Air Masses and Fronts – I

Air Masses

•A large (thousands of kms) body of air with more or less uniform properties in any horizontal direction at any given latitude♦ Temperature♦ Humidity

•Next time: fronts – the boundary between two different air masses

Origin and formation of air masses

•Source Region: the location where the air mass formed its properties♦ Large, flat areas where the air stays stagnant long enough to take on the characteristics of the underlying surface.

♦ Ideal source regions are areas of high pressure

•Types of sources.♦ Tropical: warm♦ Polar: cold♦ Continental: dry♦ Water (maritime):humid

Classification of air masses

•cP♦ continental polar♦ cold, dry, stable

•cA♦ continental arctic♦ very cold, very

dry, stable

•mP♦ maritime polar♦ cool, moist,

unstable

•mT♦ maritime tropical♦ warm, moist,

usually unstable

•cT♦ continental

tropical♦ hot, dry, unstable surface air

Stability of air masses

•Which is lighter?♦ Dry air♦ Moist air

•Dry air:♦ Nitrogen♦ Oxygen♦ Argon

•Moist air: all this plus

♦ H2O

The weight of humid air •Mean molecular mass of dry air:

•Mean molecular mass of wet air.

•Moist air is lighter and less dense than dry air at the same temperature.

•Moist air rises more readily. Evaporation enhances convection in the atmosphere.

gas m % mass of the gas mixture--------------------------------------------N2 28 78% 28x78/100+32x21/100=28.6O2 32 21%

gas m % mass of the gas mixture---------------------------------------------Dry air 28.6 90% 28.6x90/100+18x10/100=27.5H2O 18 10%

Air mass source regions and their paths

Continental Polar (cP) and Continental Arctic (cA)-winter time•Forming conditions:

♦ Above the northwest part of Canada and Alaska♦ Long clear winter nights result in radiational cooling.

•Properties:♦ Very cold temperatures;♦ Little evaporation -> low water content (low specific humidity and dew point);

♦ Temperature inversion near the ground -> stable atmosphere;♦ The cold subsiding air results in broad high P centers (anticyclones).

•Weather in the winter: ♦ Bitterly cold, dry weather, clear sky;♦ It is slightly warmer on the west and the east coast because of compressional heating of the (slightly) sinking air.

♦ The stagnant, stable ground layer can create poor visibility and air quality -> pollutants are trapped near the ground

cP, cA

Effect of cP air masses on the weather

•Upper level wind flow and surface position of anticyclones during two cold spells in 1989 and 1990. Numbers represent minimum temperatures.

Lake-Effect Snow

•Description: increased snow showers on the downwind side of large unfrozen lakes.

It is a local phenomena.

•Conditions: ♦ The water is warmer than the ground (the first half of the winter).♦ Cold dry air is moving over the lake.

•What happens? – The cold dry air is warmed up at the surface and picks up moisture. The boundary layer becomes unstable. Warm and moist air rises and forms cold cumulous clouds. There is a continuous supply of moisture from the lake. On the downwind side the air is additionally cooled and/or lifted up by topography -> precipitation.

Lake Effect Clouds – satellite image

Snow Effect on the Great Lakes

cP air masses-summer time

•Forming conditions:♦ Above the northwest part of Canada and Alaska♦ The melting ice adds moisture to the air.

•Properties:♦ Cool temperatures (the sun is never directly overhead);

♦ Increased water content and humidity;

♦ No temperature inversionnear the ground, surfaceinstability

•Weather in the summer: ♦ Cooler and comfortable temperatures, the added moisture and atmospheric instability result in fair-weather clouds;

cP air mass moving over the ocean

•Additional moisture is added to the air.

•The warm ocean surface heats up the air -> instability, cloud formation

•The cP air mass loses its characteristics and is given a new designation

Maritime Polar (mP) Air Masses•Forming conditions:

♦ It may start over Asia and the frozen polar pacific regions as cP.

♦ As it moves over the Pacific (around Aleutian low) the boundary layer warms up and accumulates significant amount of moisture.

•Properties:♦ Cold (freezing) and dry air aloft (similar to cP)♦ Milder temperatures at the ground 40-50 F (no T inversion)

♦ It is conditionally unstable

•Weather in the winter: ♦ This air enters the US from the NW coast; is forced over the mountains

♦ Rain on the upwind side

♦ Snow high inthe mountains

mPmP

Rain, snow Pacific air- cold but

not VERY cold

Cold damp air

Effect of mP air masses on the weather

Maritime Tropical (mT) Air Masses

•Forming conditions:♦ Over the subtropical east Pacific Ocean-spends a long time over the warm ocean water.

•Properties:♦ In winter it is very warm and moist♦ Increased water content and humidity♦ The atmosphere is unstable (mT is the most unstable of all air masses)

•Weather in the winter: ♦ As it moves over the colder continent it produces heavy rain.

♦ Flooding and mudslides in California in the winter.

mTmT

Winter Heavy Rain

Winter fog,

low clouds

“The Pineapple Express” –IR image

Continental Tropical (cT) Air Masses

•Forming conditions:♦ In the summer over northern Mexico and SW US.

•Properties:♦ Very dry and hot;♦ The atmosphere is unstable;

•Weather: ♦ Dry and hot.♦ Practically no precipitation.♦ Results in heat waves and droughts across the great planes.

cT

HOT & DRY

Effect of cT air masses on the weather

Motion of Air Masses

•The prevailing upper level winds determine the motion of the air masses.

•The direction of the tropospheric jet steams impacts the change in weather♦ The zonal component results in the E-W motion of the weather patterns♦ The meridional component drags tropical air masses to the North and Polar air masses to the South.

The Jet stream on Sunday

Sunday’s temperatures