|HOW DOES DRY AIR ALOFT|
MAKE STORMS MORE SEVERE?
METEOROLOGIST JEFF HABY
It would seem that having lots of
moisture in the entire troposphere would be the most beneficial to
severe thunderstorms. However, while lots of moisture near the surface is important, if
the air higher aloft is
dry it contributes to storms being stronger. Dry means that the
dewpoint depression is large. For example if
the temperature is 8 C with a dewpoint of -15 C then that air is dry. If the dewpoint depression is 5 or
less than that air is moist. The dewpoint depression is found by taking the temperature minus the dewpoint.
For example if the temperature is 20 C and the dewpoint is 17 C, then the dewpoint depression is 20 - 17 = 3.
Note that dewpoint depression is not a temperature but rather a temperature difference.
First I will explain how dry air higher aloft contributes to stronger
convective wind gusts and
larger hail at the surface.
As a cumulus
updraft rises it will become exposed to dry air higher aloft in a situation in which there
is a high dewpoint depression aloft. If there is
strong wind shear aloft
then the dry air will effectively penetrate into
the thunderstorm. The dry air will penetrate into air that is saturated and full of precipitation.
A downdraft of precipitation will fall toward the surface. When dry air mixes into the saturated air,
cooling will occur due to evaporative cooling. This makes the downdraft colder and more dense than it
otherwise would be. Cold dense air will accelerate toward the surface. Since the downdraft falls
at a faster rate the convective wind gusts experienced at the surface will be stronger. The evaporative cooling
aloft keeps the downdraft colder than it otherwise would be. This makes it more likely hail will reach the surface
and more likely the hail will be bigger once it reaches the surface.
Hail has less time to melt when the
freezing level is closer to the surface and the downdraft is colder.
Next I will explain how dry air aloft leads to higher
instability. When instability is higher the storm
updraft is stronger and this contributes to a stronger downdraft and the generation of larger hail. The term
instability is a situation in which the temperature cools with height at a rate that can support
a storm. An unstable situation is to have very warm moist air under very cold air. Since very warm air is less
dense than cold air, the warm air can rise freely. Think of steam rising from a pot of boiling water. The
steam rises to the ceiling since it is warmer than the surrounding air. For the troposphere to be unstable there
must also be adequate moisture near the surface. If the air is both very dry at the surface and higher aloft
then there will be no instability generated. When the air is warm and moist at the surface and the air is
very dry higher aloft this is a situation that is termed
convective instability. A convectively unstable
environment becomes more unstable when all the air within the troposphere is lifted (such as a low pressure
system causing the air within the troposphere to rise). Air that is dry will cool more rapidly than air that
is saturated. Thus, when lifting occurs the dry air higher aloft will become much colder as it is lifted. Instability
increases because the air at the surface is still warm and moist while the dry air higher aloft becomes much
colder. Once a storm occurs the updraft will be stronger when the surrounding air rapidly decreases in
temperature with height.