Weather

Chapter 21

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

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.

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.

Types of Air Masses

• 4 TYPES: • MARITIME (WET)• CONTINENTAL (DRY)• POLAR (COLD)• TROPICAL (WARM)• Can be different

combinations, example maritime polar - mP

North American Air Masses

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.

Types of Fronts• Cold fronts = the front edge of a moving mass

of cold air that pushes beneath a warmer air mass

Warm Front• Front edge of advancing warm air

mass that replaces colder air with warmer air

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.

Symbols Used for Fronts

Weather Instruments

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

Instruments Used to Measure Lower-Atmospheric conditions

Thermometer = measures & indicates temperature

Barometers = measures atmospheric pressure

Anemometer = used to measure wind speed

Wind vane = used to determine direction of wind

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

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.

Weather Symbols

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).

Isobars