One very important atmospheric variable to monitor as weather patterns begin to change is air pressure. Air pressure is defined as the weight of the column of air extending from any level up to the top of the atmosphere. That alone is difficult to relate to “weather”. Since the earth is heated unequally by the sun, air pressure is not the same across the earth’s surface.
Therefore, air molecules across the surface of the earth move (i.e. the wind blows) from higher surface pressure towards lower pressure. This air moving towards the center of low pressure creates surface convergence, and the air moving away from the center of high pressure creates surface divergence. As the air diverges away from high pressure, air is forced to sink at the center of the high. Likewise, air is forced to rise at the center of low pressure. Again, rising air leads to precipitation and sinking air leads to clear skies.
Monitoring changes in air pressure alone is often used to roughly speculate future weather conditions. For example, as air pressure rises, high pressure is on the way and the skies should begin to clear. As air pressure falls, a region of lower pressure is on the way which means precipitation could be imminent.
How do fronts relate to surface air pressure? Centers of high pressure often mark the center of a big blob of air, called an air mass. Air masses are marked by uniform temperature and moisture characteristics. In North America, the winters of the eastern half of the United States are marked by the dominance of a cold and dry air mass originating over Canada. As this cold blob of air moves southward just as the wind diverges from the center of this high pressure, a cold front develops along the leading edge of this colder air.
A center of low pressure is found where this moving surface air converges and starts to rise. Air pressure along a cold front often dips to its lowest value just as the cold front passes. Melding all these concepts yield the typical weather characteristics along a cold front: falling temperatures and air pressure values prior to the frontal passage; increasing cloud cover and precipitation as the front passes; the lowest surface pressure is recorded just as the front passes; after the frontal passage, air pressure starts to rise, skies begin to clear, temperatures remain cool.
Below are images of the surface conditions (brown lines on map indicate lines of equal pressure; blue line with triangles indicate a cold front; and red line with semi-circles indicate a warm front) during a strong frontal passage in April, 2005.
April 22, 2005 at 7:00am E.S.T
April 23, 2005 at 7:00am E.S.T
April 24, 2005 at 7:00am E.S.T
April 25, 2005 at 7:00am E.S.T
Questions to address about pressure
How does the pressure change in West Virginia between April 22 and 25?
Approximately when did the fronts pass through West Virginia?
How far south did the cold front reach?
Would you estimate that this was a weak or strong cold front?
Support your choice (weak or strong cold front) with your observations from the maps. Include in your discussion the maps of pressure (above), temperature and rainfall (pages linked on left).
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