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Storms
Understanding Fronts
Fronts occur at the boundaries of converging air masses
Cold Fronts
• A cold front is defined as the transition zone where a cold
air mass is displacing a warmer air mass
• Air associated with a cold front is usually unstable and
conducive to cumulonimbus cloud formation
• Usually, rainfall associated with cold fronts is in the form
of heavy deluge.
Cold Fronts
Warm Fronts
• A warm front is defined as the transition zone where a
warm air mass is replacing colder air mass.
• A warm front exists when warm air is rising over cold air
• As the air lifts into regions of lower pressure, it expands,
cools and condenses water vapor as flat sheet cloud
(altostratus),
Warm Fronts
Stationary Fronts
• A cold front is the boundary between cool and warm air
when the cool air is replacing the warm air. A warm front
is the boundary when the warm air is winning the battle.
When the pushing is a standoff, the boundary is known as
a stationary front. Stationary fronts often bring several
days of cloudy, wet weather that can last a week or more.
Stationary Fronts
Supercell
• A thunderstorm with a persistent rotating
updraft.
• Supercells are rare, but are responsible for a
remarkably high percentage of severe
weather events - especially tornadoes,
extremely large hail and damaging straightline winds.
Supercell
Tornado Formation
• Warm moist air shoots upward meeting colder, dryer air.
• Warm moist air is lighter than the cold dry air making for a
strong updraft within the thunderstorm.
• If these varying winds are staggered in just the right
manner with sufficient speed, they will act on the upward
rising air, spinning it like a top
Tornado Formation
Before thunderstorms
develop, a change in wind
direction and an increase
in wind speed with
increasing height creates
an invisible, horizontal
spinning effect in the
lower atmosphere
Rising air within the
thunderstorm updraft
tilts the rotating air from
horizontal to vertical
An area of rotation, 2-6
miles wide, now extends
through much of the
storm. Most strong and
violent tornadoes form
within this area of strong
rotation.
How are Tornadoes measured
The Fujita Scale
Tornado Rating
Wind Speed
Level of Damage
F0
40mph
Light
F1 (weak)
73-112mph
Moderate
F2 (strong)
113-157mph
Considerable
F3 (strong)
158-206mph
Severe
F4 (violent)
207-260mph
Devastating
F5 (violent)
261-318mph
Incredible
Hurricanes
• They are tropical, meaning that they are generated in
tropical areas of the ocean near the Equator.
• They are cyclonic, meaning that their winds swirl around a
central eye.
• The eye of a hurricane is always a low-pressure area. The
lowest barometric pressures ever recorded have occurred
inside hurricanes.
• The winds swirling around the center of the storm have a
sustained speed of at least 74 mph
How is a hurricane formed?
• Hurricanes usually originate as a cluster of
showers and thunderstorms in tropical waters
• Hurricanes form in tropical regions where there is
warm water (at least 80 degrees Fahrenheit / 27
degrees Celsius), moist air and converging
equatorial winds.
• Hurricane winds in the northern hemisphere
circulate in a counterclockwise motion around the
hurricane's center or "eye," while hurricane winds
in the southern hemisphere circulate clockwise.
Some important definitions:
• Tropical Disturbance: An organized area of
thunderstorms that holds itself together for at least 24
hours.
• Tropical Depression: A tropical cyclone (an area of low
pressure that develops over tropical or subtropical waters
with a definite organized surface circulation) in which the
maximum sustained wind speed is 38mph or less.
• Tropical Storm: A tropical cyclone with maximum
sustained winds of 39mph to 73mph.
The Saffir-Simpson Scale
Category
Wind Speed
Storm Surge
Damage
Category 1
74-95mph
4-5 feet
Minor
Category 2
96-110mph
6-8 feet
Considerable
Category 3
111-130mph
9-12 feet
Extensive
Category 4
131-155mph
13-18 feet
More Extensive
Category 5
Greater than 155
18 and higher
Catastrophic
Diagram of a Hurricane
Storm Surge
http://www.usatoday.com/graphics/weather/gra/gsurge/flash.htm
Nor'easter
• Gulf Stream low-pressure system - (counter-clockwise
winds) These systems generate off the coast of Florida.
The air above the Gulf Stream warms and spawns a lowpressure system. This low circulates off the southeastern
U.S. coast, gathering warm air and moisture from the
Atlantic. Strong northeasterly winds at the leading edge of
the storm pull it up the east coast.
• Arctic high-pressure system - (clockwise winds) As the
strong northeasterly winds pull the storm up the east coast,
it meets with cold, Arctic air blowing down from Canada.
When the two systems collide, the moisture and cold air
produce a mix of precipitation.