A cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in the atmosphere above the surface of a planetary body. These suspended particles are also known as aerosols. So how do they form and how fast? Researchers at the University of Bristol with collaborators from ETH-Zurich have shown that the rate of condensation of water on organic aerosol particles in the atmosphere can be very slow, taking many hours for a particle to change in size. This could have significant consequences for understanding how clouds are formed and how they affect climate.
A cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in the atmosphere above the surface of a planetary body. These suspended particles are also known as aerosols. So how do they form and how fast? Researchers at the University of Bristol with collaborators from ETH-Zurich have shown that the rate of condensation of water on organic aerosol particles in the atmosphere can be very slow, taking many hours for a particle to change in size. This could have significant consequences for understanding how clouds are formed and how they affect climate.
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In general, clouds form in the troposphere when one or more lifting agents causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. At this point cloud formation begins and aerosols form and commence growth. An aerosol is a colloid suspension of fine solid particles or liquid droplets in a gas.
The influence of aerosols (small particles less than 1 micrometer in diameter) and clouds (liquid droplets 1 – 1000 micrometers diameter) represents one of the largest uncertainties in our understanding of trends in past global climate and predicting future climate change, as recognized by the 2007 report of the Intergovernmental Panel on Climate Change.
One of the most significant 'known unknowns' is how quickly water can condense on the small aerosol particles to grow and become the larger cloud droplets, influencing the albedo (reflectivity) of clouds and cloud lifetime.
In a study published today in PNAS, Professor Jonathan Reid of the University of Bristol and colleagues show that the rate of cloud droplet growth can be strongly dependent on the composition of the aerosol.
For aerosol particles that have high viscosity (viscosity is a fluid's resistance to flow;), water evaporation and condensation can be very slow, taking many hours.
For particles that are much less viscous, evaporation and condensation can be very fast: less than 1 second.
Professor Reid said: "Although not providing all the answers, this work helps us better understand the known unknowns. Most importantly, it demonstrates that better understanding the rate at which water condenses on particles in the atmosphere is crucial for understanding clouds."
For further information see Cloud Formation.
Clouds image via Wikipedia.
