Download Fronts/Air Masses

Pressure, Fronts, air masses
WEATHER
Air Pressure
 The weight of air pressing down on a given
area of Earth’s surface
 Measured with barometer
 Changes in air pressure
 Falling: stormy weather is coming
 Rising: fair weather is coming
 Steady: existing weather will continue
LEEWARD SIDE
(other side of the mountain)
Cool dry air slides down
the other side of the mountain
creating a warm, dry climate.
WIND
WINDWARD SIDE
• Warm air cools as it goes
over the mountain.
• Moisture is released
- precipitation
LOW ATMOSPHERIC PRESSURE (less)
Air rising has released the moisture,
And the air becomes warm - less
dense. The other side of the
mountain will have a great afternoon.
Moisture is pulled down by gravity
Warm Air Contracts
Cools
Moisture condenses
Warm Air Rises
Warm Moist Air
HIGH ATMOSPHERIC PRESSURE (More)
PRESSURE SYSTEMS
HIGH - Clear/fair weather - densely packed molecules warming the air.
Warming air decreases relative humidity and water vapor evaporates,
before clouds form. Winds turn clockwise and away from center of system
LOW - As warm, less dense air rises, it creates a low pressure system.
It forms along fronts where warm air meets cold air, causing a weather
change. Stormy weather is associated with low pressure areas.
• Different air masses don’t mix together, when they meet and form a
front.
• Due to the Coriolis effect, winds begin to swirl counterclockwise in the
Northern hemisphere.
• Rising air forms a low pressure system, cools, reaches Dew point
creating clouds, storms and precipitation.
WARM
Low
pressure
COLD
Wind Currents
 Sun drives convection currents in air
 Wind is moving air, caused by differences in air
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pressure
Wind moves from high to low pressure
The greater the pressure difference the greater the
wind
Local winds=local weather (sea breezes and land
breezes)
Global winds= circle Earth in wide belts (jet stream)
Wind vane: determines wind direction; points to the
direction from which the wind is blowing
Anemometer: measures wind speed; Beaufort scale
(0-12) relates common observations to wind speed
Local Winds
 Sea breeze- convection current blows wind
from the cooler sea toward the warmer land
during the day
 Land Breeze- at night, air moves toward the
water as the land cools more rapidly than the
water
Ocean Currents
 Wind blowing over the ocean’s surface
pushing the water, producing a current of
seawater
 2 types (due to differences in density of
seawater)
 Cold: move from polar regions to equator
 Warm: move from tropical equatorial latitudes to
polar regions
Humidity and Dew point
 Humidity
 is the moisture in the air
 Relative humidity is a comparison of the amount
of water vapor in the air to the greatest amount of
water vapor that the air could hold at a given
temperature
 Psychrometer measure relative humidity
 Dew point
 Temperature when more water vapor is
condensing than evaporating-when dew forms
Air Masses

Large body of air that has the same properties as the
surface over which it develops.
 Maritime Tropical-warm, moist air-forms over
ocean
 Maritime Polar-cool, moist air-forms over cool
ocean waters (north of U.S.)
 Continental Tropical- hot, dry air-forms over
Mexico, moves north in summer
 Continental Polar- cold, dry air-moves into U.S.
from Canada in winter
FRONTS - a boundary between 2 different air masses.
Clouds, precipitation, and storms occur at frontal boundaries.
Rain occurs at all fronts because warm air is cooled, air becomes
saturated and water is precipitated.
COLD FRONT
 A cold air mass invades a warm air mass. Cold air forces the
warm air rapidly aloft along the steep front.
 Brings violent storms followed by fair cooler weather
Cumulus
COLD AIR
Cumulonimbus
WARM AIR
The front continues moving in the direction of the cold front.
Cumulus
WARM FRONT
 A warm front develops when a warm air mass meets a cold
mass.
 Warm air (less dense) advances over the colder air.
 Brings rain and showers followed by warmer, more humid
weather
CIRRUS
ALTOSTRATUS
WARM AIR
NIMBOSTRATUS
COLD AIR
HOT OVER COLD
The front continues to move along in the same direction.
A STATIONARY FRONT develops when warm or cold front quits moving
forward. Often brings many days of almost continuous precipitation.
The front does not move - it stays stationary.
Weather stays the same.
COLD or
WARM AIR
An OCClUDED FRONT develops when 2 cold air masses merge, forcing the
warmer air between to rise. Usually produces heavy rain or other precipitation.
CUMULONIMBUS
COLD AIR
WARM AIR
STRATOCUMULUS
COLD AIR
Climate vs. Weather
Climate
 General weather
Weather
 The atmospheric conditions
 Latitude
in a specific area (within 2
weeks)
 Meteorology-study of
weather; Meteorologiststudies weather (forecaster)
 Factors in weather
 Elevation
 Convection-uneven heating
conditions in broad area
over long periods of time,
includes seasonal weather
changes
 Factors in climate
 Topography
 Distance from water
 Global winds
 Ocean currents
and cooling of the Earth’s
surface
 Air masses-certain temp and
amount of moisture
WEATHER FORECASTING
Meteorologists - study and predict the weather.
Station models - show weather conditions at a specific location.
Isotherms - are lines on a weather map connecting points of equal
temperature.
Isobars - are lines on a weather map that connect points of equal
atmospheric pressure.
Indicates locations of high H and L low pressure systems.
Weather fronts move from west to east because of the prevailing
westerlies.
Weather Map symbols
STATION MODEL
WIND DIRECTION - TEMPERATURE
CLOUD COVER - BAROMETRIC PRESSURE
SEVERE WEATHER SAFETY
WATCH - means conditions are favorable for severe weather to develop.
WARNING - means that severe weather conditions already exist.
Hurricane
•huge, slowly spinning tropical storm that forms over water
•Winds of at least 74 MPH or more (large scale cyclone that
causes great destruction to coastal areas)
•Winds circulate counter-clockwise
•Need warm water temp and moist conditions
•Builds energy as it moves across the ocean, sucking up
warm, moist tropical air from the surface and dispensing
cooler air