzle or light rain. When the vertical motions are even stronger, as happens when the heat release from the condensing water causes very rapid ascent of large parcels of air, forming cumulonimbus clouds, the cloud extends into the upper troposphere where the colliding droplets freeze.
The mixing of ice and liquid droplets not only produces more rapid growth from the vapour but more efficient sticking of the colliding particles, leading to the growth of very large particles, more than 1 mm (1000 µm) in size, that fall so rapidly (more than 100 m s–1) that they can reach the surface without evaporating. These falling large droplets are known as rainfall; rainfall rates can range from very small rates of 0.01 mm hr–1 to very heavy downpours of 50 mm hr–1.
The situation at colder temperatures is similar to that described above, but there are some important differences that arise because of the peculiar properties of water and because of the difference between liquid and solid particle collisions. Because of the strong interactions of water molecules, some extra energy is needed to initiate the growth of very small water particles from vapour.
For the growth of liquid droplets in clouds near the surface, the presence of water-containing aerosol particles greatly reduces the amount of energy needed, requiring only a small excess of vapour pressure over the saturated amount (i.e., the relative humidity must only reach values of about 100.1% to form droplets).
However, at higher altitudes there are not only many fewer aerosol? available but they do not help initiate the growth of an ice crystal nearly as well as they can help droplet growth, so ice clouds do not begin to form until the vapour pressure exceeds saturation by a much larger amount (relative humidity with respect to ice usually must reach values as much as 101%). In fact, many ice clouds start instead by forming liquid droplets at temperatures well below freezing (down to as low as about – 30°C) and then freezing them.
The peculiar property of water is that at temperatures below freezing the saturation vapour pressure over liquid droplets is much higher than over ice crystals at the same temperature. Once these cold droplets begin growing, they quickly freeze, exposing them to a much higher vapour pressure. The consequence is that the ice crystals grow much more quickly to larger sizes in the range from 20- 100 µm and they keep growing below the cloud, reaching sizes of a few hundred microns, because the relative humidity is still > 100% with respect to ice below the initial cloud base.
These large particles also collide as they fall, but ice crystals have a more difficult time sticking together; nevertheless, at temperatures nearer freezing, some liquid droplets are encountered that help stick the crystals together. So when the air motions are stronger, very much larger frozen particles can be produced. In the violent vertical motions of strong thunderstorms, for example, the particles can fall and rise many times, producing large hail stones that have been known to reach sizes > 10 cm (105 µm).
The formation, evolution and motion of clouds are determined by the interaction of these cloud microphysical processes with atmospheric motions and radiation; this combination can be thought of as a kind of cloud dynamics. As the air moves past the particles in a cloud, there is a frictional force exerted, so that, even in very small clouds, the number of particles is sufficient to cause the air to move around the cloud rather than through it. Thus, smaller clouds are moved with the wind.
However, since clouds are formed by the air motions, their actual evolution is much more complex and can involve wave as well as mass motions Differences in the nature and behaviour of cloud dynamics in different meteorological situations produces different cloud types. Researchers are now studying the behaviour of these different cloud types to understand the role of each weather and climate.