The Nature of storms
I. Thunderstorms
A. At any given moment, nearly 200
thunderstorms are occurring around the world.
1. Cumulonimbus clouds produce
thunderstorms.
2. Three conditions must occur for a
thunderstorm to occur.
a. There must be an abundant source of moisture
in the lower levels of the atmosphere.
b. Some mechanisms must lift the air so that the
moisture can condense and release latent heat.
c. Portion of the atmosphere through which the cloud
grows must be unstable. The air must continue to cool
with increasing altitude for the growing cloud to stay
warmer than the surrounding air.
3. If the three conditions just described are met
the air will keep rising, causing more moisture to
condense and creating more latent heat.
4. A typical thunderstorm lasts about 30
minutes.
B. Air mass thunderstorms
1. Thunderstorms are often classified according to the
mechanism that caused the air to rise.
2. If the air rose because of unequal heating of Earth
3. Surface within one air mass, the thunderstorm is
called an air-mass thunderstorm.
4. The unequal heating of earth’s surface
reaches its maximum during mid- afternoon.
Thus air mass thunderstorms are most common
then.
5. There are two common types of air-mass
thunderstorms.
6. Mountain
thunderstorms occur
when an air mass raised
as a result of orographic
lifting. (Air moving up
the side of a mountain)
7. Sea-breeze thunderstorms
are common along coastal
areas during the summer,
especially in the tropics and
subtropics.
8. Sea-breeze thunderstorms
are local air-mass
thunderstorms caused in part
by the extreme temperature
differences between the air
over land and the air over
water.
C. Frontal Thunderstorms
1. The second main classification of
thunderstorms is frontal thunderstorms, which
are produced by advancing cold fronts and more
rarely, warm fronts.
2. This rapid upward motion can produce a line
of thunderstorms, sometimes very large.
D. Stages of Development
1. A thunderstorm usually has three stages: the
cumulus stage, the mature stage, and the
dissipation stage. The stages are classified
according to the direction in which the air is
moving
2. Cumulus stage- air
starts to rise nearly
vertically upward. This
creates updrafts, which
transport moisture to the
upper reaches of the
cloud. This begins the
mature stage of a
thunderstorm.
3. Mature stagePrecipitation in a
thunderstorm is composed of
water droplets that formed at
high, cool levels of the
atmosphere. As the
precipitation falls, it cools the
air around it. This creates
downdrafts. In the mature
stage, nearly equal amounts
of updrafts are downdrafts
exist side by side in the
cumulonimbus cloud.
4. Dissipation stage- the
production of downdrafts is
ultimately the thunderstorm’s
undoing. The convection cell
can exist only if there is a
steady supply of warm, moist
air at Earth’s surface. Once
that supply runs out, the
updrafts slow and eventually
stop.
II. Severe Weather
A. Severe Thunderstorms
1. Other factors also play a role in causing
some storms to be more severe than others.
2. The upper level usually accompanies cold
fronts. Low-pressure system that are marked by
pools of cold air
3. As the instability of the air increases, the strength of
the storm’s updrafts and downdrafts intensifies. The
storm is then considered to be severe.
4. They may develop into self-sustaining, extremely
powerful storms called super cells, which are
characterized by intense, rotating updrafts.
5. These furious storms can last for several
hours.
6. Of the estimated 100,000 thunderstorms
10% are considered to be severe.
B. Lightning
1. Lightning is electricity caused by the rapid
rush of air in a cumulonimbus cloud.
2. A lightening bolt forms when friction between
the updrafts and downdrafts within a
cumulonimbus cloud separates electrons from
some of their atoms either in the cloud or near
the ground.
3. A lightning bolt heats the
surrounding air to about 30
000 C. That is about five
times hotter than the surface
of the sun!
4. The thunder you hear is
the sound produced as this
super heated air rapidly
expands and contracts.
5. Because sound travels more slowly than light
waves, you may see lightning well before you
hear thunder, even though they are generated at
the same time.
6. Each year in the US, lightning accounts for
about 7500 forest fires, which result in the loss of
millions of acres of forest.
7. In the US lightning
injures yearly 300 people
and kills 93 people a
year.
C. The fury of Wind
1. Violent downdrafts that are concentrated in a
local area are called downbursts.
2. Based on the size of the area they affect,
downbursts are further classified as either macro
bursts or micro bursts.
D. Hail
1. Each year in the US almost
$1 billion in damage is
caused by hail.
2. Hail is precipitation in the
form of balls or lumps of ice.
3. Hail forms because of two
characteristics common to
thunderstorms.
4. Water droplets exist in the liquid state in the
parts of cumulonimbus cloud where the
temperature is actually below freezing.
5. When these super cooled water droplets
encounter ice pellets, they water droplets freeze
on contract and cause the ice pellets to grow
larger.
6. The second characteristic that allows hail to
form is an abundance of strong updrafts so that
they are constantly encountering more super
cooled water droplets.
E. Floods
1. Storms may dump its rain over a limited
location rather than spreading it over a large
area.
2. Floods occur.
F. Tornadoes
1. A tornado is a violent, whirling column of air
in contact with the ground.
2. Before it touches the ground it is called a
funnel cloud.
3. Tornadoes are often associated with super
cells, the most severe thunderstorm.
4. The air is a tornado is made visible by dust
and debris drawn into the swirling column, or by
the condensation of water vapor into a visible
cloud.
5. Tornadoes can vary greatly in size and
intensity.
6. They are classified according to the Fujita
Tornado intensity scale it ranks tornadoes
according to their path of destruction, wind
speed and duration.
7. There are some locations that are more common for
tornadoes to form.
8. Most tornadoes –especially the violent ones- form in
the spring during the late afternoon and evening, when
the temperatures contrasts between polar air, which still
has winter characteristics and tropical air, which is
steadily becoming warmer and the greatest.
9. More than 700
tornadoes touch down
each year in the US.
10. Many of these occur in
a region called “Tornado
Alley” which extends
from northern Texas
through Oklahoma,
Kansas and Missouri.
11. In the US, an average of 80 deaths and
1500 injuries result from tornadoes each year.
12. The National Weather Service issues
tornado watches and warnings before a tornado
actually strikes.
13. A watch means that the environment is
favorable for a tornado.
14. A warning means a tornado has been
spotted and touched down.
III. Tropical Storms
A. During summer and fall, the sunny tropics
are birthing grounds for large, rotating, lowpressure storms called tropical cyclones.
1. The strongest of these cyclonic storms
are known in the US and other parts of the
Atlantic Ocean as hurricanes.
2. Cyclones thrive on the tremendous amount of
energy in warm, tropical oceans.
3. Tropical cyclones require two basic
conditions to form: an abundant supply of warm
ocean water and some sort of disturbance to lift
warm air and keep it rising.
4. Tropical cyclones move according to the wind
currents and steer them.
5. A traveling tropical disturbance, which can
cause air in a developing tropical cyclone to rise,
is the first stage of a tropical cyclone.
1 6. Once winds reach speeds another
phenomenon takes place- the development of a
calm center of the storm called the eye.
7. The strongest winds in a hurricane usually
concentrated in a band immediately surrounding
the eye called the eyewall.
8. The Saffir Simpson
hurricane scale classifies
hurricanes according to
wind speed, air pressure
in the center and
potential for property
damage.
9. Category 1 hurricanes, which have a
minimum, wind speed of 74mph, to the
monstrous category 5 storms, which have winds
in excess of 155mph.
10. A hurricane will last until it can no longer
produce enough energy to sustain itself. This
usually happens when the storm moves over
land and no longer has access to the warm
ocean surface from which it draws its energy, or
when the storm moves over colder waters.
11. Hurricanes can cause a lot of damage, particularly
along coastal areas where human populations have
increased.
12. Most of the damage is associated with wind.
13. Strong winds moving onshore in coastal areas are
partly responsible for another major hurricane threat:
storm surges.
14. A storm surge occurs when hurricane force winds
drive a mound of ocean water toward coastal areas,
where it washes over the land.
15. The national hurricane center which is responsible
for tracking and forecasting the intensity and motion of
tropical cyclones in the western hemisphere, issues a
hurricane warming at least 24 hours before it strikes.
IV. Recurring Weather
A. Floods and droughts
1. Droughts are extended periods of wellbelow normal rainfall
2. Thunderstorms can produce enough rain to
produce floods.
3. An unpleasant side effect of droughts often
comes in the form of heat waves, which are
extended periods of above-normal temperatures.
4. Cold wave is an extended period of belownormal temperatures.
5. Wind transports heat away from the body,
the effects of cold air is worsened by wind.
6. This phenomenon is known as a wind-chill
factor.
THE END