Trade wind cumulus (or trade cumulus) clouds are formed by cooling and moisture absorption of the dry

trade winds The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere ...

over the relatively cold sea surface in the eastern parts of the oceans. These are clouds, typically Cumulus humilis or Cumulus mediocris, which are considered as fair weather clouds. Characteristic for trade wind clouds is the uniform height of the upper cloud limit, which typically lies between 1000 and 1500 meters and thus indicates the altitude of the trade wind inversion. Due to

orographic lift Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and cr ...

at mountains, the clouds can also rise higher, but the trade wind inversion also limits a further rise here, so that even in this case a light

drizzle Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation r ...

can occur at best. At night, the trade wind clouds usually dissipate again, especially over land.

Details of the development

The trade wind inversion is strongest in the eastern areas of the tropical oceans. Here, the sinking air masses of the Hadley or

Walker circulation The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical dir ...

and the

sea breeze A sea breeze or onshore breeze is a wind that blows in the afternoon from a large body of water toward or onto a landmass. By contrast, a land breeze or offshore breeze is a wind that blows in the night from a landmass toward or onto a large ...

ensure that the subsidence inversion lies particularly deep. Below this, a thin, very uniform layer of

stratocumulus cloud A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and t ...

s forms in the marine atmospheric boundary layer. In these areas, the water surface temperatures are relatively low due to the upwelling of cooler, deeper water. As a result of the low-lying inversion and cool water temperature, moisture content increases within the marine boundary layer and, with saturation, clouds form over a wide area of the eastern tropical oceans. Further west, away from the coast, the subsidence weakens, the sea surface temperature rises and the clouds in the boundary layer become more cumulus-like, but often remain covered by stratocumulus at first. Even further downwind, the stratocumulus clouds disappear and the classic trade wind clouds develop, i.e. puffy cumulus clouds with active vertical transport of moisture and heat. These become the predominant phenomenon and extend over a large region until extensive

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

dominates in the

Convergence Zones Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...

Significance for the climate

Trade wind clouds are found over about 20 percent of the Earth's surface. Since they reflect sunlight on their upper surface, they reduce the warming of the Earth's surface by

solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

. Numerical

climate model Numerical climate models (or climate system models) are mathematical models that can simulate the interactions of important drivers of climate. These drivers are the atmosphere, oceans, land surface and ice. Scientists use climate models to st ...

s have great difficulty with the simulation of low clouds over the subtropical oceans, in particular the trade wind clouds. Previously, it was expected that these clouds would decrease due to

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

and thus strengthen the latter, meaning that this development represents a positive feedback loop. However, a study published in 2022 based on empirical data concludes that the trade wind clouds are less sensitive to climate change than previously assumed.

References

Joachim Blüthgen, Wolfgang Weischet: ''Allgemeine Klimageographie''. 3. Auflage. de Gruyter, 1980, ISBN 3-11-006561-4
Vorschau
University Corporation for Atmospheric Research (UCAR): ''Introduction. Atmospheric Structure. The Trade Wind Inversion.''

{{Cloud types Climate variability and change Cumulus