Friday Feb. 10, 2006

Optional Assignment due next Monday at the beginning of class.  You should have the assignment completed before coming to class.

The crossectional structure of warm and cold fronts was discussed briefly.  That information was tacked on at the end of the Feb. 8 class notes.

 A short time lapse video of a cold front passing through Tucson on Easter Sunday, 1999 was showed in class.

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We have covered some of the important features found on surface weather maps.  Today we will look at upper level charts.  Before that however we will return to the surface map that has been used as an example.


A warm and cold front have been added to the surface weather map.  The approaching cold front is probably what is causing the rain shower at the city along the Gulf Coast.  The clouds, rain, and drizzle in the NE corner of the map is what you might expect to find ahead of a warm front.  Some of the stormy weather is probably being caused by the nearby surface low pressure center (converging winds will produce rising air motions, rising air motions can produce clouds and precipitation)



Now we'll take a look at upper level charts, charts that depict atmospheric conditions above the ground.  Before you go any further you need to remember that pressure decreases with increasing altitude in the atmosphere (pressure at any level is determined by the weight of the air overhead, as you move upward there is less and less air left overhead and pressure decreases).  Also, the rate of pressure decrease depends on the air's density (if you move upward through dense air you are quickly moving weight from overhead and putting it underneath you; this causes a rapid rate of pressure decrease). 



One way of depicting upper level conditions would be to measure pressure values at some fixed altitude above the ground.  The pressure pattern could then be plotted on a constant altitude chart using isobars.  Note the lowest pressures would be found in the cold air, higher pressures would be found in the warm air.



Rather than plotting conditions at a constant altitude above the ground, meterologists measure and plot conditions at a particular reference pressure level above the ground.  Every point on the sloping surface above has the same pressure, 850 mb.  The altitude above the ground is what is changing.  You would find contours of altitude or height contours plotted on a one of these constant pressure charts. 

Note, at the bottom of the figure above, that the the two kinds of upper level charts (constant altitude vs constant pressure) have the same overall pattern.



A slightly more complicated example - a wavy surface instead of a flat sloping surface.



In this last example we have added a south (warm) to north (cold) temperature change to the west to east temperature variations that were in the last example.  We end up with a wavy surface that slopes from front to back (high in the south, closer to the ground in the north).  The "topographic map" that represents this surface is much different from our earlier examples but resembles more closely what you would see on a real upper level weather map.  Ridge and trough features are clearly defined.  You can now understand why the ridges are called ridges.  They should have called troughs valleys.

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A final example not shown in class that you can use to check your understanding of upper level charts.  This is a 500 mb constant pressure chart not an 850 mb chart like shown in the previous section.  The numbers on the contours are altitudes in meters.  The 500 mb pressure level is found at higher altitude than the 850 mb pressure level. 

Pressure at sea level is typically around 1000 mb.  You find the 850 mb level at about 1500 meters altitude, the 700 mb level at 3000 meters and the 500 mb level at around 5500 meters altitude. 

All four points on the map above have one thing in common: the pressure at all four points is 500 mb.

Points C and D lie at about the same latitude.  Point C is found at higher altitude (5640 m) than Point D (5580 m).  The air below Point C is warmer than the air below Point D.  Similarly the air below Point A is warmer than the air below Point B.

Points A and C both lie in a ridge.  The altitude at Point A (5460 m) is lower than the altitude at Point C (5640 m).  The air below Point A in the north is colder than the air below Point C near the southern edge of the map.  Similarly the air below Point B is colder than the air below Point D.  Both Points B and D lie in a trough.

Pressure is decreasing most rapidly in the cold dense air below Point B.  Point B is closest to the ground.  Pressure is decreasing most slowly with increasing altitude in the warm low density air below Point C.  Point C is furthest from the ground.