1. Pressure, Fronts, air masses
Weather

2. 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

3. 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

4. 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)

5. 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

6. Wind Currents Sun drives convection currents in air
Wind is moving air, caused by differences in air 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

7. 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

8. 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

9. Humidity and Dew point Humidity Dew point 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

10. 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

11. 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.

12. 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
COLD AIR
WARM AIR
Cumulus
Cumulonimbus
Cumulus
The front continues moving in the direction of the cold front.

13. 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
COLD AIR
WARM AIR
HOT OVER COLD
NIMBOSTRATUS
ALTOSTRATUS
CIRRUS
The front continues to move along in the same direction.

14. 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.
COLD or
WARM AIR
Weather stays the same.
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

15. Climate vs. Weather Climate Weather
The atmospheric conditions in a specific area (within 2 weeks)
Meteorology-study of weather; Meteorologist- studies weather (forecaster)
Factors in weather
Convection-uneven heating and cooling of the Earth’s surface
Air masses-certain temp and amount of moisture
General weather conditions in broad area over long periods of time, includes seasonal weather changes
Factors in climate
Latitude
Elevation
Topography
Distance from water
Global winds
Ocean currents

16. 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.

17. Weather Map symbols

18. STATION MODEL

19. WIND DIRECTION - TEMPERATURE CLOUD COVER - BAROMETRIC PRESSURE

24. SEVERE WEATHER SAFETY
WATCH - means conditions are favorable for severe weather to develop.
WARNING - means that severe weather conditions already exist.

25. 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