|WX CHANGERS- SHALLOW INVERSION
METEOROLOGIST JEFF HABY
A shallow inversion can cause a dramatic change in the weather when it develops and when it mixes out. A shallow inversion can be
created by overnight cooling or a shallow layer of cool air moving in at the surface. There is a much different air mass above the
inversion. For example, it may by 25 F at the surface while it is 45 F just a few hundred feet up.
One way the shallow inversion can be mixed out is through solar heating. When the sun heats the surface, it will cause vertical
circulations that cause the air to mix. This can cause the shallow layer of cold air at the surface to mix with much warmer air
aloft. Since the volume of the warmer air aloft is much greater, the temperature of the mixed air will be closer to that of
the warm air. This can cause the surface temperature to sky rocket in the morning. For example, warming from 20 F to 45 F
in just a couple of hours in the morning.
Other ways a shallow inversion can be set up include shallow cold fronts and cold air ponding into a valley. Strong winds and solar
heating are two processes that can mix out this shallow cold air and cause the temperature to jump up. What can start out as a
cold morning can quickly turn into a mild day when the rapid warming takes place.
What can be thought of as the opposite of an inversion is a dry adiabatic layer or superadiabatic layer. This layer at the surface will
show a rapid decrease of temperature with height (warm at ground surface with rapid cooling aloft). This can be caused by intense
solar radiation when winds are weak. When the sun sets, a rapid cooling can take place. In the evening, the temperature can rapidly
decrease as a cool inversion layer forms at the surface. This can commonly occur in dry areas in summer such as in desert and
semi-arid location. This is why desert and semi-arid locations can have such a big temperature difference between the high
and low temperature. It can occur to a lesser extent in other areas also when the weather is sunny and dry.