Air Masses and Fronts

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Air Masses and Fronts
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Air Masses

• An extremely large body of air whose properties
  of temperature and moisture content are similar
  in any horizontal direction
• Usually cover hundreds of thousands of square
  miles
• Form when air stagnates for long periods of time
  over a uniform surface
• Over time, the air mass acquires the temperature
  and moisture properties of the underlying surface

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CLASSIFICATION

• FOUR BASIC CATEGORIES OF AIR MASSES
• POLAR - P
• ARCTIC - A
• TROPICAL - T
• EQUATORIAL - E
• TWO DESIGNATIONS OF SURFACE IN SOURCE REGION
• MARITIME - m
• . CONTINENTAL - c

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Source Regions

• The birthplaces of air masses
• Must be dominated by light winds (or none at all)
• Must have an extensive, uniform surface
• Good regions are found
• Subtropics
• Interior of large continents

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Air Mass Modification

• When an air mass moves away from its source
  region, it will begin to change its temperature
  and moisture properties to that of the new
  underlying surface
• If an air mass is heated from below, that will
  lead to instability
• If an air mass is cooled from below, stability
  will increase

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Maritime Tropical air mass
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Fronts

• Temperature and moisture characteristics do not
  often change gradually over distance
• The transition zone between two different air
  masses is called a front
• So named because clashing air masses result in
  inclement weather like 2 armies
• Frontal zone the point where the front meets
  the ground

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Types of Fronts

• Fronts are classified by the temperature changes
  that result after an air mass passes over a given
  region
• Cold front colder air will follow the fronts
  passage
• Warm front warmer air will follow the fronts
  passage
• Two other fronts exist as well stationary front
  (no movement of air) and occluded front (cold air
  forcing all the warmer air aloft)

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Warm Front, moist air is slowly raised as it
flows over the cold air
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Warm Front, moist air is forced upward by cold
front
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Occluded Fronts
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Cold Fronts

• Marked on weather maps as a blue line with blue
  triangles pointing in the direction of movement
• Temperature and humidity quickly drops after the
  cold front passes
• A decrease in pressure is also observed before
  the front, and an increase afterward
• Winds ahead of the front blow south/southwesterly
  while winds behind the front blow
  west/northwesterly
• Lifting of warm air along the cold front is
  pronounced
• Precipitation tends to be showery due to
  cumuliform clouds and limited to near the front
  itself
• Travel faster than any other kind of front

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Warm Front

• Marked on a weather map as a red line with red
  semicircles pointing in the direction of frontal
  movement
• Temperature and humidity increase after frontal
  passage
• Front is located in a region of lower pressure
  than the surroundings
• Winds ahead of the warm front are easterly, while
  winds behind the front are southerly
• Has a gentler slope than a cold front
• Warm air cant push the cool air in front of it
  out of the way
• The warm air rises over the cooler air
  Overrunning
• Clouds tend to be stratiform, and precipitation
  is more moderate and steady
• The height of the clouds decrease as the front
  approaches

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Occluded Fronts

• Marked on a weather map with a purple line
  containing alternating warm and cold front
  symbols (both in purple) on the same side of the
  line indicating the movement of the front
• Because warm fronts travel slower than cold
  fronts, cold fronts can overtake warm fronts
• When this happens, the warm air is forced aloft
  as cold air surrounds the low pressure
• A cold occlusion results if the cold air behind
  the cold front is the coldest air on the map
  (front will resemble a cold front at the surface)
• A warm occlusion results if the cold air behind
  the front is warmer than the cold air ahead of
  the warm front (front will resemble a warm front
  at the surface)

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Stationary Fronts

• Marked with a line containing alternating warm
  and cold front symbols (in their respective
  colors) showing the way the warm air or cold air
  would move if it could
• Little or no movement of the frontal zone occurs
  at the surface
• The air aloft will usually be overrunning,
  producing clouds
• Weather conditions are usually similar to a warm
  front, but milder
• Extended periods of cloudiness and light
  precipitation can develop along the cooler air

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Dryline

• The dryline is a front separating cT air from mT
  air
• Found in the south-central US
• Temperature differences are usually small across
  the dryline
• Humidity contrasts are very significant
• Often provides a focus for thunderstorm
  development over Texas, Oklahoma, and Kansas in
  the spring

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Cyclones Anticyclones
Air motion in cyclones and anticyclones.
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Summary

• Air Masses are extremely large body of air whose
  properties of temperature and moisture content
  are similar in any horizontal direction.
• Fronts are boundaries between Air Masses.
• Air that rises will cause insatiability in the
  atmosphere.