Convection is the process whereby hotter, less dense parcels of air rise and colder, denser parcels sink resulting in the transfer of heat. Convection can occur in both liquids and gases and is a major factor in the weather. Convection occurs in both moist and dry atmospheres as well as in the oceans and plays an important role in determining the weather.
Moist convection occurs when rising air contains large amounts of water vapour, which condenses to form clouds. As condensation happens, heat is released, which enables the air parcel to continue to rise. It is this kind of convection that plays a part in the formation of cumulus and cumulonimbus clouds. These are the type of clouds that are often associated with heavy rain and thunderstorms. Dry convection can occur when the rising air parcel is sufficiently warm that condensation into clouds does not occur.
There are two main situations which can trigger convection in the atmosphere:
- Surface heating driven convection
- Air mass driven convection
Heating of the land surface by the sun causes warm columns of air to rise; known as thermals. As air rises, it cools and water vapour contained in the air condenses into droplets to form clouds. If the air does not cool sufficiently the water vapour will be unable to condense and clouds will not form. This surface heating driven convection is beneficial for glider pilots who are able to use thermals for extra lift in-flight. Additionally, in a dry atmosphere convection can result in an overall cooling of the surrounding environment. This is because as the warmer air at the surface rises colder air above sinks producing a cooling effect at the surface. In a similar manner, differences in temperature at the surface drive convection in the ocean forming currents.
Secondly, convection can occur when a warm air mass meets a cold air mass. As the two air masses meet, such as a weather front, the cold air mass passes under the warm air mass forcing the warm air upwards. As the air rises, large, deep, and often widespread clouds develop producing heavy rain and thunderstorms.
Katie Brown- Aviation Scientist