Re: What CAUSES low barometric pressure?

Vivian,

There are several forces which can factor into the development of an area of low (or high) barometric pressure near the earth's surface. Unfortunately some of these are not so easy to explain or understand. But I'll give it a try anyway.

First you should understand that surface cyclones (low pressure areas) form in areas of surface convergence underneath regions of rising motion. Bear with me here for just a second.

From Bluestein (1993), there are 4 forcing functions that contribute to rising motion and therefore pressure falls at the surface. They are 1)Vorticity advection becoming more cyclonic with height, 2) a local maximum in temperature advection, 3) the vertical component of the curl of the frictional force becoming more cyclonic (or less anticyclonic) with height, and 4) a local maximum in diabatic heating. There are mathematical equations which relate these 4 forcing functions to surface pressure falls or rises. Rarely does one of these forces act alone (although one can be much more dominant than the others).

If you're still reading, I guess I'm lucky. Some examples of processes that can cause surface pressure falls are: warm advection along warm-frontal zones (causes air to rise), latent heating due to processes such as cumulus cloud formation (condensation produces heat), and downslope motion (subsiding air) causes air to compress (the compressed air warms and rises, causing low pressure).

These are only a few examples of processes which can contribute to barometric pressure falling at the surface, and each of the forcing functions can act together, or against each other.

Some areas of frequent cyclogenisis in the United States during the winter are the lee of the Rocky Mountains and offshore from the Mid-Atlantic states and southern New England.

Hope this helps! For more in-depth information, you can reference
Synoptic-Dynamic Meteorology in Midlatitudes, Volume II
Howard B. Bluestein
Oxford University Press, 1993.