Air Masses and Weather Patterns


Air Masses

An Air Mass is a large (>1000sq kilometer) body of air with uniform properties.

Air Masses form where there is persistant atmospheric conditions. This is primarily where the upper air is dominated by anticyclones. In regions where cyclone dominate the air near the surface is constantly changes as air is brought into the region from surrounding regions. Regions dominated by anticyclones tend to have light winds and generally clear skys. These regions are shown above.

The middle latitudes are not places generally where air masses form because this is where cold polar and warmer subtropical/tropical air tend to clash.


Polar Front Theory

The polar front theory was set forth by Norwegian scientists durng World War I to explain where storms originate. The idea setforth was that cyclone orginate at the polar front where the polar easterlies and the westerlies meet. This front is located in the middle latitudes. The theory is that waves start to develop along the polar front. As the wave develops it takes on a cyclonic motion.


Fronts are boundaries between contrasting air masses

One air masses is warmer than the other and therefore more bouyant. Where the two air masses meet the warmer, more bouyant air mass with be uplifted relative to the colder, more dense air mass. Depending on which of the air masses is advance there will be either an and Advancing Warm Front or an advancing Cold Front


Warm Front

Warm Fronts have broader, less steep slopes

Because of their slow rate of advance and less steep slopes, they tend to have moderate precipitation spread out over a broad area

Cold Front

Cold Front have narrower, more steep slopes. Because of their steep slope, air rises quickly, condenses and cause large rain storms but they are limited in areal extent.

Occluded Fronts

Here and advancing cold front takes over a slower moving warm front.


A cold type and a warm type.

Stages in the cycle of a middle-latitude cyclone.


Divergences and Convergence Aloft

In our discussion of sinking and rising air we showed how air flows in (lows) and out (highs) in all direction near the surface. However, aloft the converging air and the diverging air move in a more or less straight path. See above.

Actually, the air mostly move in one direction. Where the air converges and diverges aloft there are waves which produce Ridges and Troughs.

Note how the air slows down on the down wind side of the high. This supports the build up of the ridge because air is piling up over Montana. Note also how the air speeds up as it leaves the trough. This adds to the uplift of air over Indiana.



Note how the sags in the jet stream set up vorticity in the flow. Vorticity provided by the trough is a major contributor to the cyclonic flow near the surface in middle latitudes.