Air Masses and Fronts II

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Air Masses and Fronts II
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Brief review

• An air mass is a large body of air whose
  properties of temperature and humidity are fairly
  similar in any horizontal direction
• Source regions for air masses tend to be
  generally flat, of uniform composition, and in an
  area of light winds, dominated by surface high
  pressure
• Continental air masses form over land. Maritime
  air masses form over water. Polar air masses
  originate in 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

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Air mass source regions and their paths
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What is an atmospheric front?

• A front is a transition zone between two air
  masses of different densities.
• 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.

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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 the two air
  masses is not moving.
• The symbols on a map are in the direction of the
  air mass motion.

• Occluded front when a cold front catches up with
  a warm front

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Weather Map

• Shown surface-pressure systems, air masses,
  fronts, isobars, winds and air flow (large
  arrows)
• Green-shaded area precipitation

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What are the Signs of a Passing Front?

• Signs
• Sharp temperature changes over a relatively short
  distance.
• Changes in the airs 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

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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)

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Characteristics of a Cold Front

• Steep leading edge the vertical slope of a cold
  front surface is 150 - 1100 (ratio of vertical
  rise to horizontal distance). For comparison
  warm fronts have ratios 1200 1300.
• 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.

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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).

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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

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Characteristics of a Warm Front

• The slope of a typical warm front is 1200 (more
  gentle than cold fronts) -gt 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 -gt stable atmosphere

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Warm Fronts cloud and precipitation patterns

• Although they can trigger thunderstorms, warm
  fronts are more likely to be associated with
  large regions of stratus clouds and light to
  moderate continuous rain.
• Warm fronts are usually preceded by cirrus first,
  then altostratus or altocumulus, then stratus and
  possibly fog.
• At the warm front, gradual transition.
• Behind the warm front, skies are relatively clear.

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Occluded fronts.
Cold occlusion

• 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

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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

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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).

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Weakening/Strengthening of the Front

• Frontolysis
• 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

• Frontogenesis
• The front intensifies.
• Why? The temperature (humidity) contrast across
  the front is increasing.
• Example cP air mass moves over warm ocean
  water.