Cumulus ( Latin for accumulation , abbreviation: Cu) or also cumulus is the name of a cloud shape . The common term for this is heap cloud or cumulus cloud . The classic, unmistakable “picture book cloud” (also known as “sheep cloud”) with its flat underside and gleaming white cauliflower heads on the top consists of water droplets and can be found in the lower layers of clouds.
Cumulus clouds usually appear in sunny weather when the air is a little more humid. They arise from locally limited updraft , such as thermals or orographically- induced slope updrafts: air masses rise, expand and cool down in the process. At a certain height, when the dew point is reached, the moisture that is carried along condenses. For this reason, the underside of the cumulus clouds is flat and the undersides of the clouds in the area are - assuming the same weather conditions or air stratification - roughly at the same height.
In the early morning they can be taken as an indication that thunderstorms will form. In most cases, cumulus clouds only appear during the day when the sun has heated the ground sufficiently. If there is sufficient humidity and convection , cumulonimbus clouds develop from the cumulus . A sign of this is a tower-like shape of the clouds. If the clouds are rather spherical, there is not enough humidity for the thunderstorm to form or barriers prevent their further vertical expansion. It can also rain from mighty cumulus clouds (Cumulus congestus), but only in small amounts and not continuously. There is neither lightning nor thunder.
Cumulus clouds often dissolve again in the second half of the night.
Structure and appearance
Cumulus (Cu) are dense, sharply demarcated clouds. In contrast, Stratocumulus clouds have structures that have grown together. Most typical cumulus clouds have a smooth horizontal border on the underside that appears shady and relatively dark. Snow-white "cauliflower heads", which are very puffy in shape, arch above it. The shadowy foot of the cloud can be just a few hundred meters above the ground if the air has a high water vapor content . Coming in from the sun bright white sunlit upper area is still mostly in the deepest of the three clouds Floor , namely at an altitude between 600 m and 2 km, but can also (as with Cumulus congestus) up to 6 km height range. As high cumulus clouds, they are evidence of strong upward movements in the air.
Cumulus clouds are pure water clouds (in contrast to the ice clouds of higher regions) and have temperatures of −10 ° C to 0 ° C in their interior .
In meteorology , four types of cumulus cloud are distinguished according to their size:
- Cumulus congestus (Cu con) ( Latin for "piled up") represent the next stage in the development of heap clouds. They can be recognized by their massively inflated peaks and sharp outlines, with the cloud reaching heights of up to 6 km due to strong convection currents, so that C. congestus appears higher than it is broad from the ground. A sustained deterioration in the weather with showers and gusts of wind is to be expected in Central Europe within a few hours when congestus clouds move in from the southwest. They are therefore assigned to weather phase 5 (weather change). They can be the preliminary stage of a cumulonimbus (Cb), one of the typical summer thunderclouds, which are characterized by an anvil-like eruption on the upper edge. In aviation, Cumulus congestus are also known as Towering Cumulus (TCU).
- Cumulus humilis (Cu hum) ( lat. "Weak, low") are the smallest representatives of their kind. They are small, flaky and mostly large-scale fair weather clouds of weather phase 1 (medium fair weather). They arise with relatively weak convection and therefore only have a small height extension (600 m to 1 km). From the ground they appear to the viewer to be wider than they are high.
- Cumulus mediocris (Cu med) ( Latin for "moderate") are medium-high (600 m to 1.2 km) cloud clusters of weather phase 2 (increased fair weather). They often form a short-lived transitional form between the less developed C. humilis and the more swollen C. congestus and show only slight swellings on the upper side. In Central Europe, this stage of cloud development is usually only reached in the afternoon. They are often visible after a cold front passage. The earlier it happens, the greater the likelihood that a thunderstorm will occur. From the ground, these clouds seem as high as they are wide.
- In stronger winds, heap clouds can be torn apart. These wisps of cloud, which are driven rapidly across the sky with constantly changing outlines, are called Cumulus fractus (Cu fra) ( Latin for "broken"). They are typical for weather phase 6 (calming weather).
If the wind is strong enough, the updraft fields and thus the cumulus clouds are organized in rows or cloud streets parallel to the wind direction.