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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

Weather 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:

    • A cold front is when a mass of cold air moves 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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