1. Weather
California Standards: 5a & 6a
San Francisco fog

2. Air Masses
A large body of air which has the same properties as the surface over which is developed is called an air mass.
Changes in weather from one day to the next are due to these air masses.
Tropical air masses contain more moisture than those formed over a desert region.

3. Air Masses are named for the regions from which they are formed.

4. Air Pressure Variations in atmospheric pressure affect the weather.
Air that is sinking is called high pressure.
Air that is rising is called low pressure.
High pressure systems make it hard for clouds to form thus they indicate good weather.

5. Fronts The boundary between two air masses is called a front.
When a high pressure air mass, moves under a low pressure air mass the low pressure air mass is pushed up into the atmosphere.
As this air moves upward it cools and forms clouds and rain, resulting in storm clouds.

6. Fronts Warm front: warm air moves over cold air
Cold front: cold air moves under warm front
Occluded front: two cold fronts force warm air up.
Stationary front: warm and cold fronts stop moving.

7. Warm Front:
warm air over cold air

8. Cold Front:
cold air under warm air

9. Occluded Front:
cold air lifts warm air off ground

10. Severe Weather
Thunderstorms occur when warm, moist air moves rapidly upward.
Lightening occurs when electrically charged particles collide releasing electrical current.
Thunder is caused by the rapid heating of the air which causes it to expand and contract rapidly causing the molecules to move back and forth rapidly releasing the thunder.

13. Tornadoes A tornado is a violent, whirling wind caused by wind shear.
This wind shear causes the rotation of air masses.

15. Hurricanes Hurricanes are the most powerful and destructive.
Most hurricanes occur in the tropics between 5 and 20 degree N & S latitude.
Hurricanes form over tropical water where they pick up warm air with a lot of moisture.

16. Hurricane Katrina, 2005

17. Weather Instruments: 1. Thermometer: measures temperatuer.
2. Barometer: used to measure atmospheric pressure.
3. Anemometer: used to measure wind speed.
4. Wind vane: used to measure wind direction.

18. Weather Instruments: Wind speed Wind direction Relative Humidity
Atmospheric Pressure

19. Forecasting the Weather
Station Model is a set of symbols which represent the weather at a particular weather station.
Information as to wind direction, weather conditions, cloud coverage, etc.

20. Station Model Symbols

21. Weather Station Model A – cloud coverage B – temperature C – dew point
D – atmos pressure
E – wind direction
F – wind speed
present weather

22. The End