Air masses

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Air masses and Fronts
UPDATED 28 FEB 2000
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An air mass is an extremely large body of air
whose properties of temperature and moisture are
fairly similar in any horizontal direction at any
given altitude.
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A source region is a large area of a uniform
surface (land, sea or ice) of uniform temperature
and humidity. c -Continental air mass
originating over land or ice. m -Maritime air
mass originating over water.
Air masses have different characteristics
according to their source region. A- Arctic-
originating near the North or South Poles. P-
Polar- originating from 40ºto 60º North and
South. T- Tropical- originating from 25º North
and South. E- Equatorial- originating from 10º
North and South.
Air masses are further classified according to
Thermodynamic Process or temperature compared to
the surrounding air. k -air mass is colder then
the underlying air. w -air mass is warmer then
the underlying air.
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Visible satellite picture showing the
modification of cP air as it moves over the
warmer Gulf of Mexico and the Atlantic Ocean.
cPk
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HOW TO IDENTIFY A COLD FRONT
- SHIFTING OF THE WIND DIRECTION. COLD FRONTS
ARE LOCATED IN THE AREA OF MAX TROUGHING
OR COUNTERCLOCKWISE ROTATION OF THE WINDS. -
SHARP TEMPERATURE CHANGES OVER A SHORT DISTANCE.
- CHANGE IN THE AIR MOISTURE CONTENT. INDICATED
BY DIFFERENCE BETWEEN TEMPERATURE AND THE
DEWPOINT. - CHANGE IN THE SURFACE PRESSURE. -
WEATHER ASSOCIATED WITH A COLD FRONT. SHOWERS
AND THUNDERSTORMS
mTw air mass
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A front is a zone of transition between two air
masses of different density and temperature.
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A COLD FRONT IS THE LEADING EDGE OF A MASS OF
COLD AIR THAT IS UNDERRUNNING WARMER
AIR. FRICTION AT THE SURFACE CAUSES THE COLD
FRONT TO HAVE A STEEPER SLOPE THEN THE WARM
FRONT. WINDS BACK (ROTATE COUNTERCLOCKWISE) WITH
HEIGHT.
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SLOPE- DISTANCE IN THE VERTICAL TO THE
DISTANCE IN THE HORIZONTAL FAST MOVING COLD
FRONT- 150 SLOW MOVING COLD FRONT - 1100
Warm air
Cold
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AVERAGE SPEED IS 15 KTS. WEATHER IS LOCATED AT
AND BEHIND THE SURFACE FRONT
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AVERAGE SPEED IS 25 KTS WEATHER IS LOCATED AHEAD
OF THE SURFACE FRONT
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COLDER AIR RUSHES AHEAD OF THE COLD FRONT LIFTING
THE WARMER AIR IN FRONT OF THE SURFACE COLD
FRONT. WARMER AIR IS UNSTABLE CAUSING THE AIR TO
RISE UNTIL IT BECOMES A THUNDERSTORM.
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POSSIBLE
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A weakening cold front over land intensifies
into a vigorous front as it crosses the warmer
waters of the Gulf Stream. Warmer air at the
surface causes instability.
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A warm front is the line of discontinuity where
the foward edge of an advancing mass of
relatively warm air is replacing a retreating
relatively colder air mass.
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SLOPE- for a warm front averages between
1150 and 1300 Average Speed of a Warm Front Is
10 Kts
Cold
Warm
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HOW TO IDENTIFY A WARM FRONT
- MUCH HARDER TO LOCATE THEN A COLD FRONT. -
WINDS SHIFT FROM SOUTHERLY BEHIND THE FRONT TO
EASTERLY AHEAD OF THE FRONT.
- WINDS VEER (ROTATE CLOCKWISE) WITH HEIGHT. -
WEATHER ASSOCIATED WITH A COLD FRONT. FOG,
DRIZZLE AND RAIN
mTw air mass
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Weather expected is fog and low ceilings.
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Possible embedded thunderstorms
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A stationary front has essentially no
movement. Surface winds blow parallel to the
stationary front, but in opposite directions.
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Stationary fronts oscillate between the warmer
and colder air.
Cold
Warm
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An occluded front is a composite of two
fronts. They form when a cold front overtakes a
warm front and one of these two fronts is lifted
aloft.
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COLD OCCLUSION
COLD OCCLUSIONS FORM WHEN A COLD FRONT UNDERRUNS
THE WARM FRONT FORCING IT ALOFT. THIS IS THE
MOST COMMON TYPE OF OCCLUSION. THE CLOUDS AND
WEATHER ASSOCIATED WITH A COLD OCCLUSION ARE
USUALLY THE SAME AS A COLD FRONT. WITH THE
SHOWERS OCCURING JUST AHEAD OF THE SURFACE FRONT.
CLEARING OF THE CLOUDS USUALLY OCCURES AFTER
THE PASSAGE OF THE COLD FRONT, BUT SOMETIMES CAN
LAST UNTIL THE PASSAGE OF THE WARM FRONT ALOFT.
COOL AIR
COLD AIR
WARM AIR
CROSS-SECTION VEIW OF THE TRIPLE POINT
WARM AIR
COLD AIR
COOL AIR
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WARM OCCLUSION
WARM OCCLUSIONS OCCURE WHEN COOL AIR OVERTAKES
COLDER AIR AND IS FORCED ALOFT. THE WEATHER
ASSOCIATED WITH A WARM FRONT HAS THE
CHARACTERISTICS OF BOTH A COLD FRONT AND WARM
FRONT. THE CLOUDS A HEAD IF THE FRONT ARE
SIMILAR TO THE CLOUDS OF A WARM FRONT. THE
INTENSITY OF THE WEATHER DEPENDS ON THE DISTANCE
FROM THE APEX OR TRIPLE POINT. THE WEATHER
CHANGES RAPIDLY AND ARE USUALLY MOST SEVERE
DURING THE INITIAL STAGE OF DEVELOPMENT.
COLD AIR
COOL AIR
WARM AIR
CROSS-SECTION VEIW OF THE TRIPLE POINT
WARM AIR
COOL AIR
COLD AIR
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References
Meteorology Today Aerographer's Mate Second
Class, Volume 2 USA Today's homepage and Universit
y of Illinois homepage
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This Concludes Training