Air Masses and Fronts

Two fundamental assertions about weather and why there is weather:

1.) Differential heating of the surface of the earth. 2.)Heating of the lower atmosphere by energy transfer from the surface.

How do you identify a front on a surface weather map or by your own weather observations? Look for:

1.) Sharp temperature changes over a relatively short distance 2.) Change in moisture content 3.) Rapid shifts in wind direction 4.) Pressure changes 5.) Clouds and precipitation patterns

Classification:

4 general air mass classifications categorized according to the source region.

Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.

Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.

cT

Continental tropical air usually only influences the US in summertime as warm, dry air is pumped up off of the Mexican Plateau. It is usually fairly stable and dry, and if it becomes stagnant over the midwest, results in a drought. Deaths associated with the 1995 heat wave in the midwest were the result of cT and mT air which stagnated over the central and eastern part of the US this last summer.

The differences in density are most often caused by temperature differences. Separate air masses with different humidities as well. We identify fronts by the movement of this transition zone and the properties that move over a geographical location. What weather changes do you expect when TV weather person says a cold front is moving through the area?

The differences in density are most often caused by temperature differences. Separate air masses with different humidities as well. We identify fronts by the movement of this transition zone and the properties that move over a geographical location. What weather changes do you expect when TV weather person says a cold front is moving through the area?

There are two kinds of occluded fronts

There are two kinds of occluded fronts

We can then make combinations of the above to describe various types of air masses.

We can then make combinations of the above to describe various types of air masses.

mP

Winter cP air moves over a region such as the NE Pacific, picking up some warmth and moisture from the warmer ocean. In the case of the Pacific NW mountains force the air to rise (orographic lifting) causing rain.

Source Regions

are simply geographic areas where an air mass originates. Should be: uniform surface composition - flat light surface winds

stationary front

characterized by no movement of the transition zone between two air masses.

cold front

cold air displacing warm air.

Cold occlusion

cold front catches up with warm front. Ns, Tcu,Cb warm very cold

cP

continental polar cold, dry, stable

cT

continental tropical hot, dry, stable air aloft--unstable surface air

Steep leading edge

friction slows surface advance, moves quickly--25 knts up to 40 knts faster=steeper

Air Mass

is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction. can cover hundreds of thousands of square miles. there can be small variations

Front

is the transition zone between air masses with distinctly different properties.

continental c

located over large land masses--dry

marine m

located over the oceans----moist

polar latitudes P

located poleward of 60 degrees north and south

tropical latitudes T

located within about 25 degrees of the equator

mP

maritime polar cool, moist, and unstable

mT

maritime tropical warm, moist, usually unstable

Warm occlusion

mostly in NW. Warm cool mP off ocean cold cP warm occlusion. Precipitation is similar to the cold occlusion

warm front

warm air displacing cool air diagram. Shallow leading edge warm air must "overrun" cold air--cold air recedes moves slow 10-15 knts

cP

wintertime bitter cold can extent to Southern US and even Florida causing crop damage. Require long, clear nights, which means strong radiational cooling of air near the surface. A stable air mass. Little moisture added so air is dry

mT

wintertime source for the SW US is the subtropical East Pacific Ocean. mT air that influences weather east of the Rocky Mountains comes from the Gulf of Mexico, but only influences winter weather in the SE states. Occasionally, slow moving weather systems in SW flow aloft can draw up moisture at mid and low levels producing precipitation.