5.2 – Air Masses & WeatherDigging Deeper

Air Masses:

• Huge volume of air in the lower atmosphere with similar temperature and moisture characteristic throughout.

How & Why Air Rises:

1. Convective Uplift

2. Orographic Uplift

3. Frontal Wedging

4. Convergence

Upward and downward motions of air caused by differences in temperature CONVECTION

Narrow zone of transition between different air masses where colder, denser air forces warmer, less dense air upwardEx. warm and cold fronts

When wind blowing from different directions converge and go upwardEx. ITCZ

Mountain RangesWinds carrying warm, moist air is forced to rise. As it rises it cools, causing water vapor to condense forming clouds and precipitation.

The winds now are drier. As the air fall/ sinks it compresses and warms.

In general, the windward side of a mountain will receive more precipitation and have greener, more lush vegetation than the drier leeward side of the

mountain. The same is true for islands.

Where Does Precipitation Occur?

• Precipitation forms from the rising and cooling of moist air.

• The warmer the air before it rises, the more moisture it can hold. In addition, the higher air rises the more precipitation it can release.

As air rises it cools, when it cools to it’s dew point, water vapor condenses to form clouds

Occurs everywhere on Earth BUT not equally

• Therefore, areas that receive the most precipitation are those where warm, moist air rises in large quantities.

1. The Equator –

2. Mid-Latitudes –

3. Windward side of mountain ranges – (Orographic Uplift)

Heat from the Sun produces high temps causing air to rise producing clouds that produce daily rain/ thunderstorms

EX. ITCZ/Doldrums

In storm areas where cold and warm air meet forming low pressure systemEX. Mid-latitude Cyclone (Low)

Notes: Fronts & Lows

Describe the following types of fronts:1. Cold Front

2. Warm Front

3. Stationary Front

4. Occluded Front

Front: A boundary where two air masses meet

• Most common in the mid-latitudes where polar air meets tropical air (polar front).

• Since the air masses have different densities (temperatures), the warmer air will be forced upward; as it cools to its dew point clouds form with the possibility of precipitation.

Cold Front: boundary between an advancing cold air mass and the warmer air it is displacing; causing the warmer, moister air to rise rapidly

• When the temperature difference is large between the 2-air masses, strong thunderstorms can occur with heavy rain, hail, strong winds, and possibly tornadoes.

Warm Front: boundary that occurs when warm advances on cooler air. The less dense warmer air, gradually rises above the cooler air.

• Forms a wide area of clouds prior to the front arriving, with cirrus, cirrostratus, altostratus – then nimbostratus clouds and rain.

Stationary Front: is a boundary when neither air mass is moving forward, but sliding past each other on opposite directions

• Warmer air rises over denser colder air, and clouds and precipitation may occur. If a stationary front stays in one place for too long, flooding may occur.

Occluded Front: occurs when a faster moving cold front over-takes a slower moving warm front; trapping the warmer air above both colder masses of air.

• Often causes cloudiness and precipitation

Mid-Latitude Cyclones• Large traveling low pressure systems (cyclones)

Mid-Latitude Cyclones• Form along the polar

front where colder air from the north meets warmer air from the south

• Occur between 30°-60°N/S