1. Weather
The present state in the atmosphere at a given location for a short period of time.

2. Atmosphere Consists mostly of nitrogen and oxygen
Can be divided into layers by temperature
Almost all weather takes place in the lowest layer, the troposphere

3. The Sun Primary source of energy for the Earth
Weather changes occur due to uneven heating of Earth’s surface by the sun

4. Air masses
Large bodies of air with similar temperature and humidity conditions
Weather changes are due to the movement of air masses

5. Layers of the atmosphere
Troposphere (where weather occurs,
lowest)
Stratosphere
Mesosphere
Thermosphere (highest)

6. Composition of Air Nitrogen 78% Oxygen 21% Carbon Dioxide 0.03%
Other gases 0.17%
Water vapor (varies) 1 to 3%

7. Weather instruments Thermometer – measures air temperature
Barometer – measures air pressure
Hygrometer – measures humidity (water
vapor in the air)
Anemometer – measures wind speed
Wind vane – measures wind direction
Rain gauge – measures amount of rainfall
Cloudiness – amount of sky covered by clouds

8. WEATHER <font face=Arial,Helvetica size=-1> A weather symbol is plotted if at the time of observation, there is either precipitation occurring or a condition causing reduced visibility. Below is a list of the most common weather symbols:
Station Models
What are the characteristics of the station model above?
Temperature
Dew point
% Cloud Cover
Air Pressure
Barometric Trend
Wind Direction
Wind Speed

9. Earth’s surface
Not even or flat so some surfaces (ex. Pavement) heat up faster than others (ex. Water)
Heats up depending on the angle the sun’s rays strike the surface
Near the equator the rays are nearly vertical
Near the poles the rays are more angled
High
Angle
Low Angle
Low
Intensity
High
Intensity

10. Air masses Form over warm water are warm and moist
Form over land near the poles are cool and dry
Warm and moist – maritime tropical
Warm and dry – continental tropical
Cool and moist – maritime polar
Cool and dry – continental polar

11. High Pressure Systems
Surface air pressures are usually highest in the centers of the air mass
Air tends to sink and blow outward in a clockwise direction (in Northern Hemisphere)
Brings clear skies, dry weather and gentle winds
H indicates high pressure on a weather map
Are large, move slowly, weather changes gradual

12. Low Pressure Systems
Surface air pressures are lower toward the edges of air masses
Air tends to rise, wind spirals in toward the center in a counterclockwise directions (in the Northern Hemisphere)
Usually bring cloudy, wet weather, strong winds
L indicates low pressure on a weather map

13. Fronts When 2 air masses meet changes in weather occur
Cooler, drier air masses tend to sink and warmer, moister air masses rise
This causes clouds, strong winds and precipitation

14. Cold Front When cold air pushes into and under a warm air mass
Brief, heavy downpours, gusty winds, cooler temperatures
Can bring thunderstorms and decreases in temperature and humidity

15. Warm Front When warm air pushes into and over a cold air mass
Brings light precipitation that lasts a day or two
Brings warmer temperatures

16. Clouds Are formed based on how air rises
Cumulus – produced if air is pushed straight up
Stratus – produced if air rises at a low angle
Cirrus – produced if formed high in the atmosphere, made up of ice crystals, usually indicate a warm front is approaching

17. Storms
Involve low air pressure, clouds, precipitation and strong winds
Thunderstorms
Hurricanes
Tornadoes
Winter/ice storms

18. Thunderstorms Brief, intense storms that affect a small area
Produced when rapidly rising warm air causes cumulus clouds to build upward into a thunderhead
Characterized by thunder and lightning, strong gusty winds, and sometimes hail
Lightning is a huge electrical discharge
Avoid hilltops, open fields, beaches and bodies of water

19. Hurricanes
Huge rotating storms that form over the ocean near the equator
Take about a day to pass
Produce strong winds, heavy rains, and large powerful waves
Calm region in center is called the eye
Can cause severe flooding and damage from high winds
Coastal regions are flood-prone and should seek higher ground

20. Tornadoes
Violently whirling winds sometimes visible as a funnel shaped cloud
Produced by severe thunderstorms
Appear suddenly, carve a narrow path of destruction and disappear suddenly
Spiraling high speed winds and extremely low pressure

21. Winter storms Include blizzards and ice storms
Blizzards are fierce storms with strong winds, blowing snow, and very cold temperatures
Ice storms occur when falling rain freezes at Earth’s surface, coating everything with ice
Remain indoors and do not attempt to travel