|WHAT THERMAL ADVECTION IS NOT
METEOROLOGIST JEFF HABY
Thermal advection is the synoptic scale movement of air of
a different temperature into the forecast area. This
air must be moving nearly parallel to the earth's surface (no strong vertical motion). The term
advection means a horizontal transport of air. The two types of thermal advection
are cold air advection and warm air advection. An example of cold air advection
is the temperatures cooling after a cold front passage. An example
of warm air advection is an influx of warmer air coming from the south that replaces
cooler air. When thermal advection is first taught many students can get confused between
thermal advection and other processes that cause a temperature change to the air. The following
is a list and description of what thermal advection is not. Once you understand what thermal
advection is not you will better identify when thermal advection occurs.
a. Thermal advection is not solar warming or radiational cooling. Temperatures will often
warm during the day due to solar radiation. This warming by itself is not warm
air advection since the air is warming in-place and not warming by warming air
advecting in. At night temperatures will often cool due to radiational cooling especially
on clear nights with light wind. This cooling by itself is not cold air advection.
b. Evaporative cooling is not cold air advection. Temperatures will often cool when
rain or snow occurs especially if the air is dry to begin with. Evaporative cooling
is in-place cooling and not a horizontal transport of cooler air.
c. When thunderstorms occur often cool air will rush out of them. This cooling is
not classified as cold air advection since
thunderstorm convection occurs on a local scale. Thermal
advection is analyzed at the synoptic scale.
d. Adiabatic warming and cooling is not thermal advection. When air rises it cools adiabatically
and when air sinks it warms adiabatically. This temperature change process is not
classified as thermal advection since this temperature change is being produced by
vertical motions and not by a horizontal movement of air.