On clouds: where they come from, where they go

Atmospheric rivers, Hadley cells, jet streams, rainforests, and land degradation

Clouds, soft and ethereal, an apparition of water vapor condensed, hover in the atmosphere, reflecting sunlight back out to space, trapping radiation beneath, blown around by the global aerial dance of pressure systems and temperature gradients, and distributing rain with its life giving properties to the life forms below. Let us examine how clouds form and move around the world, and how land degradation and the chopping down of forests can impact these formations, possibly increasing the occurence of atmospheric rivers and hurricanes.

[The distribution of clouds averaged over 1982-2020. Data from CM SAF]

Multiple global processes give rise to clouds. Clouds appear when the water vapor ascends in the upward leg of large scale atmospheric circulation patterns. These circulation patterns are the interlinked Hadley cell, Ferrel cell, and Polar cell, and the zonal Walker cell.

The sun heats up the air more in the equatorial tropics. That warm air rises and moves poleward. It then descends in the subtropics where it cools off and then circulates back to the tropics. This loop is called the Hadley cell. On the upward leg of the Hadley cell the water vapor rises, creating clouds.

Trade winds blow the warmer surface water from one side of the Pacific, Indian and Atlantic cceans to the other side. That warmer water then evaporates to create clouds. Air then moves back across the ocean the other way at higher altitudes, and descends to complete the loop called the Walker cell. In normal years, clouds appear on one side of these oceans. In El-Nino years, as trade winds weaken, the warm waters can slosh back, and the position of the clouds shifts across the ocean. El Nino can bring clouds and rain to South America, and drought to Australia and Indonesia.

Jet streams are narrow bands of fast moving winds in the upper atmosphere, that move from west to east while forming a wavy pattern that is a result of cold air from the poles meeting warm air from the equator. As the thermal gradient weakens, the jet streams buckle into a curvier line, bringing along with it low pressure zones and its accompanying clouds and rain. In 2021, snowstorms in Texas, a place which rarely gets snow, arrived because of the buckling of the jet stream.

Atmospheric rivers are long narrow bands of moisture and clouds that transport water from the tropics to higher latitudes. The combination of the forces of the earth spinning and of global temperature gradients create wind patterns that string the clouds into these narrow bands. When these atmospheric rivers hit land they can dump huge amounts of rain, like that in the recent Australian floods.

[the tendril like areas are the atmospheric rivers]

Clouds are more likely to form above forests, because the evapotranspiration of the forests combines with the moisture blowing in from the ocean to increase the humidity to the point where clouds can form. In addition forests slow the wind (see Francina Dominguez’s research), increasing the chances water vapor molecules will find each other.

While rain and clouds are blown across the globe in ever shifting patterns, large rainforests are able to maintain clouds over them constantly. Precipitation recycling aka the water cycle, can keep the clouds there. In this time lapse video of global rain patterns I made, you can see there are three places in the world where the rain remains more stationary - in the Amazon rainforest, the Congo rainforest, and the Indonesian rainforest.

There are interesting open questions about how land degradation affects the formation of atmospheric rivers with their cloud bands, and hurricanes with their rotating clouds. Physicist Anastasia Makarieva, the co-founder of the biotic pump theory, physicist Andrei Nefiodov, and hydrologist Michal Kravcik, are amongst those who believe that land degradation has led to more moisture blowing out to the oceans and affecting the creation of extreme weather formations like atmospheric rivers and hurricanes. When forests are chopped down, a lot of the moisture in the air is not able to brought down as rain onto the land, and instead blows out to sea to increase the moisture there. Atmospheric rivers and hurricanes form from the rise of moisture to create clouds. Makarieva and Nefiodov argue that the lack of hurricanes in the Atlantic Ocean east of Brazil and west of the Congo, is due to the Amazon rainforests and the Congo rainforests absorbing all that moisture before it blows back out to the ocean. Climate scientist Francina Dominguez, of the Univ of Illinois, has modelled how chopping of forests in Amazon keeps air moisture and wind flowing fast in South America and out to the adjacent ocean.

[tropical storm/hurricane map from GeoAwesome]

Climate scientist Millan Millan, who is working on an ambitious project to regreen the Sinai desert, believes that growing trees there will bring down the moisture blowing in from the Mediterranean Sea, and thus have less of that moisture blowing out to the Red Sea and Indian sea, which means less hurricanes there. (See our previous article about this)

There is also intriguing research being done on how land degradation can affect jet streams and large scale atmospheric circulations, and thus cloud and rain patterns around the globe. Climate models studying these effects have been carried out by climate scientists like Abigail Swann.

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