2. AIR MASSES

3. Effects of Earth Earth is a sphere  uneven heating  convection currents  world-wide wind patterns Rotation of Earth  Coriolis Effect  wind direction changes to the right (northern hemisphere) and to the left (southern hemisphere) Tilt of Earth  seasons change as move through orbit

4. Air is usually in motion (Convection and the Coriolis effect) BUT Large portions of air often remain in nearly the same place long enough to take on the temperature and moisture characteristics of the land or ocean below

5. These characteristics vary greatly Differences between the land and ocean can have a significant influence on the properties of the air above When a large portion of the air is nearly uniform in temperature and humidity, it is considered an air mass

6. Classification Air masses can be classified due to where on the planet they form - 1. The amount of moisture in the air continental or maritime (depends on location – over land or ocean) 2. The temperature of the air polar (cool) tropical (warm)

7. Air masses are not stationary Air masses are not stationary Often move according to prevailing wind patterns Often move according to prevailing wind patterns Air masses can interact as they move and even collide= weather. Air masses can interact as they move and even collide= weather.

8. Highs, Lows, and Fronts

9. Frontal Systems “An air mass is a parcel of air that spends enough time over a region to take on the temperature and humidity of the region.” “An air mass is a parcel of air that spends enough time over a region to take on the temperature and humidity of the region.” Can remain in the same place for some time Can remain in the same place for some time Eventually moves due to worldwide wind currents Eventually moves due to worldwide wind currents The zone b/w interacting air masses is called a front The zone b/w interacting air masses is called a front There are 4 general types: There are 4 general types: Warm Warm Cold Cold Stationary Stationary Occluded Occluded

10. Cold Fronts 1. 1. Cold air mass overtakes a warm air mass 2. 2. Cold air is more dense 3. 3. Moves underneath, pushing warm air upwards 4. 4. Warm air rises rapidly forming a cumulus cloud 5. 5. Vapour condenses If this occurs long enough, a cumulonimbus cloud is created Rain often comes with a cold front If the cold front is fast moving intense thunderstorms may develop sometimes bringing hail and tornadoes

11. Warm Fronts Warm air mass overtakes a cooler one Warm air mass overtakes a cooler one Warm air gently rides up over the trailing end of the cold air mass Warm air gently rides up over the trailing end of the cold air mass Stratus clouds usually form (altostratus and cirrus clouds form as warm air climbs higher) Stratus clouds usually form (altostratus and cirrus clouds form as warm air climbs higher) Nimbostratus clouds form close to the front bringing light rain or snow Nimbostratus clouds form close to the front bringing light rain or snow Precipitation may last longer than with a cold front Precipitation may last longer than with a cold front Once it passes, the air is left warmer than before the front arrived Once it passes, the air is left warmer than before the front arrived

13. STATIONARY FRONTS Two air masses (warm/cold) in contact but neither is moving Two air masses (warm/cold) in contact but neither is moving Some warm air rises and condenses creating stratus clouds and often rain Some warm air rises and condenses creating stratus clouds and often rain The front may remain this way for several days The front may remain this way for several days = dreary, gloomy sky = dreary, gloomy sky OCCLUDED FRONTS A faster moving cold front catches up to a slower moving warm front A faster moving cold front catches up to a slower moving warm front All the warm air is pushed up, away from the ground, contacting the leading cold air mass All the warm air is pushed up, away from the ground, contacting the leading cold air mass Many types of clouds are present, one after another Many types of clouds are present, one after another Often cause a period of steady precipitation Often cause a period of steady precipitation

17. Investigating Weather Fronts Investigating Weather Fronts

18. High Pressure Systems Any region where air is descending (putting more pressure on the region below) is a HIGH PRESSURE SYSTEM. Any region where air is descending (putting more pressure on the region below) is a HIGH PRESSURE SYSTEM. An air mass cooled by conduction and convection when it forms over cold ground becomes more dense and settles lower to the ground An air mass cooled by conduction and convection when it forms over cold ground becomes more dense and settles lower to the ground Creates more pressure, continues to settle, draws more air from above Creates more pressure, continues to settle, draws more air from above Air is also pushed out toward low pressure areas at the perimeter in all directions Air is also pushed out toward low pressure areas at the perimeter in all directions This air curves to the right due to the Coriolis Effect This air curves to the right due to the Coriolis Effect = ANTICYCLONE = clockwise rotation of air due to the high pressure cell = ANTICYCLONE = clockwise rotation of air due to the high pressure cell

20. High pressure systems create a clear sky (since air is descending not rising) High pressure systems create a clear sky (since air is descending not rising) Air contains little moisture due to cold in upper atmosphere = Clouds do not form Air contains little moisture due to cold in upper atmosphere = Clouds do not form High pressure systems are usually located within a single air mass High pressure systems are usually located within a single air mass May be 100’s of km across May be 100’s of km across Due to size, weather conditions are stable Due to size, weather conditions are stable Prevents most turbulence Prevents most turbulence

21. Low Pressure Systems 1. Very intense heating of the ground heats air above it (conduction) Air rises, pulling more, less dense air in beneath it Air rises, pulling more, less dense air in beneath it Air curves to the right (Coriolis Effect) so rising air rotates = CYCLONE (in the N.hemisphere rotates counterclockwise) Air curves to the right (Coriolis Effect) so rising air rotates = CYCLONE (in the N.hemisphere rotates counterclockwise) 2. Warm air rises in a frontal system creating low pressure systems also Generally smaller than high pressure systems Generally smaller than high pressure systems Likely to occur b/w air masses Likely to occur b/w air masses Bring unstable weather conditions (cause precipitation and often storms) Bring unstable weather conditions (cause precipitation and often storms)

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