Download SUPPLEMENTAL LECTURE MATERIALS

SUPPLEMENTAL LECTURE MATERIALS...
Air Masses
- An air mass is a large body of air with similar properties of temperature and moisture in any horizontal
direction at any given altitude.
- The source region is the location where an air mass originates and picks up the properties of the land or
ocean surface over which it has been sitting.
- Properties of the ideal source regions:
Area must be physically uniform, i.e., all land or all water
Area must have a stagnant circulation.
- Stagnant circulation is characteristic of high pressure - so where are the major source regions? Answer.
Air Mass Characteristics
1. Temperature
Arctic (A) - very cold
Polar (P) - cold
Tropical (T) - warm
Equatorial (E) - very warm (no equatorial air masses affect North America)
2. Moisture (and source region surface material)
Continental (c) - dry (forms over land areas)
Maritime (m) - moist (forms over ocean areas)
Take one letter from the temperature group and one from the moisture group to define an air mass. Lower
case moisture is written first followed by the upper case temperature letter. Example: cP - continental polar cold and dry
There are five air masses that affect North America - see diagram below.
1. cA (Continental Arctic) - very cold, dry.
Source region: northern Canada near the Arctic Circle.
2. cP (Continental Polar) - cold, dry
Source region: central and southern Canada.
3. mP (Maritime Polar) - cool, moist
Two source regions: Atlantic and Pacific
Pacific air mass affects the U.S. more than the Atlantic mP air mass. Why is that so? Hint .
4. cT (Continental Tropical) - hot, dry
Source region: SW deserts U.S./Mexico.
Least important air mass because it only exists in the summer and it is the smallest air mass.
5. mT (Maritime Tropical) - warm, moist
Two source regions: Gulf of Mexico/Subtropical Atlantic and Pacific
Gulf of Mexico/Subtropical Atlantic
Air is more unstable than the mT air mass in the Pacific.
mT air moving northward meets cP air coming down from the north. When they meet, warm air rises which
leads to clouds, rain, storms, etc
Also mT air moving over cold land in the winter will often cool to the dewpoint resulting in fog. Which type of
fog results from warm air moving over a cool/cold surface? Answer.
Pacific Source Region
Cool waters offshore make the mT air mass more stable. Air is therefore less likely to rise and produce
clouds. This contributes to the dry summers in California.
Fronts
A front is a boundary between air masses with different properties (like temperature, moisture, pressure, wind
direction, etc). The type of front is determined by which air mass is advancing: e.g., cold front = cold air
advancing.
There are four types of fronts (pictured below) stationary front (no movement of either air mass)
cold front (cold air advancing)
warm front (warm air advancing)
occluded front (cold front overtakes the warm front - signals the weakening phase of a mid-latitude
cyclone).
Stationary Front
No movement
Surface winds blow in opposite directions on either side of the front
Weather: clear to partly cloudy
Precipitation: little to none
Location: commonly found along the eastern edge of the Rockies with colder air on the eastern side of the
front
Cold Front
Occurs where cold, dry, stable cP or cA air moves in and replaces warm, moist, unstable mT air.
Cold front moves faster than the warm front
Frontal boundary is steep which causes the warm air to rise quickly
Clouds: cumulonimbus
Precipitation intensity and duration: heavy/severe intensity and of short duration. Precipitation occurs AT the
frontal boundary.
The following table summarizes weather conditions before and after the passage of a classic cold front
Weather Element
Before Passage
After Passage
Temperature
Warm
Colder
Dew point
High
Lower
Wind direction
South
West-Northwest
Pressure
Falling
Rising
Clouds
Cirrus, then Cumulonimbus
Clear or Cumulus
Precipitation
Heavy AT front
decreasing or none
The image below (left) shows the cross-section of the cold front. Using the picture on the right, we are
basically moving from letter "B" to "C" across the cold front, where "B" is in the cold air mass and "C" is in the
warm air mass.
Warm Front
Occurs where warm, moist mT air replaces the retreating cool mP air.
Frontal boundary has a gentle slope so air does not rise as quickly
Clouds: stratus group, getting lower and thicker as the front gets closer
Precipitation intensity and duration: light to moderate intensity and of long duration. Precipitation occurs
AHEAD of the frontal boundary.
The following table summarizes weather conditions before and after the passage of a classic warm front
Weather
Element
Before Passage
After Passage
Temperature Cool - cold
Warmer
Dew point
moderately moist, rising
Higher
Wind
direction
East
South
Pressure
Falling
Rising
Clouds
Stratus group: cirrus, cirrostratus,
altostratus,
Precipitation
Clear with
scattered
nimbostratus, stratus, possible fog
stratocumulus
possible
Light to moderate AHEAD of the
front
None
The image below (left) shows the cross-section of the warm front. Using the picture on the right, we are
basically moving from letter "C" to "E" across the cold front, where "C" is in the warm air mass and "E" is in
the cool air mass to the north of the warm front boundary.
Occluded Front
An occluded front occurs when a cold front catches up to and overtakes a warm front. The result is that the
warm air that was at the surface before occlusion will be lifted off the ground. Widespread cloudiness and
precipitation are associated with the occluded front. Occlusion marks the point where the cyclone will start to
die since the warm, moist air (fuel supply) is cut off.
Life Cycle of a Mid-Latitude Cyclone
Mid-latitude cyclones begin as low pressure areas along the polar front (stationary portion) where warm air
meets cold air.
The wave develops initiating a separate warm and cold front and pressure starts to decrease.
As the storm intensifies, pressure continues to lower and cyclonic (counterclockwise flow in the N.H.)
increases.
At the point of occlusion, the storm is at its greatest strength and will then decrease in intensity after
that time.
Storm dissipates and fronts disappear.
These storms travel in a general west to east direction.
Storms last a few days to a week and go through their life cycle as they travel across the U.S.
The Mature Mid-Latitude Cyclone
Examine the picture of the mature mid-latitude cyclone
The area of lowest pressure is marked with an "L"
Circulation is counterclockwise in the N. Hemisphere
Clearly defined cold front and warm front
Fronts separate three air masses, cP, mT, and mP
The warm air (warm sector) resembles a triangular region that gets smaller as the storm matures and
approaches occlusion
Upper Air Flow and Linkage to Surface Highs and Lows
Air flow aloft in the mid-latitudes usually consists of a series of troughs and ridges. These are called Rossby
waves or longwaves.
We encounter the terms convergence and divergence again here:
Convergence: piling up of air above a region.
Divergence: spreading out of air above a region.
Where are these zones of upper level convergence and divergence?
Approaching the trough (between ridge and trough or west of trough) is a region of upper level convergence
which leads to sinking air and the development of surface high pressure below that area. In the area
approaching the ridge (between trough and ridge or east of trough), you will find upper level divergence which
encourages air to rise from below. This often initiates or strengthens existing low pressure areas at the
surface.
These winds aloft steer the surface pressure systems. So given the upper level winds, surface low pressure
areas should move to the northeast and surface high pressure areas should move to the southeast.