1. Air Masses and Fronts
Earth Science
Chapter 24

2. Origin of an Air Mass
Air mass: a huge section of lower troposphere having same kind of weather (temp & air pressure) throughout.
Air masses can be several thousand kilometers in diameter & several kms deep.
Air masses originate where winds are light, typically polar & subtropical high-pressure belts.

3. Kinds, Sources, & Paths of Air Masses
Temperature of air masses depends upon where it comes from (polar or tropical).
Humidity of air mass depends on whether it comes from land or sea.
Are named for source regions (or places of origin).

4. Naming Air Masses
Common names for air masses and their characteristics:
Maritime Tropical (mT): air mass from tropical seas, typically warm and humid.
Continental Tropical (cT): air mass from tropical land area, typically hot and dry.
Maritime polar (mP): air mass comes from cold ocean waters, typically cold and humid.
Continental polar (cP): mass comes from land areas in high latitudes, typically cold and dry.
Continental Artic (cA): air masses come from ice-covered Artic regions, and are very cold and dry

6. Weather in an Air Mass
So large they take days to pass over a location.
Weather inside is typical of the area over which air mass originated.
Changes in the air mass depend upon how much time that has passed since it left the source.

7. Skies in an Air Mass
Meteorologists have found that the conditions in an air mass depend upon the difference in temperature between the air mass and the ground.
If the ground is warmer:
Bottom layer is unstable and convection form cumulus clouds, visibility is good
If the air mass is dry, weather should be dry
If the air mass is humid, showers may occur
If the ground is colder:
Bottom layer is stable
Inversions form & visibility is poor
Condensation & precipitation will likely occur.

8. Observing an Air Mass
Meteorologists use a balloon-carried package of instruments (rawinsonde) to observe air masses.
The rawinsonde contains a radio transmitter that sends signals about temperature, air pressure, and relative humidity.
The balloon can carry the rawinsonde up to heights of more than 30 km.
Tracking of the rawinsonde also gives info on wind speed and direction.

9. What is a Front? A front is the boundary between two air masses.
The front is where the temperature and humidity of the two air masses meet & change.
An approaching front means a change in weather is coming.
The greater the difference between the air masses the greater the change in the weather.

10. Front Symbols

11. Characteristics of Fronts
Observations have shown that fronts have certain characteristics
Fronts tend to have gentle slopes.
Frontal surfaces may reach heights of 5 km, affect weather in an area several hundred kilometers wide.
Fronts almost always bring precipitation because warm air is rising and experiences cooling / condensation.

12. Review Symbols

13. Kinds of Fronts There are four kinds of fronts: Cold front:
cold air displaces warm air
Typically has a steeper slope and moves slower due to friction with the ground surface
Warm front:
Warm air is pushing ahead and displacing colder air
Has a gentle slope and less friction
Stationary front:
Neither air mass is being displaced, and the front is not moving
Similar weather to a warm front
Occluded front:
Cold front overtakes a warm front
Warm air is lifted off the ground by the cold front

14. How Mid-Latitude Lows Form
A low starts as a kink or wave in the polar front that moves from east to west.
A cyclone or low forms when the warm air bulges over into the cold region of low pressure.
This causes an occluded front which can last for three days or more.

15. Weather Associated with Lows, Fronts, & Highs
The weather changes that occur when a low passes north of a location:
Long period of steady precipitation in advance of warm front.
Warming and slow clearing after warm front passes.
Showery precipitation as the cold front passes
Cooling and rapid clearing associated with new cold air mass.

16. Front Symbol Quiz!!! 1. 2. 3. 4.

17. Warm-Front Weather
Warm front affects weather long before rains arrive.
Clouds begin arriving: cirrus, cirrostratus then stratiform clouds.
Next, nimbostratus clouds arrive followed by precipitation.
Warming occurs once front has passed.
These conditions also typical of stationary fronts.

18. Cold-Front Weather Cold fronts: steeper & faster moving.
Air ahead of a cold front is unstable, therefore cumulonimbus clouds form & thunderstorms & precipitation are likely.
Once front passes, temperatures drop rapidly & wind speed increases.
Rain usually ends shortly after the front passes.

19. Weather in a High Pressure area
Between lows & their fronts lie anticyclones: areas of high pressure.
A high represents a single air mass with winds blowing outward.
Bright clear weather usually present with small cumulus clouds & heavy dews and frosts at night.
Inversions may form in mornings, trapping pollution.

20. Weather Report