Download ES Meteology ppt

The Causes of Weather
What we now know:
Difference between weather and climate.
Weather-current state of the atmosphere,
Climate-average weather over a long period of
time (30 years or more)
The Causes of Weather
We know the atmosphere redistributes
heat around the world to bring it into
balance.
– Weather is part of a constant redistribution of Earth’s
heat energy
What we know about weather:
• High pressure air masses sink/low pressure air
masses rise
• Air moves from high pressure to low pressure to
create wind
• High and low pressure air masses are created
by differences in pressure, temperature, density
• Different types of air masses affect weather in
the United States
The Causes of Weather
Different Air Masses
Form Over Different
Regions
– An air mass has
similar temperature
and
moisture content as
the area over which
it formed.
The Causes of Weather
We Know Some Air Masses Are More
Stable Than Others
The Causes of Weather
We Know That Air Masses Move To
Transfer Heat, Including Latent Heat
– An air mass exchanges heat or moisture with the surface
over which it travels until its characteristics are about the
same as the new surface over which it is traveling.
Weather Systems
We Know a Front Separates Two Different
Air Masses That Are Colliding
•
That the two air masses are of different densities
caused by differences in temperature, pressure,
and humidity
• That the interaction between the colliding air
masses can bring dramatic changes in weather
Weather Systems
We Know About Cold Fronts:
• Cold, dense air displaces warm air and forces the
warm air up along a steep front.
• Clouds, showers, and
sometimes thunderstorms
are associated with
cold fronts.
• A cold front is represented
on a weather map as a solid
blue line with blue triangles
that point in the direction of
the front’s motion.
Weather Systems
We Know About Warm Fronts:
• Advancing warm air displaces cold air.
– The warm air develops a
gradual frontal slope rather
than a steep boundary.
– A warm front is
characterized by extensive
cloudiness and light
precipitation
– On a weather chart, a warm
front appears as a solid red
line with regularly spaced, solid red semicircles
pointing in the direction of the front’s motion.
Weather Systems
About Stationary Fronts:
– A stationary front is the result of two air masses meeting
and neither advancing into the other’s territory, stalling
the boundary between them.
– Stationary fronts seldom
have extensive cloud and
heavy precipitation patterns.
– A stationary front is
represented on a weather
map by a combination of
short segments of cold- and
warm-front symbols.
We Know the Earth Rotates
• East to West (counterclockwise)
• How does the rotation of Earth affect the
movement of air?
Weather Systems
Earth’s Rotation Creates the Coriolis
Effect
Coriolis effect- causes moving
particles such as air to be
deflected to the right in the
northern hemisphere and to the
left in the southern hemisphere.
-combines with the heat
imbalance found on Earth to
create distinct global wind
systems that transport colder
air to warmer areas and
warmer air to colder areas.
Weather Systems
Global Wind Systems
Weather Systems
Global Wind Systems
• Three basic wind systems, in each hemisphere.
The trade winds- flows at 30° north and south
latitude, where air sinks, warms, and returns to
the equator in a westerly direction (convection
current)
• Around 30° latitude the sinking air associated
with the trade winds creates a belt of high
pressure that in turn causes generally weak
surface winds.
Weather Systems
Global Wind Systems
• Near the equator, an area of low pressure is
created over a large area called the doldrums.
• This area is characterized by a band of
cloudiness and occasional showers.
Weather Systems
Global Wind Systems
Other Wind Zones
– Prevailing westerlies, flows between 30° and 60°
north and south latitude in a circulation pattern
opposite that of the trade winds.
– The prevailing westerlies are responsible for much
of the movement of weather across the United
States
and Canada.
– The polar easterlies, lies between 60° latitude and
the poles. In both hemispheres, the polar easterlies are
characterized by cold air.
Weather Systems
Jet Streams: high-altitude, westerly
winds
– The polar jet stream
separates the polar
easterlies from the
prevailing westerlies.
– The subtropical jet stream
is located where the trade
winds meet the prevailing
westerlies.
Weather Systems
Jet Streams
Large-Scale Weather Systems
– The position of the jet stream varies, and it can split into
different branches and later reform into a single stream.
– The jet stream represents the strongest core of
westerly winds.
– Weather systems generally follow the path of the
jet stream.
– The jet stream affects the intensity of weather systems
by moving air of different temperatures from one region
to another.
Weather Systems
Pressure Systems
• At Earth’s surface, rising air is associated with
low pressure and sinking air is associated with
high pressure.
• Rising or sinking air, combined with the
Coriolis effect, results in the formation of
rotating low- and high-pressure systems in the
atmosphere.
• Air in these systems moves in a general
circular motion around either a high- or lowpressure center.
Weather Systems
Pressure Systems
High-Pressure Systems
– In a high-pressure system, air sinks, so that when it
reaches Earth’s surface it spreads away
from the center.
– The Coriolis effect causes the
overall circulation around a
high-pressure center to move
in a clockwise direction in
the northern hemisphere.
– High-pressure systems
rotate in a counterclockwise
direction in the southern
hemisphere.
Weather Systems
Low-Pressure Systems
– In a low-pressure systems, air rises, causing an inward
net flow toward the center and then upward.
– In contrast to air in a highpressure system, air in a lowpressure system in the northern
hemisphere moves in a
counterclockwise direction.
– This movement is reversed
in the southern hemisphere.
Weather Systems
Low-Pressure Systems: The centers of all winter
storms are areas of low pressure systems.
•
•
•
•
A COLD, dry Canadian air mass moves south and interacts with a
warm, moist air mass moving north from the Gulf of Mexico to form a
front. Winter storms usually form along a stationary front.
As the atmosphere tries to even out the pressure difference between
the cold and warm air masses, an area of lower pressure develops
along the front .
This creates wind (air blowing from high pressure towards low
pressure) to try and move enough air to even out the pressure
difference.
As the air moves toward the low-pressure area, it has nowhere to go
but up into the colder regions of the atmosphere (LIFT). Water vapor
in the air condenses (MOISTURE) and the condensed water falls as
snow to the north of the storm, where temperatures are colder,