Clouds: understanding shapes and heights

From moisture to a cloud

Understanding a cloud starts with its ingredients. Cloud droplets develop when water vapour condenses onto tiny particles already present in the atmosphere, known as condensation nuclei. Dust, salt particles and smoke can provide these surfaces. Moisture and these microscopic particles therefore work together in the formation of droplets, rather than the droplets appearing without anything on which to condense. [s1]

A common route towards condensation involves air moving upwards. As a parcel encounters the lower pressure above, it expands and becomes cooler. If it reaches saturation, cloud droplets can develop. Saying that air is rising therefore does not yet describe a cloud: the cooling must also bring that parcel to the condition in which condensation can produce the droplets. [s1]

Recognising the forms

The outline offers clues about this process. Cirrus-like forms are high, fine and threadlike, with ice crystals making up the cloud. Cumulus-like forms instead appear as separate elements with distinct edges and vertical growth. Their bases are often flat, marking the level where moisture in the rising air begins to condense and the cloud becomes established. [s2]

Stratus-like forms spread into a broad covering, with more diffuse edges than the sharply defined outlines of cumulus. In the historical grouping explained by NOAA, nimbus refers to rain-bearing clouds. These form-based families offer a useful starting point for looking at the sky, but they should not be mistaken for a complete catalogue of the genera in modern cloud classification. [s2] [s3]

Reading heights and changes

Height also needs a careful reading. The elevations associated with cloud levels vary between geographical regions. Low cloud bases are usually within the first two thousand metres above the surface, but there are exceptions: the bases of cumulus and cumulonimbus can be higher. A typical height boundary is therefore a guide, rather than an absolute limit applying to every cloud. [s2]

As a hypothetical observing exercise, imagine a cloud growing and later becoming thinner. Its droplets are continually forming and disappearing: growth is favoured when condensation exceeds evaporation, while dissipation is favoured when evaporation dominates. The changing outline is consistent with this continuing exchange. There is no need to imagine the cloud as a fixed object whose shape must remain unchanged. [s1]

Sources consulted

1. How Clouds Form — NOAA JetStream — Source: NOAA / National Weather Service
2. The Four Core Types of Clouds — NOAA JetStream — Source: NOAA / National Weather Service
3. Ten Basic Clouds — NOAA JetStream — Source: NOAA / National Weather Service