GEO 125

Physical Geography

Air Masses and Fronts

Characteristics of Air Masses

• Macroscale or subcontinental in size (approximately 1,000 miles in diameter 
• Extend above the earth's surface for several miles
• Uniform properties in the horizontal dimensionTenmperature
• Humidity
• Stability
• Move as distinct units from their source areas

Classification of Air Masses

Location of source area over land or water indicated by the first letter written in lower case

• c (continental = land)
• m (maritime = water)
• if no designation is given then it is assumed that the source area of the air mass is over water

Latitudinal location of the source area is indicated by the second leeter that is written in upper case

• A (arctic found at 75 degrees north latititude or higher)
• P (polar found between 60 and 75 degrees north latitude)
• T (tropical found between approximately 15 to 30 degrees north latitude)
• E (equatorial found between approximately 5 to 15 degrees north latitude)

Air Masses that Affect the Weather of the Continental United States

• Arctic (A)
• Continental polar (cP)
• Maritime polar (mP)
• Continental tropical (cT)
• Maritime tropical (mT)
• Equatorial (E)

The Relationship Between Air Masses and Fronts

• Weather systems in the midlatitudes by and large are caused by the collision of air masses between 30 and 60 degrees north latitudeNote that none of the air masses affecting the weather of the continental United States has a source area between 30 and 60 degrees north latitude 
• Fronts mark the boundaries between air masses and are defined by the type of air on either side of them

Types of Fronts

Warm fronts

• Indicated by half circles facing the direction of movemnet
• Normally cool dry air on the eastern side of the front and warm moist air on the western side of the front
• Gradient of the front is very gentle
• Associated with stratus clouds
• Produces prolonged soaking rains (36 to 48 hours normally)
• Temperature will increase with the passage of a warm front
• Barometric pressure will drop with the passage of a warm front

Cold fronts

• Indicated by triangles facing in the direction of movement
• Normally warm moist air is on the eastern side of the front and cool dry air on the western side of the front
• Gradient of the cold front is very steep
• A cold front is often associated with the occurrence of cumulonimbus clouds
• A strong cold front will produce thunderstorms and perhaps tornadoes
• Temperature will drop with the passage of a cold front
• Barometric pressure will rise with the passage of a cold front

Occluded fronts

• Occur when cold fronts catch up and merge with warm fronts because cold fronts do move faster than warm fronts
• Indicated by alternating half circles and triangles facing in the direction of movement
• Mixed precipitation associated with both warm fronts and cold fronts

Stationary Fronts

• Occur because warm and cold air masses do not have the strength to displace each other
• Indicated by alternating circles and triangles placed on either side of the front indicating the relative direction of incipient movement
• Oten stationary fronts are associated with stalled weather systems

Conflicting Air Masses and Associated Fronts that are Typical for Wisconsin

• Continental polar air moving southwardMaritime tropical air moving northward
• Entire system moving from the west to the east by the prevailing westerlies
• Warm front on the eastern side of the system

• Cold front on the western side of the system

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