Convection is generally the transport of moisture and heat by the movement of a fluid. In meteorology, convection describes the upward and downward vertical transport of moisture and heat in an unstable atmosphere. As the earth is heated by the sun, parcels of air warm and rise up from the surface. If a parcel is able to rise high enough to cool to its saturation point, the moisture within condenses and becomes visible as a cloud. Clouds often appear as a visible sign that convection is occurring but dry convection can also occur without any visible indicators. The terms “convection” and “thunderstorms” are often used interchangeably, but thunderstorms are only one form of convection.
Surface-based convection is convection that occurs within a surface-based layer (a layer in which the lowest portion is based at or close to the earth’s surface). Heating from the sun and instability at the surface triggers air parcels to rise. If the middle and upper levels of the atmosphere are unstable as well, the surface-based convection will build into the upper levels, producing thunderstorms. Surface-based convection tends to occur in the warm season and in the warm sector (a warm, humid area bounded by a cold front and a warm front) of a mid-latitude low pressure system.
Elevated convection refers to convection occurring within an elevated layer (a layer in which the lowest portion is based above the earth’s surface). Elevated convection occurs when the air near the surface is stable and an unstable layer of air is present aloft. It is most common in the cool season and typically occurs on the cool side of a warm front boundary, behind a cold front, or near the circulation of a mid-latitude low pressure system. Elevated convection can also occur when an upper-level low or upper-level forcing causes air to rise in the middle and upper levels of the atmosphere. Stability indices used to forecast severe weather are often calculated from surface-based measurements and typically underestimate the amount of instability present. Severe weather is possible from elevated convection, although it is less likely than it is with surface-based convection.
The diagram below shows the difference between surface-based and elevated convection.
Courtney Obergfell - NWS Milwaukee/Sullivan - SCEP
Diagrams by Jeff Craven - NWS Milwaukee/Sullivan - SOO