The Weather Makers of the Mid-LatitudesAir Masses. Fronts, and the Mid Latitude Cyclone

Air masses and their source regions that influence the weather of North America

An air mass is a body of air with similar moisture and temperatures

Air masses of N. America

cP cT cA mP mT Continental Polar Continental Tropical Continental Arctic Marine Polar Marine Tropical

Air Mass Modification

As a Continental Air mass moves across the great lakes, a Lake Effect Snow is created

p297

After crossing several mountain ranges, cool moist mP air from off the Pacific Ocean descends the eastern side of the

Rockies as modified, relatively dry Pacific air. This creates a rainshadow on the leeward side

Air Mass Modification

Weather maps provide a “synoptic” view of the weather and the locations

of air masses and frontal systems.

Surface Weather Map

500mb Weather Map

Mapping

A surface map allows for analysis of current weather variables, weather fronts, air masses, and radar imagery (in some cases)

Pressure Systems and Weather Mapping: Cyclones and Anticyclones

Locating Pressure Systems

Notice the counterclockwise flow around the Low pressure. (*)

From this pattern you should be able to

determine the center of the low. The “L” (*)

L

Frontal Boundaries• Front- the boundary

between two air masses of different densities (temp and moisture dependant)– Cold Front– Warm Front– Stationary Front– Occluded Front

• Fronts are “typically’ associated with a Mid Latitude Cyclone but may be independent

How Do You Locate Fronts on a Weather Map?

• Sharp temperature changes

• Changes in moisture content (by looking at dew point)

• Shifts in wind direction• Pressure and/or

Pressure changes• Clouds and precipitation

patterns

A Special Front: The Dryline• A dry line is a boundary between

warm, moist air and warm, dry air

Locating Drylines

Locating the Occluded Front

• Observe the conditions surrounding the Occluded Low

Figure11-25a p314

Locating the Stationary Front • For a stationary

front, temperature patterns are not only used but parallel winds are key identifiers!

Locating Fronts on a Weather Map

Notice the counterclockwise flow around the Low pressure. (*)

From this pattern you should be able to

determine the center of the low. The “L” (*)

L

Locating Fronts on a Weather Map

The next step is to determine the position of the warm and cold fronts. (*)

LL

Locating Fronts on a Weather Map

Notice the thunderstorms at ABL, OKC, and KNC. A line of thunderstorms is associated with an advancing cold front. Behind this front the air is colder and the winds are coming from the northwest. (*)

L

In front of the cold front the air is warmer and moving generally towards the north and northeast (south to southwest winds). See if you can see the leading edge of this warm air. This is the warm front. (Click to see the warm front.) (*)

Be sure and notice the changes in wind direction, temperatures, and cloud cover that these fronts separate. (*)

L

The Wave Cyclone

Where is the warm air? Decide where the mT (maritime tropical) air is located. When you have made your decision, advance to the next image. (*)

L

mT

Now decide where the cP (continental polar) air mass is located. (*)

cP

(*)

L

Locating Fronts on a Weather Map

L

mT

cP

To finish the concepts of the wave cyclone, you will make a forecast for the following cities: Springfield, MO; Oklahoma City, OK; Chicago, IL; and Buffalo, NY.

Weather Patterns of the Mid-Latitude Cyclone

The idealized life cycle of a wave cyclone in the Northern Hemisphere based on the polar front theory. As the life cycle progresses, the system moves eastward in a dynamic fashion. The small arrow next to each L shows the direction of storm movement.

Midlatitude Cyclones

What type of weather do you observe along the cold front?

What type of weather do you observe along the warm front?

Cold Fronts and Warm Fronts

Occluded Front

An occluded front occurs when a faster moving cold front catches a slower moving warm front.

Stationary Front

A Stationary front occurs when a cold air mass collides directly with a warm air mass and no low pressure center develops.

Weather Patterns of the Open Wave Cyclone

An idealized 3-D view of the formation of a mid-latitude cyclone during baroclinic instability. (a) A longwave trough at 500 mb lies parallel to and directly above the surface stationary front. (b) A shortwave (not shown) disturbs the flow aloft, initiating temperature advection (blue arrow, cold advection; red arrow, warm advection). The upper trough intensifies and provides the necessary vertical motions (as shown by vertical arrows) for the development of the surface wave cyclone. (c) As the surface storm moves northeastward, it occludes, and without upper-level diverging air to compensate for surface converging air, the cyclonic storm system dissipates.

L

mT

cP

The Wave CycloneForecast for Springfield, Missouri. (*)

Currently the conditions in Springfield, MO are clear with winds out of the southwest at 11 miles per hour. The temperature is 56oF. BUT, with the advancing cold front, Springfield can expect thunderstorms within a few hours, just as Kansas City, MO is experiencing. Following the thunderstorms, the weather will clear off, winds will shift and come in from the northwest and temperatures will cool into the high 30’s. (All this happens due to the advancing cP air.) (*)

Think about the forecasts, then advance to the next image. (*)

L

mT

cP

The Wave CycloneOklahoma City, OK (*)

Oklahoma, OKC is currently experiencing a thunderstorm. These storms will end soon. Following the thunderstorms, the weather will clear off and temperatures will cool into the high 30’s. (All this happens due to the advancing cP air.) (*)

L

mT

cP

The Wave CycloneChicago, IL (*)

Chicago has partly cloudy skies with a temperature of 55oF and winds out of the southwest. This will change in the next few hours as the low pressure system moves in from the west. Clouds will be on the increase and rain is likely. The passage of the cold front will be followed by much cooler temperatures. Within 24 to 36 hours the temperatures will drop into the high 20’s. (*)

L

mT

cP

The Wave CycloneBuffalo, NY (*)

Buffalo is experiencing scattered showers, completely overcast skies, with temperatures only in the high 30’s. But relief is on the way with the advancing warm front to the south. Skies will soon clear off, temperatures will be on the rise as winds shift and come in from the southwest. (*)

Tracking a Storm

The 500mb Steering Rule states that a surface Low pressure systems will move at a pace ½ of the upper level wind speeds and travel parallel to the warm sector isobars . Example: If upper level winds are 70 knots, the surface low will move forward at 35 knots.

The Jet StreamJet streams are relatively narrow bands of strong wind

in the upper levels (stratosphere) of the atmosphere.

Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter.

Jet streams are the strongest for both the northern and southern hemisphere winters.

H L H L

Cyclone & Anticyclone Paths

Upper level convergence and divergence play a large role in the strength of the pressure system.

Putting them Together: Relationships of the Surface and Upper Level (500mb)

The surface low-pressure area and fronts are shown for April 17, during an unseasonably hot spell in the eastern portion of the United States. Numbers to the east of the surface low (in red) are maximum temperatures recorded during the hot spell, while those to the west of the low (in blue) are minimums reached during the same time period. The heavy arrow is the average upper-level flow during the period. The faint L and H show average positions of the upper-level trough and ridge.

This Concludes the unit on Air Masses, Fronts,

and the Mid Latitude Cyclone