Air Masses and Fronts – II
Brief review
•
• Source regions for air masses tend to be generally
• masses form over water. Polar air masses originate in
An air mass is a large body of air whose properties of
temperature and humidity are fairly similar in any
horizontal direction
flat, of uniform composition, and in an area of light
winds, dominated by surface high pressure
Continental air masses form over land. Maritime air
•
cold, polar regions, and extremely cold air masses
form over arctic regions. Tropical air masses
originate in warm, tropical regions.
Continental polar (cP) air masses are cold and dry;
continental arctic (cA) air masses are extremely cold
and dry; continental tropical (cT) air masses are hot
and dry; maritime tropical (mT) air masses are warm
and moist; maritime polar (mP) air masses are cold and
moist
Air mass source regions and their paths
The weather today
Water Vapor in the Atmosphere
TODAY
What is an atmospheric front?
• Adensities.
front is a transition zone between two air masses of different
• The density contrast results from:
♦ Difference in temperature;
♦ Difference in humidity.
• The frontal zone (surface) is the
upward extension of the front.
• Sometimes the frontal zones can be very sharp.
• The
intensity of the weather along the front depends on the
contrast of the air mass properties.
• The
type of front depends on both the direction in which the air
mass is moving and the characteristics of the air mass.
•
Types of Fronts
Cold front: cold, dry stable air is
replacing warm, moist unstable air.
•
Warm front: warm, moist unstable
air is replacing cold dry stable air.
• Stationary front: boundary between • Occluded front: when a cold front
the two air masses is not moving.
•
The symbols on a map are in the
direction of the air mass motion.
catches up with a warm front
•
•
Weather Map
Shown: surface-pressure systems, air masses, fronts,
isobars, winds and air flow (large arrows)
Green-shaded area: precipitation
• Signs
•
What are the Signs
of a Passing Front?
♦ Sharp temperature changes over a relatively short
distance.
♦ Changes in the air’s moisture content (as indicated
by changes in the dew point).
♦ Shifts in wind direction
♦ Pressure and pressure changes.
♦ Clouds and precipitation patterns.
The location of the front is not always very obvious!
Even meteorologists sometimes disagree…
Cold Fronts
• Cold
front- a front in which cold air is
replacing warm air at the surface.
• Notice the difference in
•
•
♦ Temperature
♦ Dew point
♦ Wind direction
♦ Pressure
Associated with low pressure centers
(low pressure troughs): follow the dashed
line
The pressure is minimum as the front
passes (first decreases as the front
approaches and then increases behind
the front)
Characteristics of a Cold Front
• Steep
leading edge: the vertical
slope of a cold front surface is
•
•
•
•
1:50 - 1:100 (ratio of vertical
rise to horizontal distance). For
comparison: warm fronts have
ratios 1:200 – 1:300.
The steeper the edge, the
faster the front (the effect of
surface friction).
Cold fronts tend to move faster than all other types of fronts.
Cold fronts tend to be associated with the most violent
weather among all types of fronts.
Cold fronts tend to move the farthest while maintaining their
intensity.
Cold Front: cloud and precipitation patterns
• The
warm, moist air ahead of
the front is forced upward
and condenses
♦ Cirrus clouds well ahead of
the front
♦ Strong thunderstorms with
heavy showers and gusty
winds along and ahead of
the front: squall lines
♦ Broad area of cumulus
clouds immediately behind
the front (although fast
moving fronts may be
mostly clear behind the
front).
Warm Fronts
• Warm
front - in which warm
air replaces cooler air at the
•
•
surface
Notice the difference in
♦ Temperature
♦ Dew point
♦ Wind direction
♦ Pressure
Notice the presence of
precipitation well ahead of
the front
Characteristics of a Warm Front
• The
slope of a typical warm front is 1:200 (more gentle than cold
fronts) -> warm fronts tend to advance more slowly.
• Warm fronts are typically less violent than cold fronts.
• Overrunning:
warmer, less-dense air rides up and over the colder,
more-dense surface air.
• Frontal
inversion: temperature inversion at the front -> stable
atmosphere
Warm Fronts: cloud and precipitation
patterns
•
clouds and light to moderate continuous rain.
•
•
• Behind the warm front, skies are relatively clear.
Although they can trigger thunderstorms, warm fronts are
more likely to be associated with large regions of stratus
Warm fronts are usually preceded by cirrus first, then
altostratus or altocumulus, then stratus and possibly fog.
At the warm front, gradual transition.
Occluded fronts.
• Cold
fronts move faster than warm
fronts. They can catch up and overtake
•
•
their related warm front. When they do,
an occluded front is formed.
Cold occlusion: very cold air behind, not
so cold air ahead of, the warm front
The upper warm front follows the
surface occluded front
Cold occlusion
Warm Occlusion
• Very
cold air ahead of, not so cold air
behind, the warm front
• The
cooler air from the cold front
cannot lift the very cold air ahead,
•
rides “piggyback”
The warm front aloft precedes the
surface occluded front
Stationary Front
• Stationary front- a front which does not move or barely moves.
• Stationary
fronts behave like warm fronts, but are more
quiescent.
• Many
times the winds on both sides of a stationary front are
parallel to the front and have opposite direction.
• Typically
stationary fronts form when polar air masses are
modified significantly so as to lose their character (e.g., cold
fronts which stall).
• Typically
there is no strong precipitation associated with
stationary fronts (why? – no big contrast in the air mass
properties, no air uplifting and condensation).
Weakening/Strengthening of the Front
• Frontolysis:
•
Frontogenesis:
♦ The front weakens and dissipates
♦ Why?-the air masses start losing
their identities.
♦ The temperature (humidity)
contrast across the front is
decreasing.
♦ Typical for slow moving fronts
♦ The front intensifies.
♦ Why? – The temperature
(humidity) contrast across
the front is increasing.
♦ Example: cP air mass moves
over warm ocean water.