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Interactions between aerosols, clouds, and precipitation are the most uncertain mechanism by which human activities feedback onto the Earth’s energy budget.

Given that about two-thirds of the planet is cloudy at any given moment, understanding and quantifying aerosol-cloud-precipitation interactions is critical to producing more accurate climate predictions.

Aerosols directly affect the energy cycle by scattering sunlight, which can be seen as haze. Indirectly, aerosol particles affect the Earth’s climate by acting as the seeds on which clouds form. More aerosol particles can lead to more, but smaller, cloud droplets. This changes the appearance of clouds, like brightness, but may also reduce the likelihood of rain.

Clouds can cool the planet by reflecting sunlight into space: think of standing in the shade on a hot day. Clouds can also warm the planet by blocking the heat its emits: have you noticed that in winter overcast nights are warmer than clear skies? Which of these two processes dominates depends on the cloud’s location, both geographically and in altitude, phase (is it water, ice or a mixture) and microphysics (for example, the size of the droplets making up the cloud).

The role of clouds in the Earth’s energy cycle is intricate. They have a cooling effect on the planet by reflecting sunlight back into space. Simultaneously, they contribute to atmospheric warming by absorbing thermal radiation from the Earth’s surface. The dominance of either cooling or warming depends primarily on factors such as cloud altitude, phase (water, ice, or mixed), and the physical properties, like size, of individual ice crystals or cloud droplets.

Aerosols directly affect the energy cycle by scattering sunlight, which can be seen as haze. The aerosols we release can alter the clouds that form around us, changing their brightness or the likelihood of rain. For example, clouds produced by ‘ship tracks’ form in the exhausts of large ships. These indirect effects on the energy and water cycles are highly uncertain as they are almost impossible to measure in a laboratory and very difficult to isolate in nature.

NCEO researchers at the universities of Oxford, Leicester, Reading and RAL Space are working together to better understand and quantify these aerosol-cloud-precipitation interactions by using:

• satellite imagery
• sampling on research flights
• monitoring at ground stations

One focus is on deep convective “anvil” clouds as these are a major source of extreme precipitation, including destructive hailstorms, while being susceptible to many conflicting aerosol-cloud interactions.  At the University of Leicester, we developed tools to monitor extreme weather events at a global scale.