CH
The Nature of Storms
Section 13.1: Thunderstorms
Section 13.2: Severe Weather
Section 13.3: Tropical Storms
Section 13.4: Recurrent Weather
Section 13.1
Thunderstorms
Objectives
Identify the processes that form thunderstorms.
Compare and contrast different types of
thunderstorms.
Describe the life cycle of a thunderstorm.
Review Vocabulary
latent heat: stored energy in water vapor that
is not released to warm the atmosphere until
condensation occurs
Section 13.1
Thunderstorms
The intensity and duration of thunderstorms
depend on the local conditions that create them.
New Vocabulary
air-mass thunderstorm
frontal thunderstorm
mountain thunderstorm
stepped leader
sea-breeze
thunderstorm
return stroke
Section 13.1
Thunderstorms
Overview of Thunderstorms
At any given moment, nearly 2000
thunderstorms are in progress around
the world.
Section 13.1
Thunderstorms
Overview of Thunderstorms
Both geography and air mass
movements make thunderstorms
most common in the southeastern
United States.
Section 13.1
Thunderstorms
Overview of Thunderstorms
How thunderstorms form
For a thunderstorm to form, three
conditions must exist:
1.a source of moisture,
2.lifting of the air mass,
3.and an unstable atmosphere.- clouds
can grow into a cumulonimbus cloud.
Section 13.1
Thunderstorms
Overview of Thunderstorms
How thunderstorms form
For a thunderstorm to form, there must be
an abundant source of moisture in the
lower levels of the atmosphere.
Section 13.1
Thunderstorms
Overview of Thunderstorms
How thunderstorms form
For a thunderstorm to form, there must be
some mechanism for moisture to
condense and release its latent heat. This
occurs when a warm air mass is lifted into
a cooler region of the atmosphere.
Section 13.1
Thunderstorms
Overview of Thunderstorms
How thunderstorms form
If the surrounding air remains cooler than
the rising air mass, the unstable
conditions can produce clouds that grow
upward. This releases more latent heat
and allows continued lifting.
Section 13.1
Thunderstorms
Overview of Thunderstorms
Limits to thunderstorm growth
Because the rate of condensation
diminishes with height, most cumulonimbus
clouds are limited to about 18,000 m.
Thunderstorms are also limited by duration
and size.
Section 13.1
Thunderstorms
Types of Thunderstorms
Thunderstorms are often classified
according to the mechanism that causes
the air mass that formed them to rise.
There are two main types of
thunderstorms:
1.air-mass and 2. frontal.
Section 13.1
Thunderstorms
Types of Thunderstorms
Air-mass thunderstorms
When air rises because of unequal heating of
Earth’s surface beneath one air mass, the
thunderstorm is called an air-mass
thunderstorm.
There are two kinds of air-mass thunderstorms.
Section 13.1
Thunderstorms
Types of Thunderstorms
Air-mass thunderstorms
1.Mountain thunderstorms occur when
an air mass rises by orographic lifting,
which involves air moving up the side of a
mountain.
2.Sea-breeze thunderstorms are local
air-mass thunderstorms that occur
because land and water store and release
thermal energy differently.
Section 13.1
Thunderstorms
Types of Thunderstorms
Sea breeze thunderstorms
During the day, the temperature of land
increases faster than the temperature of
water. At night, conditions are reversed.
Section 13.1
Thunderstorms
Types of Thunderstorms
Frontal thunderstorms
Frontal thunderstorms are produced by
advancing cold fronts and, more rarely, warm
fronts.
Thunder storms due to cold fronts are severe
and hundreds of kilometers long.
Thunder storms due to warm fronts are less
severe.
Section 13.1
Thunderstorms
Thunderstorm Development
A thunderstorm usually has three stages:
1.the cumulus stage- air starts to rise
vertically. This creates updrafts.
2.the mature stage,
3.and the dissipation stage.
The stages are classified according to
the direction the air is moving.
Section 13.1
Thunderstorms
Thunderstorm Development
Mature stage
In the mature stage, updrafts and downdrafts
exist side by side in the cumulonimbus cloud.
The updrafts and downdrafts form a
convection cell which produces the surface
winds and rain associated with
thunderstorms.
Section 13.1
Thunderstorms
Thunderstorm Development
Dissipation stage
In a thunderstorm, the cool downdrafts spread
in all directions when they reach Earth’s
surface. Downdrafts cool the areas from
which the storm draws its energy, the
updrafts cease, and clouds can no longer
form. The storm is then in the dissipation
stage.
Section 13.1
Thunderstorms
Lightning
Lightning is the transfer of electrical charge
due to friction between the updrafts and
downdrafts within a cumulonimbus caused
by the rapid rushes of air.
Section 13.1
Thunderstorms
Lightning
Friction between the updrafts and downdrafts
within a cumulonimbus cloud removes
electrons from some of the atoms in the cloud.
Atoms that lose electrons become positively
charged ions, and atoms that receive the extra
electrons become negatively charged ions.
Section 13.1
Thunderstorms
Lightning
Eventually, the differences in charges break
down, and a branched channel of partially
charged air, called a stepped leader, is
formed between the positive and negative
regions.
Section 13.1
Thunderstorms
Lightning
When the stepped leader nears the ground, a
branched channel of positively charged particles,
called the return stroke, rushes upward to meet
it and illuminates the connecting channel with
about 100 million volts of electricity.
Section 13.1
Thunderstorms
Lightning
Thunder
A lightning bolt heats the surrounding air to
about 30,000C, about five times hotter
than the surface of the Sun.
The thunder you hear is the sound
produced as this superheated air rapidly
expands and contracts.
Section 13.1
Thunderstorms
Lightning
Lightning variations
There are several names given to lightning
effects: sheet lightning, heat lightning,
spider lightning, ball lightning, blue jets, and
red sprites.
Section 13.1
Thunderstorms
Lightning
Thunderstorm and lightning safety
Each year in the United States,
lightning causes about 7500 forest fires
and an average of 300 injuries and 93
deaths to humans.
CH
The Nature of Storms
13.1 Section Questions
Thunderstorms in the United States are most
common in the Midwest.
a. true
b. false
CH
The Nature of Storms
13.1 Section Questions
A mature thunderstorm has a region of updraft
and a region of downdraft.
a. true
b. false
CH
The Nature of Storms
13.1 Section Questions
What is the difference between air-mass
thunderstorms and frontal thunderstorms?
Answer: Air-mass thunderstorms form as
a result of uplift of air within one air mass.
Frontal thunderstorms form as a result of
uplift of air along frontal boundaries.