Chp 9 Air Masses Fronts Fronts A FRONT

Chp 9 Air Masses & Fronts

Fronts A “FRONT” is the intersection of air masses. “Mass” implies high pressure.

Source Regions • High or Low latitudes. • Not middle latitudes. • Middle latitudes are too variable & high pressure does not tend to develop & persist for long periods.

Source Characteristics • High & low latitudes. • 20, 000 or more km 2. • Several days required to form. • Develop in all seasons.

Air Mass Source, N Am. m. P c. A c. P c. T m. P 4 or 5 effect U. S. at once m. T

Air Mass Types • Continental • Maritime • Tropical • Polar • Arctic Of the 6 possibilities, only 5 can exist.

Arctic & Polar Air Masses large vertical Temp. gradient Arctic air is “freeze dried. ” 60°N 40°N

c. P Air Invades c. P tends to move South

Minn. Cools 36°F

Day 3, Alabama cools 18°F

m. T • develop over “warm” water • are not stable • develop thunderstorms • impact SW U. S. in El Niño. • become more unstable over land.

m. T vs m. P 1994 m. T moves N m. T dominates the SE U. S.

Convergence day 2

Day 3 14 -90 inch of snow

1995, 50 mph, 16”snow

Four Fronts • separate air masses of differing Temp. • cause uplift. • induce precip. • mix very slowly.

mid-latitude cyclone Usually cold & warm fronts. 2 -3 air masses.

Cold Front Stormy, slope backward, cumuliform clouds

Cold Front

Warm Front The boundary of warm air moving against cold air.

What’s wrong with this picture? Can you find 6 things ?

Overrunning Sometimes both masses move, warm overruns cold May bring heavy, general rain/snow.

Occlusion

Occlusion cont.

Alternative Occlusion Cyclonic low elongates to become a trough

Alternative Occlusion 2 No “side view” is given, (& perhaps is not known).

Dry Line c. P c. T m. T Where the dry air meets the moist.

1996