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Weather Fronts

Introduction:

  • Fronts are the boundary layer between air masses (where weather happens)
  • Fronts are named according to the temperature of the advancing air, relative to the air it is replacing
  • The word 'front' came about during WWI during the times of trench warfare where the two opposing fronts came out and battled, similar to how the two air masses battle in a weather front

Air Mass:

  • Uniform temperature moisture
  • Classified by temperature on surface relative
  • 4 discontinuities between Air masses:
    • Temperature
    • Dew Point
    • Wind
    • Pressure

Fronts:

  • The severity of the clouds and precipitation occurring along a front are dependent on:
    1. The amount of moisture available (shown by the dew point)
    2. The degree of stability of the lifted air
    3. The slope of the front
    4. The speed of the frontal movement
    5. The contrast in the amounts of temperature and moisture between the two air masses

Cold Front
Figure 1: Cold Front

Cold Front:

  • Cold air is moving to displace warm air
  • Denser cold air moves along the surface and displaces the warmer air aloft
  • Movement is usually eastern while the front is usually a NE-SW line
  • Move slower in the winter
  • Precipitation always falls in front of a cold front
  • Fast Moving Cold Fronts:
    • Pushed by intense high pressure systems
    • As surface friction slows a front, the leading edge tends to bulge out and steepen the front's slope
    • These contain squall lines and severe weather
  • Slow Moving Cold Fronts:
    • Less than 15 knots
    • More like a warm front in properties and weather


Prior to Passage During Passage After Passage
Clouds Cirriform
Towering Cumulus/
Cumulonimbus
Towering Cumulus/
Cumulonimbus
Cumulus
Precipitation Showers Heavy showers
Possible hail/Lightning
Slowly decreasing showers
Visibility Fair/haze Poor Good
Wind Parallel to front Variable/gusty Perpendicular to front
Temperature Warm Suddenly cooler Continued cooler
Dew point High Rapidly dropping Continued drop
Pressure Falling Bottoms out, then rises rapidly Rising

Warm Front
Figure 2: Warm Front

Warm Fronts

  • When warm air overtakes and replaces cooler air
  • Typically move at a slower rate than cold fronts
  • The slope of a warm front is very gradual
  • Precipitation always behind in front of a cold front


Prior to Passage During Passage After Passage
Clouds Cirriform/Stratiform Fog/Cumulonimbus Stratiform Stratocumulus, possible Cumulonimbus
Precipitation Light to moderate rain, drizzle, snow Drizzle, if any Rain or showers, if any
Visibility Poor Poor but improving Fair in haze
Wind Parallel Variable Perpendicular
Temperature Cold to cool Rising steadily Warming, then steady
Dew point Rising steadily Steady Rising, then steady
Pressure Falling Becoming steady Slight rise, then falling

Stationary Front
Figure 3: Warm Front

Stationary Front:

  • No apparent movement because the opposing forces of the two air masses are relatively balanced
    • Less than 5 knots of movement
    • Winds blow 180° off from one another on each side
  • Can remain stationary and affect local flying conditions for several days
  • Weather in a stationary front is a mix of cold and warm front characteristics
  • Wind blows parallel to fronts, but opposite directions to one another

Occluded Front
Figure 4: Occluded Front

Occluded Front:

  • Occurs when a fast-moving cold front catches up to a slow-moving warm front
  • The temperature within each front is the primary determinant as to the type of front/weather to be expected
  • Cause persistent weather lasting over 24 hours
  • Classified cold or warm based on what is in contact with the ground
  • Cold Front Occlusion:
    • A cold occlusion results when the coldest air is behind the cold front
    • The cold air replaces the cooler air at the surface and forces the warm air aloft
    • High potential for serious weather
  • Warm Front Occlusion:
    • Warm occlusion results when the coldest air is ahead of the warm front
    • The cold air forces the cooler air of the advancing front aloft

Prior to Passage During Passage After Passage
Clouds Cirriform/Stratiform Nimbostratus, possible towering cumulus/
Cumulonimbus
Nimbostratus
Altostratus
Possible Cumulus
Precipitation Light to heavy Light to moderate Light to moderate then clearing
Visibility Poor Poor Improving
Wind Parallel to front Variable Perpendicular to front
Temperature:
Cold
Warm
Cold to cool
Cool
Falling
Rising
Colder
Milder
Dew point Steady Slight drop Rising, then steady
Pressure Falling Becoming steady Slight drop, then possible rise

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