Resource: Convective Cloud Systems

The Sun, Earth's main source of energy, continuously showers the planet with electromagnetic radiation. About 31% of this solar radiation is reflected back towards space, about 23% is absorbed by the atmosphere, and about 46% is absorbed by Earth's surface. Earth's surface re-emits some of this absorbed energy as infrared radiation, and this heat energy is then absorbed by greenhouse gases in the atmosphere. Energy is also radiated from the atmosphere and warms Earth.

Air near the ground is also warmed by conduction — the direct transfer of energy between objects in contact by the collision of their molecules. However, air is not a very good conductor. A more efficient mode of energy transfer within the atmosphere is convection — the transfer of heat through the movement of matter. As a region of surface air is warmed, the molecules move faster and the air expands and becomes less dense. Less dense air rises through the cooler and denser air above it in the process of convection. These parcels of rising warm air, called thermals, can lead to the formation of cumulus clouds.

As air rises, it moves into an environment of lower atmospheric pressure, and as a result, it expands further and cools. When the temperature of the air drops below the dew point point, the rate at which invisible water vapor condenses onto cloud condensation nuclei — tiny particles in the air such as dust, sea spray, and industrial aerosols — exceeds the rate of evaporation of liquid water. Billions of these tiny water droplets or ice crystals may develop and collect to form a cloud. When the droplets or ice crystals grow too large to remain suspended in the air, they fall to the ground as precipitation.

Because warm air is less dense than cold air, and moist air is less dense than dry air, the warm and moist air of the tropics provides an ideal environment for convection. Along coastal locations in the tropics, convection is often easy to observe on a warm, sunny day. While cumulus clouds are found over both land and water, they tend to form over land earlier in the day than over water. This is because water has a higher heat capacity than land — it takes a lot more energy to raise the temperature of water than to raise the temperature of land. As a result, land warms more rapidly than water and is able to heat the surface air to a higher temperature.

Whether or not cumulus clouds bring about thunderstorms depends on the stability of the surrounding atmosphere. If the air around the cloud is stable, vertical air motion and cloud growth is inhibited. However, if the air around the cloud is unstable, vertical motion is enhanced and the cloud continues to grow, potentially into a thunderhead.

To learn more about energy transfer, check out Cooking Cookies with Solar Power.

To learn more about clouds, check out Cloud Types.

To learn more about energy and the atmosphere, check out Water Vapor Circulation on Earth.

To learn more about weather in the tropics, check out Ocean Temperatures and Climate Patterns.

• What is "convection"? Explain how the process of cloud formation depends on the process of convection.
• What do you think needs to happen to warm rising air for a cloud to form?
• Compare and contrast cloud formation over islands and large bodies of water. Give two similarities and two differences.
• Predict how these patterns in storm development can influence the tropical climate.
• In what ways do you think the process of cloud formation over water and islands at other latitudes would be similar to or different from the process in tropical latitudes?