Clouds form when water vapour condenses into droplets around aerosol particles which act as ‘condensation nuclei’. Cloud droplets are typically a few hundredths of a millimetre in size and condensation nuclei are far smaller. Various types of aerosol act as condensation nuclei, including particles of ice, salt and soot. Clouds formed around many nuclei tend to have a larger number of smaller droplets, making them more concentrated. Clouds formed around fewer nuclei are sparser, with fewer, larger droplets. Most clouds are found in the troposphere, though some extend into the lower stratosphere. Low- and high-level clouds have distinct characteristics, owing to the contrasting temperatures at different heights in the atmosphere.

Up to atmospheric heights of about 6 km, the relative abundance of both aerosol particles and moisture evaporated from the surface creates the conditions for dense clouds such as cumulus to form. These typically have a ‘fluffy’ appearance and can build up over significant heights. Their thickness and high density gives these clouds a high albedo – seen from above, they appear very white and can reflect about 80% of the sunlight hitting them. Seen from below, these clouds can appear grey or dark, because very little sunlight can penetrate through the cloud’s depth. Low- and mid-level clouds also contribute to the greenhouse effect by trapping heat given off by the Earth’s surface. However, this is outweighed by the amount of incoming sunlight reflected and overall these clouds have a cooling effect on the climate.

Above heights of about 6 km, the atmosphere is relatively dry and the clouds that form here – such as cirrus – tend to be sparse and ‘wispy’. Owing to the low temperatures at these altitudes, high-level clouds are primarily composed of ice crystals rather than water droplets. Because these clouds are sparse and thin, they have low albedo and reflect as little as 10% of the sunlight hitting them. This has only a slight cooling effect on the climate, which is outweighed by the contribution of high-level clouds to the greenhouse effect, as they trap outgoing heat given off by the Earth’s surface. As a result, high-level clouds have an overall warming effect on the climate.

Air temperature controls many aspects of the climate, so temperature changes would be expected to lead to changes in cloud cover. If a temperature rise increased low-level cloud cover, the cooling effect of these cloud types would be enhanced, resulting in a negative feedback. In contrast, increased high-level cloud cover would result in a positive feedback, enhancing the warming effect of high-level clouds. By examining climate model simulations and measurements of cloud responses to past temperature trends, scientists have concluded that the overall cloud feedback is likely to be positive. However, there are many challenges both in modelling clouds and in estimating cloud feedbacks from atmospheric measurements. The overall strength of cloud feedbacks is one of the greatest remaining uncertainties in climate science.

Scientists use computer models to investigate the behaviour of the climate system. Climate models divide the globe into grid boxes about 100 km across and use the laws of physics to simulate the atmospheric conditions in each grid box. Cloud processes occur on a smaller scale than a typical model grid box, so scientists use measurements of real clouds to instruct the models how to simulate cloud behaviour. If the atmospheric conditions in a model grid box meet the observed criteria for cloud formation, the model adds cloud cover to that grid box. The presence of cloud cover affects the grid box’s other simulated characteristics, such as the amount of sunlight reaching the surface. If grid box conditions change, this can trigger the simulated clouds to produce rain or to disperse.

Scientists use a range of measurements to investigate cloud characteristics and behaviour. Ground-based observations of cloud cover have been recorded since the 1950s. Instruments known as radiometers are stationed on the surface to measure the amount of incoming solar energy. Because cloud cover plays a key role in sunlight levels at the surface, scientists can use these measurements to track variations in cloudiness over time. Some scientists also use ‘Light Detection and Ranging’ instruments (LIDARs) carried onboard scientific aircraft to take atmospheric measurements of cloud cover and other cloud characteristics. More recently, instruments carried on board satellites orbiting the Earth have been measuring cloud cover from space.

Many different factors influence the Earth’s climate. Professor Graf’s work focuses on a range of these factors, from human activity to volcanic eruptions. ‘Understanding what drives our climate and its variability is both fascinating (just look at clouds or volcanic eruption plumes) and essential to the society.’ Professor Graf’s work has resulted in the first successful simulation of the climate effects of a big volcanic eruption. However he’s quick to emphasise the complexity involved in such work. ‘There are always struggles with the big computer models and our work would benefit from more reliable observations. However, our understanding of the basic physics involved in the field is quite good.’

• Climate research and modelling at CEA
• Met Office: Climate Feedbacks
• Koshland Science Museum: Clouds