The physics of clouds and their role in our climate have perplexed scientists for decades. Karin Ranero Celius investigates.
Do clouds affect our climate, or does the climate have an effect on clouds? Both are true. However, clouds are so difficult to understand that they are not yet well incorporated into climate models. That would require us to understand how clouds form, why they appear and disappear, and why and when they precipitate. To this end, scientists must analyse the behaviour of individual clouds, their components and their effect on the surroundings.
Essentially, clouds are visible masses of water droplets (or even crystals), suspended in Earth’s atmosphere. To study them, researchers divide them into categories. Thin and wispy clouds are called cirrus clouds. Cotton-puffs, or heaps of cotton-puffs, are cumulus clouds. On a cloudy day, the sky is usually covered by flat, hazy and featureless stratus clouds. Each cloud can be classified as one or a combination of these types of clouds (see diagram below).
Furthermore, clouds are classified according to their altitude into: low-level (up to 2000 m), medium-level (2000-6000 m), and high-level clouds (over 6000 m). The height of the clouds determines their temperature, which then determines how much energy they radiate. High-level clouds are cold and radiate little heat into space; instead, they reflect heat radiation back towards Earth’s surface, warming the atmosphere and magnifying the greenhouse effect. Low-level clouds, more compact and warmer, emit more heat radiation into space than back towards Earth. They act as a parasol, reflecting sunlight and thus cooling Earth’s surface.
A cloud’s altitude, therefore, determines its effect on the climate. By studying the occurrence of high-level and low-level clouds, we can begin to understand the roles they play. If both types of clouds were to occur equally, then the warming and cooling effects would counteract each other, resulting in little heating or cooling of Earth’s surface. If the occurrence of high-level clouds increased while that of low-level clouds decreased, there would be a notable increase in atmospheric temperature.
So, why do clouds appear and disappear, and what determines their formation and precipitation?
All clouds have one thing in common: they form from cloud seeds – also known as aerosols. When water evaporates under the Sun’s heat, the vapour rises into the air and the water molecules condense on the aerosols – which can be natural, such as salt, or anthropogenic, such as sulphates. If the cloud seed is at least 60-80 nm in diameter, a water envelope can collect around it, forming a droplet. The cloud consists of many of these droplets. If the droplets reach a diameter of 0.5-1 mm, they will fall – colliding with other droplets, assimilating them and swelling into raindrops, which can fall at up to 35 km/h.
Aerosols play an important part not only in cloud formation, but also in precipitation: they determine when and where it rains. In a pristine atmosphere with very few particles, sunlight evaporates a great deal of water. As it rises, the vapour finds very few condensation seeds in the air, so the drops forming around those few seeds are very large, and it rains heavily.
In a polluted atmosphere, the large number of aerosol particles prevents much of the Sun’s radiation from reaching the ground, causing less water to evaporate. As the vapour rises, it finds many seeds, creating more, but smaller droplets. This slows down the formation of raindrops (droplets only fall once they reach 0.5-1 mm), so it does not rain on the point of origin. More water condenses on the seeds as the cloud continues to rise and the low temperatures freeze the drops. So the cloud does not rain, but continues to climb higher.
High concentrations of aerosols can inhibit precipitation and even cloud formation entirely. The aerosol concentration in northern China, for example, is thought to be the cause of the major shift in the frequency of precipitation. On a global scale, of course, all water that evaporates must eventually precipitate. Thus clouds that only rarely empty their contents will do so in the form of heavy rainfall, causing floods, landslides and mudslides.
Although aerosols explain the formation of clouds, and to some extent the occurrence of precipitation, they do not explain another crucial factor in climate: why do clouds change shape, and why do they appear and disappear?
The shape and lifespan of clouds, and thus their influence on climate, is determined by turbulence. At the edges of a cloud, turbulence mixes the dry surrounding air with the moist cloud air. This is called ‘mixing entrainment’. On a microscopic scale, entrainment changes the distribution and size of the cloud droplets, affecting the cloud’s tendency to rain or dissipating the cloud completely. But its effect can also be global. For example, if clouds over the frequently cloud-covered southeast Pacific Ocean disperse, there will be more solar radiation, contributing to phenomena like El Niño, which is characterised by an increase in the ocean’s temperature.
To forecast the fate of a cloud, scientists must know how turbulent the droplets are: this determines how fast the raindrops form and fall. However, studying turbulence in clouds is a complex task due to the different dimensions of clouds’ components (a tiny droplet, a larger raindrop, an air current) and the physical processes taking place within and between them.
To understand the effect of turbulence, not only the droplets’ velocity and trajectory but also their acceleration is important. Acceleration fluctuates considerably, and can peak at more than 20 times gravity. The frequency of collisions, which increase the chances of precipitation, is determined by particularly strongly accelerated groups of droplets. These strong fluctuations in acceleration could explain why droplets are found to collide more quickly than conventional physics theories allow.
While some scientists try to recreate the conditions of turbulence in clouds using huge wind canals, others study it using computer simulations and fieldwork.
For example, Björn Stevens, a researcher at the Max Planck Institute for Meteorologyw1 in Hamburg, Germany, studies marine stratocumulus clouds, which form over cold regions of the subtropics, such as off the Californian and South American Pacific coasts and over the Atlantic coastline near Namibia. They exert a great influence on the global climate, covering more than one tenth of the oceans’ area. Stevens found out that these clouds are quite peculiar: satellite images show ‘holes’ in the solid cloud blanket and although the clouds do not normally cause heavy rain, they do so around the holes. And when these clouds rain, turbulence – the circulation of air between the ocean and the cloud – can change radically.
Stevens and his colleagues are now incorporating their newly gained information on cloud behaviour into the global climate computer models. In these models, the atmosphere is divided into grid boxes; for each box, the computer calculates average values of temperature, humidity and other characteristics of the atmosphere, and predicts cloud formation. Although the models are not yet precise enough to predict the exact locations where the clouds will form, they can now calculate the degree of cloud cover and type of clouds in each grid box, thus allowing the influence on heat and solar radiation on cloud formation to be quantified.
Scientists, therefore, are still investigating the relationship between cloud cover, precipitation, aerosols and the properties of air surrounding clouds – all fundamental for understanding the link between clouds and climate change. So far, they are not even close to deciphering all the different mechanisms involved in the behaviour of clouds and thus, their effect on our climate. But the scientists will not give up, because, as the French Renaissance philosopher and naturalist René Descartes said: “Clouds provide the key to understanding all things wonderful on Earth.”
This article was compiled from three articles published in Max Planck Research: Meier (2010), Hergersberg (2010) and Wengenmayr (2010). Max Planck Research is published by the Max Planck Society and describes – in simple language – the work of its research institutes. The quarterly publication is freely available to downloadw2.
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