Transcript Low Pressure and Severe Weather

Severe Weather
Some Meteorology Basics
Atmospheric Heating
Atmosphere is heated from the bottom
Solar energy absorbed by the Earth is re-
radiated as heat
Warm air is less dense so it rises
This mixing of the air produces our
weather
Atmospheric Heating is Uneven
Latitude (position relative to the poles and
the equater) influences amount of solar
energy received
Land re-radiates heat much faster than
water
Moisture also plays a role
– Evaporation requires energy
– Condensation releases energy
Uneven Heating and Air
Pressure
Areas where air is heated
Air becomes less dense (weighs less)
Air rises
Combination is measured as Low Pressure
Uneven Heating and Air
Pressure
Areas where air is less heated
Air becomes more dense (weighs more)
Air sinks
Combination is measured as High
Pressure
Air Pressure and Weather
Low Pressure areas have rising air
Air rises to areas where it is cooler
Cooling may produce condensation
Condensation may result in precipitation
Lows are generally associated with stormy
weather
Air Pressure and Weather
High Pressure areas have sinking air
Air sinks to areas where it is warmer
As air is heated, it can hold more moisture
If more moisture is not available, the effect
is that the air become drier (less humid)
Highs are generally associated with clear
weather
Air Pressure and Wind
Warm air rises in areas of Low Pressure
Cool air sinks in areas of High Pressure
To equalize the pressure, air must move
from areas of High Pressure to areas of
Low Pressure
This movement of air is called wind
Simple, but not so simple
Air moves from areas of high pressure
(sinking) to areas of low pressure (rising)
However, the air doesn’t follow a straight
path
Because the Earth spins on its axis, the
wind is “deflected” in a slightly different
direction
Coriolis Effect
The “deflection” of the wind, or any
moving object, because of the Earth’s
rotation is known as the Coriolis Effect
In the Northern Hemisphere, moving
objects are deflected to the right of their
intended path
Highs, Lows and Around It Goes
Wind blows from High Pressure toward
Low Pressure
We can also think of this as blowing
“outward” or away from the High
And, “inward” or toward the Low
The Coriolis Effect deflects these winds,
so…
Highs, Lows and Around It Goes
Winds blow clockwise and out from Highs
Winds blow counterclockwise and in
toward Lows
Counterclockwise winds around
a Low: Hurricane Fran
Counterclockwise winds around
a Low: “The Perfect Storm”
Low Pressure and
Severe Weather
Air rises, cools and may promote
condensation in Lows
Lows are generally associated with stormy
weather
Some storms are quite severe
Examples of Severe Weather and
Low Pressure Systems
 Tropical Depressions (Lows)
 Tropical Storms (Stronger Lows)
 Hurricanes (Very Strong Lows)
 Mid-Latitude Cyclones (Strong Lows with
associated warm and cold fronts)
 Nor’easters (Strong Lows that move off the East
Coast
 Tornadoes (Small, powerful Lows associated
with fronts)