Meteorologists define fronts as the boundary between two air masses. We have cold fronts where cold air is replacing warm air and warm fronts where warm air is replacing cool air. Then there are stationary fronts where the two air masses on either side of the dividing line are moving very little - usually no movement but perhaps some slight drift. There are also occulded fronts.
Notice I used the term "line." Actually, fronts are drawn on weather maps with a line, usually drawn blue with pointed symbols for cold fronts and red with rounded symbols for warm fronts. But in the real world, fronts are usually not lines. They are boundaries that can often be tens of miles across. And I think that is the situation we are dealing with today.
If you take a look at surface map that includes observations such as one of my favorites at www.rap.ucar.edu/weather/surface you can see how the 'front' has a fairly wide zone to it. The wind was generally out of the north to northwest from about Birmingham northward. So the wind shift often associated with the position of the front would be drawn at about 11:30 am from near Chattanooga to about Birmingham to south of Jackson. The Apalachian Mountains often make it difficult to precisely locate fronts northeast of Chattanooga.
If you look at dewpoints, you see that values south of the wind shift are in the lower 70s. From Birmingham north to Muscle Shoals and Huntsville, dewpoints are running in the 60s with upper 60s at Birmingham and lower 60s at Muscle Shoals. This is the frontal zone and you'd probably be justified in putting the front anywhere within this zone. Once you get into Tennessee, especially Memphis, you see some much drier air with dewpoints in the 50s.
Satellite images are also helpful in identifying the frontal zone. Current visible satellite imagery at that same site I noted above show a limited cumulus field behind the wind shift and almost no lower clouds when you get into the 50-degree dewpoint air.
Weather really is fascinating.
-Brian-
Notice I used the term "line." Actually, fronts are drawn on weather maps with a line, usually drawn blue with pointed symbols for cold fronts and red with rounded symbols for warm fronts. But in the real world, fronts are usually not lines. They are boundaries that can often be tens of miles across. And I think that is the situation we are dealing with today.
If you take a look at surface map that includes observations such as one of my favorites at www.rap.ucar.edu/weather/surface you can see how the 'front' has a fairly wide zone to it. The wind was generally out of the north to northwest from about Birmingham northward. So the wind shift often associated with the position of the front would be drawn at about 11:30 am from near Chattanooga to about Birmingham to south of Jackson. The Apalachian Mountains often make it difficult to precisely locate fronts northeast of Chattanooga.
If you look at dewpoints, you see that values south of the wind shift are in the lower 70s. From Birmingham north to Muscle Shoals and Huntsville, dewpoints are running in the 60s with upper 60s at Birmingham and lower 60s at Muscle Shoals. This is the frontal zone and you'd probably be justified in putting the front anywhere within this zone. Once you get into Tennessee, especially Memphis, you see some much drier air with dewpoints in the 50s.
Satellite images are also helpful in identifying the frontal zone. Current visible satellite imagery at that same site I noted above show a limited cumulus field behind the wind shift and almost no lower clouds when you get into the 50-degree dewpoint air.
Weather really is fascinating.
-Brian-
on July 23, 2006, 1:24 pm
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