Download Air Masses

What
Influences
Weather?
What is weather?
• Weather is the short-term
state of the atmosphere,
including temperature,
humidity, air pressure, wind
and visibility.
How does the water cycle affect weather?
• Liquid water from the oceans evaporates into water vapor
which enters the atmosphere affecting humidity levels.
• Clouds and fog form as a result of condensation of water
vapor. Fog limits visibility.
• Precipitation occurs when rain, snow, sleet or hail falls
from the clouds to the Earth’s surface.
How do air masses affect weather?
• An air mass forms when the air over a
large region of the Earth’s surface stays in
place for many days.
• The air gradually takes on the
temperature and humidity of the land or
water below it.
• The weather changes when a new air
mass moves into your area as the one that
was there moves out.
Air Masses
•
A huge body of air with similar characteristics:
 Humidity
 Temperature
 Air Pressure
•
Classified according to two characteristics:
 Temperature: Where did the air mass form?
 Humidity: Did the air mass form over water or
land?
Maritime Air Masses
•
Maritime air masses form over oceans.
•
Water evaporates from the ocean
creating a humid air mass.
Tropical Air Masses
•
Warm air masses form in the tropics as a
result of direct rays from the sun. These
air masses have low air pressure.
Polar Air Masses
•
•
Cold air masses form north of 50°N
latitude and south of 50°S latitude.
Polar air masses have high air pressure.
Continental Air Masses
•
•
Form over land, in the middle of
continents.
Continental air masses have very dry air.
Air Masses
Continental
Polar
Maritime
Polar
Maritime
Polar
Maritime
Tropical
Continental
Tropical
Maritime
Tropical
Maritime Tropical
• mT- originates over a warm tropical
ocean.
• Warm and Moist (humid air)
• Causes high heat and humidity in the Midwest and Eastern US.
• Thunderstorms form due to the moist air
and heat available, or heavy rain or snow
in the winter.
Air Masses
Maritime Polar
• mP- Originates over the ocean in a
polar region.
• Cold, Damp (humid) air.
• Not as cold as cP
• If cooled to the dew point,
precipitation occurs, bring fog, rain
and cool temps.
Air Masses
Continental Tropical
• cT- Originates over a desert region.
• The air mass is hot and dry air.
• Produces tremendous heat waves.
• Produces drought like conditions.
Air Masses
Continental Polar
• cP- Originates over Alaska and
Canada. (Those that affect North America)
• The air mass is cold and dry air.
• Can create precipitation by picking up
moisture over the Great Lakes.
Air Masses
How do air masses move?
•
•
•
•
Global winds bring air masses of different
temperatures and moisture content to a region.
The Prevailing Westerlies (a major global wind
belt) generally push air masses from west to
east across the United States.
Jet streams are long-distance winds that travel
above global winds for thousands of kilometers.
Jet streams can affect temperatures in the
United States by pulling cold air masses down
from Canada or by pulling warm air masses up
from the tropics.
Where do weather fronts form?
• When two air masses meet, density
differences caused by temperature
differences prevent the air masses from mixing.
• A boundary, called a front, forms between the
air masses.
• Fronts result in a change in the weather, usually
bringing unsettled weather conditions as they
pass.
• Unsettled weather conditions mean changeable
or stormy.
Fronts
• There are four
different types of
weather fronts.
• They are determined
by the characteristics
of the air masses and
how they are moving.
Warm Fronts
•
•
•
A moving warm air mass collides with a
slower moving cold air mass.
The warm air moves over top of the cold
air. (warm air is less dense)
Types of weather produced:
–
–
–
–
Clouds, storms and rain.
If the rising air is humid, light rain forms.
If the rising air is dry, scattered clouds form.
After the front passes, it is likely to be
warmer and more humid.
A Warm Front
Cold Fronts
•
•
•
•
•
Cold air is very dense and tends to sink.
Warm (less dense) air is pushed up.
As it rises it cools and forms clouds which
may bring heavy rain or snow.
These fronts move very quickly, and can
cause abrupt weather changes including
violent thunderstorms.
After the front moves through, cool, dry air
creates clear skies and cooler temps.
A Cold Front
Stationary Fronts
•
•
•
Stationary fronts occur when cold
and warm air masses meet, but
neither can force the other to move.
Water vapor in the warm air
condenses into rain, snow, fog or
clouds.
If the front stalls over an area, it can
bring these conditions for several
days.
Stationary Fronts
Occluded Fronts
•
•
An occluded front is a complex situation
that occurs when a cold front catches up
to a warm front.
Occluded fronts bring rainy or snowy
weather.
Occluded Fronts
What are pressure systems and how do they interact?
• Areas of different air pressure cause changes in
weather.
• In a high pressure system, the air sinks and
warms bringing clear skies and calm air or
gentle breezes.
• In a low pressure system, the air rises and cools
bringing overcast skies and stormy weather.
How do ocean currents affect weather?
• The same global winds that blow across the
surface of the Earth also push water across the
oceans, causing ocean currents.
• Ocean currents carry warm water or cold water.
• The air above these currents becomes warm or
cold depending on the temperature of the
current.
• The air over ocean currents can change local
temperatures, humidity levels, and affect local
weather and climate.
The North Atlantic Current
• In areas where
warm ocean
currents flow,
coastal cities have
warmer winter
temperatures than
inland cities at
similar latitudes.
The California Current
• The California
Current is a coldwater current that
keeps the average
summer high
temperature cooler
than inland cities at
similar latitudes.
It’s time for a formative
Assessment.
Grab a white board, marker and eraser.
It is 98°F and humid outside. A front
is approaching and the sky begins to
darken. Heavy rain and hail fall
during a severe thunder and lightning
storm. After the passing of the front,
the temperature was 87°F and the air
was less humid.
Which type of weather front
passed through the area?
A. Warm Front
B. Cold Front
C. Stationary Front
D. Occluded Front
An air mass moves into your area
bringing hot, dry conditions that
last for several days. You learn
from the news that you are not
allowed to wash your car or water
your lawn.
Which type of air mass moved
into your area?
A. Maritime Tropical
B. Maritime Polar
C. Continental Tropical
D. Continental Polar
An air mass forming over the Gulf of
Mexico moves northwest toward the
central part of the United States. It
collides with a continental polar air
mass moving down from Canada
resulting in the formation of a tornado.
Which type of air mass moved
northwest from the Gulf of Mexico?
A. Maritime Tropical
B. Maritime Polar
C. Continental Tropical
D. Continental Polar
Two air masses collide over
your town. They push
against each other without
moving. Rain falls for
several days.
Which type of weather front
formed over your area?
A. Warm Front
B. Cold Front
C. Occluded Front
D. Stationary Front
A maritime polar air mass is
pushed from over the northern
Pacific Ocean into the west
coast of the United States.
Which global wind system
is responsible for the
movement of this air mass?
A. The Polar Easterlies
B. The Prevailing Westerlies
C. The Trade Winds
According to the weather map, there are two pressure
systems affecting the weather over the United States.
The western and central U.S. are experiencing clear
skies, and gentle breezes. The northeastern U.S. is
experiencing rain, ice and snowfall.
Which type of pressure system is affecting the
weather over the northeastern part of the United
States?
?
?
A. A Sprinkler Pressure System
B. A High Pressure System
C. A Compact Pressure System
D. A Low Pressure System