AIR MASSES, FRONTS, AND STORMS
Weather forecasting requires an understanding of:
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Cyclones and anticyclones
Air masses and fronts
AIR MASSES
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An air mass is a large body of air
 Subcontinental in size
 Several miles high
With uniform temperature and moisture conditions
SOURCE REGIONS - WHERE AIR MASSES ACQUIRE THEIR PROPERTIES
Air Masses are Classified According to:
Surface of source region
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Continental (c) Forms over land - dry
Maritime (m) Forms over ocean - moist
Temperatures
 Tropical (T) -Low latitudes - warm
 Polar (P) - High latitudes –
 coldArctic (A) - Poles - very cold and
dry
FRONTS
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Fronts are boundaries that separate air masses
There are four types of front:
 Stationary front
 Cold front
 Warm front
 Occluded front
STATIONARY FRONTS
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Cold and warm air masses next to each other
Neither air mass advances
COLD FRONT
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Cold air replaces warm
Much steeper than warm fronts
Advances faster than warm front
More violent weather -cumulonimbus clouds
Short, sharp showers
WARM FRONT
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Warm air replaces cold
Gentle slope (1:200)
Covers a wide area with its weather
Stratus clouds get lower as front approaches
Drizzle or steady rain
OCCLUDED FRONTS
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Cold front overtakes the warm front
Lifts the warm air off the ground
Clouds and rain possible
STRUCTURE OF A MID-LATITUDE CYCLONE
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Low Pressure at the center of the storm
Counterclockwise winds
A warm front
A cold front
A pie-shaped wedge of warm air
Surrounded by cold air mass
CHANGES IN WEATHER AS A STORM MOVES EAST (A-E)
MID-LATITUDE STORMS EVENTUALLY DISSOLVE
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Occlusions close up the storm
Becomes a stationary front
WHAT DO HURRICANES AND TORNADOES HAVE IN COMMON?
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Low Pressure
Latent Heat of Condensation
Strong Winds
Hazardous to Life and Property
HOW DO THEY DIFFER?
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Location
Size
Duration
Season of Occurrence
Distinctive Hazards
WHEN DO THEY OCCUR?
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Hurricane season?
Late Summer and Fall
Tornado season?
Spring and Early Summer
LOCATION - WHERE DO STORMS FORM?
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Hurricanes form over warm (80 °F) tropical seas
Latitude 7-15 degrees North and South of the equator
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Tornadoes form over land in midlatitudes
SIZE AND DURATION OF HURRICANES
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Hurricanes 100-300 miles wide
Last 10-14 days
Rate of travel 5 mph
Travel with Trade Winds
HURRICANE STRUCTURE
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Intense low pressure system
Steep pressure gradient - over 74 mph winds
Strong convectional lifting
Eye with calm, clear conditions
HURRICANE DAMAGE
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High winds
75-125 mph
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Torrential rains
Flash floods
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Storm surges as sea level rises
LOCATION OF TORNADOES
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Most common in the United States
Plains States, Midwest, and Florida
In advance of a cold front
Embedded in hurricanes
SIZE AND DURATION OF TORNADOES
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Narrow intense storm
1/4 mile wide
Path of 6 miles SW to NE
A few minutes to three hours
STRUCTURE OF A TORNADO
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Cumulonimbus cloud
A narrow funnel
A dust cloud
TORNADO HAZARDS
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Wind Speed 150-450 kph
Strong updraft in the funnel
Explosions due to pressure contrast