Chapter 12, Part 2
Fronts
Fronts
• A transition zone between two air masses of
different densities is called a front.
• There is usually a temperature difference
across a front as well.
• The humidities may differ as well.
Types of Fronts
Stationary front
Occluded front
Warm front
Cold front
1
Stationary Fronts
• Do not move.
• On the map, semicircles point to warmer air
and triangles point to cooler air.
• Here, air blows parallel to the fronts.
Cold Fronts
• Cold air is replacing warm air.
• The triangles indicate the direction the front
is moving.
Determining Location of Front
1.
2.
3.
4.
5.
Sharp temperature changes over short distance
Changes in the air’s moisture content (dew pt.)
Shifts in wind direction
Pressure and pressure changes
Clouds and precipitation patterns
2
Cold Front Example
Lower T, Dew Point
Isobar kink at front
(low pressure – trough)
Wind northwesterly
Pressure rising
Higher T, Dew Point
Wind southwesterly
Pressure lowering
Typical Airflow at a Cold Front
• Cold dense air forces warm air upwards.
• Warm moist air rises and condenses into clouds,
producing rain showers at the front.
• Leading edge of front is steep (1:50 for a 25 knot front).
Regenerated Fronts
• Frontolysis – temperature contrast lessens
and front weakens
• Frontogenisis – temperature contrast
increases and front strengthens
3
Typical Weather at a Cold Front
Before
During
After
Wind
S or SW
Shifting
W or NW
Temperature
Warm
Dropping
Cooler
Pressure
Falling
Minimum
Rising
Precipitation
Heavy
showers
Poor
Clearing
Visibility
Short
showers
Fair to poor
Dew point
High
Sharp drop
Lowering
Good
Warm Fronts
• Warm air is replaces cold air.
• The semicircles indicate the direction the
front is moving.
• Front moves slowly (10 knots), about half
the speed of an average cold front.
Warm Front Example
Lower T, Dew Point
Pressure rising
Wind southeasterly
Higher T Dew Point
Pressure lowering
Wind southwesterly
,
Isobar kink at front
(low pressure – trough)
4
Typical Airflow at a Warm Front
•
•
•
•
Warm air rises over cold air,
creating clouds and rain showers ahead of the front.
Leading edge of front has a gentle slope (1:300).
There is a temperature inversion (frontal inversion) just
ahead of the front.
Typical Weather at a Warm Front
Before
During
After
Wind
S or SE
Shifting
S or SW
Temperature
Cool
Rising
Warmer
Pressure
Falling
Minimum
Rising
Precipitation
Drizzle
Visibility
Light to
moderate
Poor
Dew point
Steady rise
Steady
Usually
none
Poor, improving Fair
Rise
Occluded Front
• When a cold front catches up to and
overtakes a warm front, the boundary is
called a occluded front (or occlusion).
• On a weather map, there are alternating
cold-front triangles and warm-front
semicircles pointing in the same direction.
5
Cold-Occluded Front
• The cold air of the
cold-front lifts both
the warm and cold air
ahead of it.
Warm-Occluded Front
• The cold air of the cold front rises over the colder air
ahead of it.
Typical Weather - Occluded Front
Wind
Before
During
After
E, SE, or S
Variable
W or NW
Temp. (Cold) Cold or cool Dropping
Temp. (Warm) Cold
Rising
Pressure
Falling
Low point
Colder
Milder
Rising
Precipitation
Yes
Yes
Clearing
Visibility
Poor
Poor
Improving
Dew point
Steady rise
Slight drop
Slight drop
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Summary
• When two air masses meet, a front is created.
• There are four kinds of fronts:
–
–
–
–
Stationary – not moving
Cold – cold air replaces warm air
Warm – warm air replaces cold air
Occluded – cold front catches up with warm front
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