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DEFINE the term air mass
A large body of air that has essentially uniform temperature and moisture conditions (in the horizontal plane)
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DEFINE the term front
An area of discontinuity that forms between two contrasting air masses
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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
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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
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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
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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
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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
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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
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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
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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
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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
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