|TEMPERATURE PROFILE OF THE TROPOSPHERE
METEOROLOGIST JEFF HABY
On average, temperature decreases with height in the troposphere. The troposphere is bounded by the earth's
surface and the tropopause (located at about 150 millibars in the tropics and 300 mb at high latitudes). In the
troposphere on any given day, there are various layers
between the surface and the tropopause where the
lapse rates are far from average (the
lapse rate is near 6.5 degrees C/km). Rawinsonde weather balloons measure temperature as they rise (this is the plot of
the environmental lapse rate). Under certain conditions, the lapse rate in the atmosphere (defined as the change
in temperature with height) is much above or much below normal.
An example of a lapse rate much below normal is an
inversion. In an inversion the
temperature actually increases with height. An inversion promotes atmospheric
stability (stability is common when
warm air is vertically higher than cooler air). The location of the inversion
when it does exist is within or near the boundary layer (but can vary depending on elevation and the current weather
pattern). An inversion can exist at the surface also on a clear night. The surface cools through longwave radiation
emission. This results in an inversion since the air near the surface is cooler than the air higher up (i.e.
The lapse rate can be well above normal also. An example is in the case of solar heating of the
earth's surface. The sun warms the earth's surface resulting in daytime temperatures that are much warmer near
the surface and cool significantly with height. Cold air advection into the mid-levels of the atmosphere and
warm air advection
into the low levels can also lead to a steep (much greater than normal) lapse rate. A steep
lapse rate is indicative of an
unstable troposphere. Warm air under
cold air will support convective mixing.
Convection is not only the development of thunderstorms. Any positively buoyant rising air can be
labeled convection. Even though
you can not see it with your eyes, convective rising is occurring everywhere in the low levels of the troposphere
(especially during the day). If you could see the convection, it would look like a slowly boiling pot of water.
This convection mixes air from the surface to higher in the troposphere. In summary, although the average environmental
lapse rate is about 6.5 C/km, the lapse rates in the actual troposphere can vary dramatically day to day and day versus
night and weather situation to weather situation.