1. Weather
Chapter 21 1

2. Air Masses
Differences in air pressure are caused by unequal heating of Earth’s surface.
Heated equatorial air rises & creates a low-pressure belt.
Cold air near the poles sinks & creates a high-pressure centers.
Differences in air pressure creates the wind patterns

3. How does air move? Moves from areas of high pressure to low pressure.
Worldwide movement of surface air from poles toward equator.
Temperature & pressure differences give us the wind belts.

4. Formation of Air Masses
Air mass = large body of air throughout which temperature & moisture content are similar.
Air masses over frozen polar regions are cold & dry; air masses formed over tropical regions are warm & moist.

5. Types of Air Masses 4 TYPES: MARITIME (WET) CONTINENTAL (DRY)
POLAR (COLD)
TROPICAL (WARM)
Can be different combinations, example maritime polar - mP

6. North American Air Masses

7. Fronts
When 2 unlike air masses meet, density differences usually keep the air masses separate.
Cool air mass is dense; doesn’t mix with less-dense air of a warm air mass.
A boundary --- known as a front --- forms between the air masses.

8. Types of Fronts
Cold fronts = the front edge of a moving mass of cold air that pushes beneath a warmer air mass

9. Warm Front
Front edge of advancing warm air mass that replaces colder air with warmer air

10. Stationary & Occluded Fronts
Stationary = a front of air masses that moves either very slowly or not at all.
Occluded = forms when a cold air mass overtakes a warm air mass & lifts the warm air mass off the ground & over another air mass.

11. Symbols Used for Fronts

12. Weather Instruments
Weather observations are based on a variety of measurements including: atmospheric pressure, humidity, temperature, wind speed, & precipitation.

13. Instruments Used to Measure Lower-Atmospheric conditions
Barometers = measures atmospheric pressure
Thermometer = measures & indicates temperature
Wind vane = used to determine direction of wind
Anemometer = used to
measure wind speed

14. Instruments Used to Measure Upper-Atmospheric Conditions
Radiosonde = package of instruments carried by a balloons to measure temp, dew point, & wind velocity
Radar = a system that uses reflected radio waves to determine velocity & location of objects
Weather satellites
Computers

15. Forecasting Weather
Data that is collected by weather stations are transferred onto weather maps.
Station model = a pattern of meteorological symbols that represents the weather at a particular observing station & that is recorded on a weather map.

16. Weather Symbols

17. Plotting Temperature, Pressure, Fronts & Precipitation
Lines that connect points of equal atmospheric pressure are called isobars.
Closely spaced isobars indicate a rapid change in pressure & high winds
Isobars that form closed circles indicate centers of high or low air pressure. (Marked with an H or L).

18. Isobars