thunderstorms in progress overEarth. Although thunderstorms can be dangerous, it
is important to note that these small disturbances help maintain the energy balance
between the surface and the upper troposphere. Moreover, there are manylocations
where thunderstorms are the primary mechanism causing precipitation.
Most thunderstorms are the result of localized instability. The most common
type of thunderstorm is called an air mass thunderstorm. These are formed in
warm, moistair massesand frequently occur in places dominated by maritime
tropical and equatorial air masses. Scattered afternoon thunderstorms are quite
common in the southeastern United States in the summer because of surface heat-
ing causing unstable maritime tropical air to rise.
There are three phases to thunderstorms. The first is the cumulus stage. In this
stage, cumulus and towering cumulus clouds are formed by condensation and sub-
limation in the strong rise of surface air through the unstable air mass. The change
of phase out of water vapor releases latent heat retarding the rate of cooling of the
rising air, keeping it warmer and spontaneously rising high into theatmosphere.
Individual small clouds eventually merge to form a cumulonimbus cloud that is
the thunderstorm. In the cumulus stage, all air motion is upward. The second phase
is the mature stage. Here, the air has risen to the top of the layer of instability pro-
ducing cloud droplets and ice crystals along the way. The ice crystals and cloud
droplets accumulate enough mass via the precipitation processes to start to fall.
The falling precipitation is the start of downdrafts that entrain surrounding air. In
that the entrained air is relatively colder and dense it accelerates the rate of motion
of the downdraft. The thunderstorm enters the mature phase when it spawns light-
ning and thunder. In the mature phase, updrafts and downdrafts are approximately
equal; this is a tenuous equilibrium usually achieved for only a few minutes. The
final phase of the thunderstorm life cycle is the dissipating stage. This occurs as
the updraft fails to keep up with the flow of the downdraft as the downdraft
entrains air. The updraft weakens to the point where the entire storm becomes
downdraft. Tops of tall storms can literally collapse and downdrafts can reach
speeds in excess of 160 km per hour. Air mass thunderstorms are usually short-
lived. Individual cells last on the order of 30 minutes. However, as one thunder-
storm dissipates others pop up as long as there is great instability.
Lightning is an electrical discharge that defines the presence of a thunderstorm.
As ice and liquid water is quickly transported through the cumulonimbus cloud
there is a net transfer of ions from warmer to colder pieces creating a large
place-to-place electrical potential. When electrical potential exceeds 3,000,000
volts over 50 meters there is an electrical discharge that averages on the order of
10,000 amperes. This is lightning that “steps” 50–100 m at a time through a narrow
channel about the diameter of the human thumb. The lightning stops for a few ten-
millionths of a second each few meters and then steps again. Only about 20 percent
Thunderstorms 341