3-3 Frontal Mechanics

  1. DEFINE the term air mass
    A large body of air that has essentially uniform temperature and moisture conditions (in the horizontal plane)
  2. DEFINE the term front
    An area of discontinuity that forms between two contrasting air masses
  3. DESCRIBE the structure of a front
    • Can be hundreds of miles long
    • 3-Dimensional
    • Surface Front: point where the new airmass makes contact with the ground - this is what is plotted on charts
    • Most active (dangerous) weather is focused along either side of the Surface Front
    • End between 15000 or 20000 AGL
  4. DESCRIBE the discontinuities used to locate and classify fronts
    • Fronts are named for the temperature change they bring
    • Fronts move across the country with their attached low-pressure system
    • All fronts are located in troughs of low pressure
    • Cold fronts move faster than warm
    • Usually see a 90° shift in wind direction as a front passes
    • Amount and rate of temp change depends on the front’s intensity
    • Pressure decreases as the front approaches, then rises right after passagebring
  5. DESCRIBE the factors that influence frontal weather
    • Amount of moisture available - moisture = weather
    • Stability of lifted air - Cloud Type
    • Speed of frontal movement
    • Contrast of Temp and Moisture
    • Slope of Front
    •  - Gradual - Slow moving = extensive cloudiness and steady precipitation
    •  - Steep - narrow bands of clouds and showery precipitation
  6. DESCRIBE the conditions associated with a cold front
    • Lift action produces violent and unstable conditions
    • Winds shift from SW to NW
    • Narrow belt of precipitation
    • Possible severe Icing
    • Post Frontal Weather: rapidly clearing skies, decreasing temp, decreasing dew point
    • Altocumulus clouds on horizon
  7. DESCRIBE the characteristics of a squall line
    • A line of violent Thunderstorms
    • Indicated by a dashed, double-dotted line
    • Develops 50-300mi ahead of cold front
    •  - Cold downdraft ahead of cold front lifts warm, unstable air
  8. DESCRIBE the characteristics of a warm front
    • Warmer, less dense air rides up and over a cold air mass
    • Broad areas of cloudiness - 500 to 700mi in front
    • Slow moving and Gradual
    • Winds shift SE to SW
    • Cirrus, Cirrostratus, Altostratus clouds
    • Precipitation gradually increases on approach
    • Low ceilings and poor visibility
    • can contain embedded thunderstorms
  9. DESCRIBE the conditions associated with a stationary front
    • Little or no movement
    • Winds can be blowing
    • Surface winds parallel on both sides of front
    • 180° wind shift
    • Symptoms similar to warm front, but less intense. Can persist for several days
  10. DESCRIBE the conditions associated with occluded fronts
    • When a Cold Front overtakes a slower moving warm front:
    • Tend to be aligned from NW to SE. Moves NE
    • 180° wind shift (from SE to NW)
    • Most severe weather 100nm S to 300nm N of intersection
  11. DESCRIBE the conditions associated with an inactive front
    • Clouds and Precipitation are not present
    • Often, not enough moisture for weather
    • Wind Shift, Change in Temp, and Change in Pressure
Card Set
3-3 Frontal Mechanics
API Weather 2017 Edition