Convection forms distinct types of clouds. Convective clouds are divided into two types (genera): cumulus and cumulonimbus. Each successive type and species display 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.
VERTICAL DEVELOPMENT STAGES
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 the Cumulus congestus stage into Cumulonimbus calvus, which may then evolve into a Cumulonimbus capillatus or a Cumulonimbus capillatus incus.
Now, as we know what convective clouds are, let us now explore each stage and then go on to numerous examples of each stage.
CUMULUS HUMILIS – fair weather clouds
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.
Cumulus mediocris develop from Cumulus humilis and display more vertical development, up to 3000 m altitude. They generally do not produce precipitation, although virga or light rain can occur. They can develop into larger Cumulus congestus clouds.
CUMULUS CONGESTUS (Towering cumulus)
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 the 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 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.
Cumulonimbus capillatus (fibrous-top) has the upper part or edge of the cloud has a fibrous, wispy structure. The term capillatus comes from Latin and means “hair”. A cumulonimbus capillatus has generally stopped rising, its top may be spreading laterally (sideways).
CUMULONIMBUS CAPILLATUS INCUS
Cumulonimbus capillatus incus (anvil-top) is a cumulonimbus cloud that has developed the characteristic flat, anvil-top form. An anvil top indicates a very strong updraft, the strongest among the Cumulonimbus species. Anvils usually top out at 10-12 km altitude, but they can reach as high as 16-18 km close to the equator.
- Intense lightning.
- Hail. Usually relatively small, 0.5 – 2 cm in diameter, but may become much bigger and damaging.
- Heavy rainfall. Very heavy rainfall and or longer downpours can lead to flash flooding.
- Strong wind. Strong straight line winds can occur. Under the right conditions they may exceed severe or even very severe criteria.
- Waterspouts and landspouts. Less frequent than the above.
- Tornadoes. Much less frequent than the above.
EXAMPLES OF CONVECTIVE CLOUDS
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?
Cumulus congestus. The top of the cloud is sharp. Vertical development is significant, however, likely less than 6 km*. A strong rain shower is ongoing beneath the cloud. Photo: Jesper Mulderij.
* – the base of the cloud is likely at <3000 m altitude and there appears about the same vertical extent from the base to the top of the cloud.
Cumulonimbus calvus. There is large vertical development in this cloud. Assuming a cloud base at 1-2 km*, the cloud is reaching 6-10 km in altitude. It is producing strong rainfall. Photo: Sandro Puncet.
Cumulonimbus capillatus (distant). The clouds are producing lightning, indicating freezing levels have been reached. Cloud tops have a wispy appearance, therefore capillatus. No distinct anvil form has formed. Photo: Sandro Puncet.
Cumulonimbus capillatus . The cloud displays large vertical development. Lightning activity indicates freezing level was reached in the upper part of the cloud, therefore it is a cumulonimbus. The top is not sharp, but fuzzy (fibrous), hence capillatus. No anvil shape is developed. Photo: Sandro Puncet.
Cumulonimbus capillatus incus. The cloud again displays large vertical development. Lightning activity indicates freezing level was reached in the upper part of the cloud, therefore it is a cumulonimbus. The top is developed into a distinct and sharp anvil shape, hence capillatus incus. Photo: Sandro Puncet.
Cumulonimbus capillatus incus. The cloud top has a fibrous (wispy) appearance, therefore capillatus. The cloud also has a well-developed anvil, therefore it is cumulonimbus incus. Note a line of cumulus mediocris clouds in front of the base of the cumulonimbus. Photo: Marin Pitton.
Cumulonimbus capillatus incus. Several well-developed anvils indicate these are Cumulonimbus capillatus incus. Photo: Sandro Puncet.
Cumulonimbus capillatus incus. Photo: Sandro Puncet.
Cumulonimbus capillatus incus. Photo: Sandro Puncet.
Cumulonimbus capillatus incus. Photo: Marko Korošec.
Cumulonimbus capillatus incus. Two distant anvil-shaped Cumulonimbus clouds. Photo: Marko Korošec.
Cumulonimbus capillatus incus. Distant, very large anvil shape, therefore Cumulonimbus capillatus incus. Photo: Marko Korošec.
Cumulonimbus capillatus incus. Textbook anvil-shaped Cumulonimbus. Photo: Devid Raziel Penguti.
We have seen how convective clouds resemble cauliflower, how buoyancy drives convection and how convection drives thunderstorms. We have learned the various types of convective clouds. Next we take a look at how thunderstorms work: how instability influences their form, how wind influences their behaviour and we will