Mid-Latitude Weather

The Polar Air Mass


Between the 60th parrallel and the poles, we have the coldest air on the planet. The air mass in these regions is referred to as the “polar cell” or the “Polar Air Mass“.

At both the North and South pole, the air is cold, dense and heavy. Basically, the air sinks and pushes the cold air out and into the mid-latitudes.

The above diagram suggests a well defined pattern, which is never the case. The boundary between the polar air mass and the sub-tropical air undulates north and south of the 60th parallel as the result of river of air in the upper atmosphere known as the “Jet Stream” or “Polar Jet“. 

Source: https://en.wikipedia.org/wiki/Jet_stream

The Polar Jet

Jet Streams are rivers of air at the top of the troposphere that flow from west to east. The more significant one is the “Polar Jet” which can flow at speeds that can exceed 100 miles per hour. The polar jet is a meandering river of air that defines the boundary between the Polar Air Mass and the sub-tropical air in the mid-latitudes.

Mid-Latitude Weather and The Polar Jet

The Polar Jet stream flows at a relatively constant speed in a straight line. If it has to make a sharp turn , it bunches up and slows down. Once back on the straight away, it speeds up. The term used for the slowdown is convergence and the speeding up is divergence.


Convergence (the bunching up) is not unlike a slowdown on the freeway. The air flow slows and is compressed, resulting in denser and heavier air. The denser, heavier air sinks to the surface resulting in a high pressure area at the surface.

When the heavier arrives at the surface, it is pushed outward. Apply the “Coriolis” effect and the outward flow turns into a clockwise spiral.

Generally, the air in a high pressure is dry resulting fair weather.


Divergence (the speeding up) creates a low pressure in the upper levels of the troposphere. The air at the surface rushes upward to fill the low pressure leaving behind a low pressure at the surface.

As the surrounding surface air moves into the surface low pressure, the “Coriolis” effect does it’s thing and a counterclockwise spiral is the result.

Highs and lows are created by the undulations in the “Polar Jet” stream. They are also towed along by the jet stream and therefore tend to move from west to east in the northern hemisphere.



The life of a mid-latitude cyclone

The polar jet stream marks the boundary between the polar air mass (cold and dry)and the sub-tropical air (warm and moist). The divergence (speeding up) of the polar jet results in the development of a low pressure area at the surface. The temperature differential together with the Coriolis effect puts the low pressure area in motion, spinning counterclockwise. If the cold air is moving towards the warm air, then it is cold front. If the warm air is moving towards the cold air, then it is cold front.