Cloud Formation, Cloud Types, and Precipitation Forms

Clouds are visible collections of tiny water droplets or ice crystals formed when moist air rises, cools to its dew point and condenses on particles, and precipitation is any form of water, such as rain, snow, hail or drizzle, that falls from clouds to the ground. In brief: air rises by convection (heating), by orographic lifting (over mountains), by frontal lifting (at fronts) or by convergence, and cools as it expands. Clouds are classified by height (high, middle, low) and shape (cirrus, cumulus, stratus). Precipitation falls when droplets grow large enough to overcome the updrafts, by collision and coalescence in warm clouds or by the growth of ice crystals in cold clouds. Fog is cloud at ground level.

Clouds and precipitation provide the fresh water on land, and their type tells a lot about the weather ahead. This guide explains how clouds form, the main cloud types, the forms of precipitation, compares the clouds in a table, works through calculations of cloud base height and mountain air temperatures, outlines the history of cloud classification and corrects common mistakes.

How do clouds form?

Air always contains water vapor. Warm air can hold more vapor than cold air. When air cools to its dew point, it is saturated (100 percent relative humidity), and further cooling causes the vapor to condense onto tiny particles in the air (condensation nuclei: dust, salt, smoke) to form droplets. Cooling usually happens when air rises, since the pressure is lower at height and the air expands and cools (adiabatic cooling): dry air cools by about 9.8 °C per km, and saturated air by about 5 °C per km (less, because condensation releases latent heat). Air rises in four main ways: convection (the ground is heated and warm air rises, giving cumulus and thunderstorm clouds), orographic (air is forced up over mountains, giving cloud and rain on the windward side), frontal (warm air rides over cold air) and convergence (air flowing together in a low-pressure area). Cloud droplets are about 0.01 mm in diameter, and it takes about a million of them to form one raindrop.

What are the types of cloud?

The classification uses Latin words: cirrus (curl of hair; wispy, high), cumulus (heap; puffy), stratus (layer; sheet-like), nimbus (rain-bearing), alto (middle level).

• High clouds (above about 6 km in mid-latitudes): cirrus (thin ice-crystal wisps), cirrostratus (thin veil that causes halos around the Sun or Moon) and cirrocumulus (small ripples).
• Middle clouds (about 2–6 km): altostratus (gray sheet, Sun dimly visible) and altocumulus (patchy “mackerel sky”).
• Low clouds (below about 2 km): stratus (uniform gray layer, drizzle), stratocumulus (lumpy layer), and nimbostratus (thick dark layer with steady rain).
• Clouds with vertical development: cumulus (fair-weather heaps) and cumulonimbus (huge towering storm clouds with an anvil top, with thunder, lightning, hail and heavy showers).

What are the forms of precipitation?

Water falls from clouds in several forms. Drizzle has droplets smaller than 0.5 mm; rain has larger drops, up to about 5 mm. Snow forms when ice crystals grow at temperatures below freezing and fall without melting. Sleet (in British usage) is rain and snow mixed, or in US usage, ice pellets formed from rain freezing as it falls. Hail forms in strong thunderstorm updrafts, where ice pellets are carried up and down through layers of supercooled water, adding layers like an onion until they are too heavy; hailstones are at least 5 mm across and can exceed 10 cm. Fog is a cloud at the surface that reduces visibility to less than 1 km; it forms when air cools to its dew point by radiation on clear calm nights (radiation fog), when warm moist air moves over a cold surface (advection fog), or when air rises up a slope (upslope fog). Dew and frost are condensation or deposition on cold surfaces. Precipitation types by cause are: convectional (heated ground), relief (mountains) and frontal.

Cloud types compared

Worked example: cloud base and mountain air

1. Cloud base height: a rule of thumb is that the base of cumulus clouds is about 125 m for each degree Celsius of difference between the air temperature and the dew point. With a temperature of 22 °C and a dew point of 10 °C, the difference is 12 °C, so the cloud base is about 12 × 125 = 1,500 m.
2. Air crossing a mountain: air at 20 °C with a dew point of 12 °C starts at sea level and is forced up a mountain of 2,000 m. The difference is 8 °C, so the condensation level is about 8 × 125 = 1,000 m.
3. Temperature at the cloud base: dry air cools by 9.8 °C per km, so at 1,000 m the temperature is 20 − 9.8 = 10.2 °C.
4. Temperature at the summit: above the condensation level the air cools at about 5 °C per km, so over the remaining 1,000 m it cools by 5 °C to 10.2 − 5 = 5.2 °C. Rain falls on the windward side.
5. Leeward side: the air descends 2,000 m, now dry, and warms at 9.8 °C per km: 5.2 + 2 × 9.8 = 24.8 °C at sea level. It is 4.8 °C warmer than the original 20 °C, and it is drier (a foehn or rain shadow effect).
6. Snow and rain: 10 mm of rain water is roughly equal to 100 mm of fresh snow, as the snow has a density of about one-tenth that of water (the ratio varies from 5:1 to 20:1).

How were clouds classified?

Luke Howard, an English pharmacist and amateur meteorologist, proposed the system of Latin names for clouds in 1802–1803 (cirrus, cumulus, stratus and nimbus), and it is the basis of today’s classification. The first International Cloud Atlas was published in 1896, and the World Meteorological Organization now maintains it, with new types added in 2017. Understanding of rain formation advanced with the work of Tor Bergeron and Walter Findeisen in the 1930s on ice crystals, and cloud seeding experiments began in the 1940s. Satellites have provided the global view of clouds since the 1960s.

Which mistakes are common?

Mistake 1: Thinking that clouds are water vapor

Water vapor is invisible. Clouds are made of tiny liquid droplets or ice crystals.

Mistake 2: Mixing up the two cooling rates

Rising unsaturated air cools by about 9.8 °C per km, but once it is saturated it cools more slowly, at about 5 °C per km.

Mistake 3: Assuming all precipitation comes from convection

Relief and frontal precipitation are equally common types.

Mistake 4: Confusing fog with mist or smoke

Fog is a cloud at the surface with visibility under 1 km. Mist has a visibility between 1 and 2 km.