What is Convection?

How Heat Plays a Role in Causing Air to Rise

convection diagram
Heat moves into and through the atmosphere in 3 ways: radiation, conduction, and convection. NOAA NWS Jetstream Online School for Weather

Convection is a term you'll hear quite often in meteorology. In weather, it describes the vertical transport of heat and moisture in the atmosphere, usually from a warmer area (the surface) to a cooler one (aloft).

While the word "convection" is sometimes used interchangeably with "thunderstorms," remember that thunderstorms are only one type of convection!

From Your Kitchen to the Air

Before we delve into atmospheric convection, let's look at an example you may be more familiar with -- a boiling pot of water.

When water boils, hot water in the bottom of the pot rises to the surface, leading to bubbles of heated water and sometimes steam on the surface. It's the same with convection in the air except air (a fluid) replaces the water. 

Steps to the Process of Convection

The process of convection begins at sunrise and continues as follows:

  1. The sun's radiation strikes the ground, heating it. 
  2. As the ground's temperature warms, it heats the layer of air directly above it through conduction (the transfer of heat from one substance to another).
  3. Because barren surfaces like sand, rocks, and pavement become warmer faster than ground covered by water or vegetation, air at and near the surface heats unevenly. As a result, some pockets warm faster than others.
  4. The faster warming pockets become less dense than the cooler air that surrounds them and they begin to rise. These rising columns or currents of air are called "thermals." As the air rises, heat and moisture are transported upward (vertically) into the atmosphere. The stronger the surface heating, the stronger and higher up into the atmosphere the convection extends. (This is why convection is especially active on hot summer afternoons.)

    After this main process of convection is complete, there are a number of scenarios that could happen, each which forms a different weather type. The term "convective" is often added to their name since convection "jumps starts" their development.

    Convective Clouds

    As convection continues, the air cools as it reaches lower air pressures and may reach the point where the water vapor within it condenses and forms (you guessed it) a cumulus cloud at its top!

    If the air contains a lot of moisture and is quite hot, it will continue to grow vertically and will become a towering cumulus or a cumulonimbus.

    Cumulus, towering cumulus, Cumulonimbus, and Altocumulus Castellanus clouds are all visible forms of convection. They are also all examples of "moist" convection (convection where the excess water vapor in the rising air condenses to form a cloud). Convection that occurs without cloud formation is called "dry" convection. (Examples of dry convection include convection that occurs on sunny days when air is dry, or convection that occurs early on in the day before the heating is strong enough to form clouds.)

    Convective Precipitation

    If convective clouds have enough cloud droplets they'll produce convective precipitation. In contrast to non-convective precipitation (which results when air is lifted by force), convective precipitation requires instability, or the ability for air to continue rising on its own. It is associated with lightning, thunder, and bursts of heavy rain. (Non-convective precipitation events have less intense rain rates but last longer and produce a steadier rainfall.)

    Convective Winds

    All of the rising air through convection must be balanced by an equal amount of sinking air elsewhere.

    As the heated air rises, air from elsewhere flows in to replace it. We feel this balancing movement of air as wind. Examples of convective winds include foehns and sea breezes.

    Convection Keeps Us Surface Dwellers Cool

    Besides creating the above-mentioned weather events, convection serves another purpose -- it removes excess heat from the earth's surface. Without it, it has been calculated that the average surface air temperature on earth would be somewhere around 125° F rather than the current liveable 59° F.

    When Does Convection Stop?

    Only when the pocket of warm, rising air has cooled to the same temperature of the surrounding air will it stop rising.