Measures of convective instability

Measures of convective instability

There are many different kinds of measures of convective instability. The most common, and you will come across in virtually an in-depth forecast or storm chaser discussions, is CAPE = Convective Available Potential Energy. Chances are you have heard storm chasers and enthusiasts talk about this “CAPE this, CAPE that…”. CAPE is fuel for convection – and thunderstorms.

CAPE is the energy a parcel of air has for upward motion. The higher the CAPE, the faster and higher the air parcel can rise. CAPE depends on the temperature and amount of moisture (humidity) in the air near the ground and the vertical temperature gradient. The hotter and more humid the air on the ground is, the higher the CAPE. Also, the faster the temperature drops with height, the higher the CAPE.

There are several variations of CAPE you are likely to come across: SBCAPE, MUCAPE, MLCAPE. We will cover this, and other measures of convective instability in detail in one of future tutorials.

Fun fact(s): under the right conditions the rising air goes up really fast! While most thunderstorms form in moderately unstable conditions (CAPE up to ~1000 J/kg), in some cases there is much more energy available. This is called extreme instability and in such cases CAPE values can reach 6000 J/kg and more. When thunderstorms form in this type of environment, the upward speed of rising air is extreme, sometimes over 150 m/s! This is enough to keep hailstones 15+ cm in diameter airborne!


1.1.3. Different types of convective clouds – from fair weather clouds to thunderstorms

Convection forms distinct types of clouds. Convective clouds are divided into two types (genera): cumulus and cumulonimbus. Each successive type and species displays more vertical development as a result of more convective energy being available. The more convective energy is available, the higher the convective cloud will reach. In terms of how high convection goes, there are two types of convection. If convective clouds reach above ~6 km high, it is called deep convection. If convective clouds do not reach as high, it is called shallow convection. Only deep convection produces strong rain showers and thunderstorms.

In convective clouds, there are five distinct stages of vertical development: three stages of cumulus clouds (humilis, mediocris, congestus) and two stages of cumulonimbus, which is already a thunderstorm. Cumulus clouds are generally ‘harmless’ billowing convective clouds with tops reminiscent of cauliflower. Only the largest of the three, Cumulus congestus can produce stronger rain showers. Cumulonimbus clouds develop from Cumulus congestus stage into Cumulonimbus calvus, which may then evolve into a Cumulonimbus capillatus or a Cumulonimbus capillatus incus.

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Let us now explore each stage and then go on to numerous examples of each stage.

             Cumulus humilis – fair weather clouds


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Cumulus humilis, also known as fair weather clouds are the smallest convective clouds and the first stage in development of convective clouds. They typically form at 500 to 3000 m altitude and have limited vertical development. Cumulus humilis rarely produce any type of precipitation. You can spot these clouds on many summer days; they often form in the late morning or early afternoon as the ground heats up. If they are present early in the day, they may be a sign of an unstable atmosphere.

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             Cumulus congestus (Towering cumulus)


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Cumulus congestus clouds form in deep moist convection, as an intermediate stage between cumulus mediocris and cumulonimbus – a cumulus mediocris will grow into a cumulus congestus if enough convective energy is available. In aviation cumulus congestus is also known as towering cumulus (International Civil Aviation Organization). Cumulus congestus is a sign of a very unstable atmosphere, and may lead to formation of thunderstorms.


  • Typically taller than wide.
  • Air in the cloud has not yet reached freezing, retaining the sharp, cauliflower shape.
  • Great vertical development.
  • Reaching up to 6 km high (20 000 ft).
  • Frequently produce rainfall, rarely produce intense showers.
  • Form from cumulus mediocris.
  • Also may produce landspouts and waterspouts.

             Cumulonimbus (calvus, capillatus, incus) – thunderstorms


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Cumulonimbus calvus is a form of Cumulonimbus cloud with a sharp, rounded, billowing top, still rising. It develops from Cumulus congestus. Cumulonimbus clouds, by definition, contain ice crystals – they are present in cumulonimbus calvus, but still in relatively small quantities.

Cumulonimbus calvus develops further into Cumulonimbus capillatus and can develop into Cumulonimbus capillatus incus (anvil-top).


  • Cumulonimbus calvus produce lightning.
  • They also produce hail.
  • And severe straight line winds (downburst).
  • Will develop into Cumulonimbus capillatus.
  • Will develop into Cumulonimbus capillatus incus if the updraft is strong. Examples of convective clouds – how to distinguish them visually


Convective clouds can present a considerable range of appearances, depending on their type, size and strength of updraft. The following examples encompass much of the variety of convective clouds. Each example includes a photo of a convective cloud or multiple convective clouds with argumentation for their name.

  • Vertical development: how high does the cloud reach.
  • Cloud top: Is the top of the cloud sharp, or
  • Precipitation: is there precipitation? Weak? Strong?

1.2.1. Typical thunderstorm – Cumulonimbus capillatus incus

The most distinctive and typical cloud shape associated with thunderstorms is Cumulonimbus capillatus incus: a convective tower (updraft) flattened into an anvil shape. As we have already seen in the previous chapter, it literally looks like an anvil. It is typically over 10 km high and sometimes reaches so high up, it turns day into night for anyone underneath it.

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If you see a cloud of this shape from a distance – that is a thunderstorm. While other varieties of cumulonimbus clouds also produce thunderstorms, the anvil shaped incus is the most distinctive – and associated with the strongest thunderstorms.

Why is it this shape? Let us see.