Air Masses and Fronts

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Air Masses and Fronts
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Air Masses and Fronts

• Cold Air Masses
• Warm Air Masses
• Cold Front
• Warm Front
• Occluded Front
• Stationary Front
• Cyclones, Anticyclones and Weather

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Air Masses and Fronts

• Air masses are characterized by their water
  content and temperature.
• The areas the air masses form above determine
  whether the mass is maritime, continental,
  arctic, polar or tropical.

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Air Masses and Fronts

• Air masses are labeled according to their area of
  formation. These labels are cA, cP, cT, mP, and
  mT.

• Continental Arctic (cA) Extremely cold
  temperatures and very little moisture. These
  usually originate north of the Arctic Circle,
  where days of 24 hour darkness allow the air to
  cool to sometimes record-breaking low
  temperatures. Such air masses often plunge south
  across Canada and the USA during winter, but very
  rarely form during the summer because the sun
  warms the Arctic.

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Air Masses and Fronts

• Continental polar (cP) Cold and dry, but not as
  cold as Arctic air masses. These usually form
  farther to the south and often dominate the
  weather picture across the USA during winter.
  Continental polar masses do form during the
  summer, but usually influence only the northern
  USA. These air masses are the ones responsible
  for bringing clear and pleasant weather during
  the summer to the North

• Maritime polar (mP) Cool and moist. They usually
  bring cloudy, damp weather to the USA. Maritime
  polar air masses form over the northern Atlantic
  and the northern Pacific oceans. They most often
  influence the Pacific Northwest and the
  Northeast. Maritime polar air masses can form any
  time of the year and are usually not as cold as
  continental polar air masses.

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Air Masses and Fronts

• Continental Tropical (cT) Hot and very dry. They
  usually form over the Desert Southwest and
  northern Mexico during summer. They can bring
  record heat to the Plains and the Mississippi
  Valley during summer, but they usually do not
  make it to the East and the Southeast. As they
  move eastward, moisture evaporates into the air,
  making the air mass more like a maritime tropical
  air mass. Continental tropical air masses very
  rarely form during winter, but they usually keep
  the Desert Southwest scorching above 100 degrees
  Fahrenheit during summer.

• Maritime tropical (mT) Warm temperatures with
  copious moisture. Maritime tropical air masses
  are most common across the eastern USA and
  originate over the warm waters of the southern
  Atlantic Ocean and the Gulf of Mexico. These air
  masses can form year round, but they are most
  prevalent across the USA during summer. Maritime
  tropical air masses are responsible for the hot,
  humid days of summer across the South and the
  East.

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Air Masses and Fronts

• Air masses are often described as LOW PRESSURE
  (cyclones) or HIGH PRESSURE (anticyclones).

• Low pressure areas are usually warmer (less
  dense) and wetter (less dense) air that moves
  upward due to low density.
• High pressure areas are usually cooler (more
  dense) and drier (more dense) air that moves
  downward due to high density.

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Air Masses and Fronts

• Low pressure/cyclones have air rising in their
  centers due to low density. As this air rises,
  air along the ground moves in to replace it. Due
  to the coriolis effect this surface air moves in
  a counterclockwise direction (in the northern
  hemisphere).

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Air Masses and Fronts

• High pressure/anticyclones have air moving
  downward in their centers which moves outward in
  a clockwise direction in the northern hemisphere.

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Air Masses and Fronts

• 1. Northwest Sector
• Usually cold with strong and gusty winds out of
  the northwest.
• Precipitation light to moderate, with heavier
  precipitation closer to the low.
• If cold enough, the snowfall can be significant.

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Air Masses and Fronts

• 2. Cold Front
• Narrow Band of showers and thunderstorms.
• Rapid change in wind direction.
• Winds increase in speed out of the W or NW.
• Rapid temperature decrease.
• Sinking air behind the front often leads to
  rapidly clearing skies.

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Air Masses and Fronts

• 3. Warm Sector
• Usually fair and warm.
• Later in the day, showers and thunderstorms may
  develop.
• Some thunderstorms may be heavy but most are of
  the 'garden variety' type.

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Air Masses and Fronts

• 4. Warm Front
• Mostly cloudy and cold.
• Precipitation often heavy and occasionally
  substantial.
• Winds often out of the east or northeast
  (Nor'Easter).
• Depending on sounding, potential for sleet and
  freezing rain.

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Air Masses and Fronts

• Fair weather generally accompanies a high
  pressure center and winds flow clockwise around a
  high. This means that winds on the back (western)
  side of the high are generally from a southerly
  direction and typically mean warmer temperatures.
  On the front (eastern) side of a high, winds are
  generally from the north and this typically
  results in colder temperatures.

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Air Masses and Fronts

• If a city is expected to be located west of a
  high pressure center then warmer temperatures are
  likely. However, if the city is expected to be in
  the northerly winds of a high pressure center,
  then forecast colder temperatures. Cities under
  the influence of high pressure centers can expect
  generally fair weather with little or no
  precipitation.

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Air Masses and Fronts

• Clouds and precipitation generally accompany a
  low pressure center and winds flow
  counterclockwise around lows. This means that
  winds on the back (western) side of the low are
  generally from a northerly direction and
  typically mean colder temperatures. On the front
  (eastern) side of a low, winds are generally from
  the south and this typically results in warmer
  temperatures.

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Air Masses and Fronts

• If a city is expected to be located west of a low
  pressure center then colder temperatures are
  likely. However, if the city is expected to be in
  the southerly winds of a high pressure center,
  then forecast warmer temperatures. Cities under
  the influence of low pressure centers can expect
  generally cloudy conditions with precipitation.

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Air Masses and Fronts

• The boundary where air masses of different
  densities and temperatures is called a front.
  In general, the front is where weather happens.

• There are four basic types of fronts.
• Cold Front
• Warm Front
• Occluded Front
• Stationary Front

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Air Masses and Fronts

• Cold Front - cold air moves under warm air and
  pushes it upward.
• Cold fronts generally bring precipitation (quick
  and heavy) and cooler temperatures.

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Air Masses and Fronts

• Warm Front - warm air gradually moves in to
  replace denser air which has moved away
• Warm fronts generally bring precipitation (slow
  and long lasting) and warmer temperatures

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Air Masses and Fronts

• Occluded Fronts - There two types, cold and warm
  occluded fronts and their typical weathers
  differ.
• The first brings weather typical of a warm front
  and then acts like a cold front. The second acts
  like a typical warm front.

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Air Masses and Fronts

• Stationary Fronts - Stationary fronts occur when
  neither warm nor cold air advances. The two air
  masses reach a stalemate. That is what stationary
  means - that neither front is moving. These type
  of conditions can last for days, producing
  nothing but Altocumulus clouds. Temperatures
  remain stagnant and winds are gentle to none.

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Front symbols