13.1 Thunderstorms
There are usually around 2000 thunderstorms around the world at any given moment.
How thunderstorms form – The cooling air masses are stable but air masses that are receiving warm air
form land or water are not. If the right conditions exist, convection can cause the cumulus cloud to grow
into a cumulonimbus cloud. Three conditions must exist in order for thunderstorms to form:
1) A source of moisture, there must be an abundance of moisture in the lower atmosphere. 2) Lifting air
mass, this happens when the cold front pushes the warm moist air up, and 3) an unstable atmosphere,
this is when the surrounding air remains cooler than the rising air.
The cumulonimbus clouds that produce thunderstorms are limited to altitudes of 7.5 miles, durations and
size.
Types of thunderstorms: air mass and frontal
Air mass thunderstorms are produced because of unequal heating of Earths surface within one air
mass. This unequal heating reaches its maximum during mid-afternoon, so it is common for “pop-up”
thunderstorms to occur at this time. There are two types of air mass thunderstorms: Mountain
thunderstorms occur during orographic lifting (over a mountain), and sea-breeze thunderstorms,
which occur, near the coasts and are common in the summer.
Frontal thunderstorms are produced by advancing cold fronts and very rarely warm fronts. These
thunderstorms do not rely on daytime heating therefore they can persist long into the night. They cause
massive flooding which is the number one cause of deaths from thunderstorms.
Thunderstorm development: they are usually three stages in the development of thunderstorms
1). Cumulus stage – air starts to rise vertically (updraft) and covers an area of about 3-5 miles. The
updrafts transfers water vapor to the upper parts of the cloud where it condenses. As the cloud droplets
coalesce, they become larger and heavier until they fall to Earth as precipitation. This takes us to the next
stage, which is the mature stage.
2.) Mature stage – updrafts and downdrafts occur side by side. Water and ice droplets in the upper part
of the cloud cools the air as it falls. This air sinks rapidly because it is cooler than the surrounding air.
This is what creates the downdrafts. The updrafts and downdrafts create a convection cell, which
produces the strong surface winds associated with thunderstorms. The mature stage covers an area of
about 5-9 miles.
3.) Dissipation stage – The
convection cell will only last as
long as there is a steady supply of
warm, moist air. Once the supply
is gone, the updrafts will slow and
eventually disappear. The cool
downdrafts will spread out
therefore ceasing the updrafts and
clouds can no longer form. This
stage will last until all the
precipitation that had been formed
has fallen. This covers an area of
about 5-7 miles.
Lightning - is the transfer of electricity generated by the rapid rushes of air in the cumulonimbus cloud.
The friction caused between the updrafts and downdrafts removes electrons from some of the atoms and
causes the cloud to become charged. The atoms that lose electrons become positively charged ions and
the atoms that receive the electrons become negatively charged ions. This creates regions of air with
opposite charges. The differences in the charges will eventually break down and creates a region of
partially charged air called a stepped leader. This will move from the center of the cloud towards the
ground. When the stepped leader nears the ground, a channel of positively charged particles called a
return stroke rushed upward to meet it. The return stroke surges from the ground to the cloud
illuminating it with 100 million volts of electricity.
Thunder – The lightning bolt heats the surrounding air to about 54,000oF. This is about 5 times hotter
than the surface of the sun. When the super heated air rapidly expands and contracts, you hear thunder.
Since sound waves travel more slowly than light waves, you see the lightning before you hear the
thunder.
Thunderstorm and lightning safety – If you are outdoors during a thunder and lightning storm, squat
on the balls of your feet and tuck your head in and make yourself as small as possible. AVOID things like
trees, small sheds, and convertible automobiles as shelters. Avoid using appliances and telephones as
well. These can lead to electric shock. Stay out of boats and away from water.