Almost every one who owns a TV set believes he knows what a weather front is. Or do they really know? Almost every one who has watched a TV weatherman has formed an opinion what a weather front is. There are some who believe that a cold air surge out of Canada is a cold front. They would be wrong. The front is the leading edge of the cold air surge. The rest of the cold air surge is a mass of cold air on the move.
Some will tell you that a weather front is a boundary between two dissimilar air masses. That definition is not bad. Indeed! All fronts are boundaries between two dissimilar air masses. However, there are boundaries between two dissimilar air masses that are not weather fronts. An example is commonly observed in the Mississippi valley during the spring when hot dry air from the desert southwest over runs warm moist air from the Gulf of Mexico. The two air masses are often separated by a horizontal inversion layer. An inversion layer is a layer where the temperatures increase with heights. Clearly this type of an inversion layer is an air mass boundary; but because the layer just described is horizontal, it is not a front.
The last sentence gives a clue to what a front is. A weather front is a zone in the atmosphere where the horizontal temperature gradient is the greatest. On level ground, finding fronts by observing surface temperature patterns is usually easy. However, in mountainous regions, surface temperature patterns are often governed more by altitude differences rather than air mass contrasts. Therefore, a better definition is needed.
The best definition of a front is that a front is a strongly baroclinic zone. A baroclinic zone? What is that? A baroclinic zone is a zone where constant density surfaces are not parallel to constant pressure surfaces. I will try to illustrate what a baroclinic zone is and how to locate the zones.
At this point, I will refer you to the NCAR site: http://www.rap.ucar.edu/weather/ Click on upper air then on the 850mb on the second row. If you know what a constant pressure chart is, skip the next paragraph.
A constant pressure chart must be defined. A constant pressure chart is an analysis where pressures every where are the same. It is an analysis that includes the heights of the pressure surface, temperatures, humidity, and winds. A millibar (mb) is equals 1,000dynes. A dyne is the force that causes one gram mass to accelerate 1cm/sec/sec if there are no other forces acting on the mass. Gravity applies approximately 980dynes force on one gram mass. Constant pressure charts commonly used are the 1,000mb, the 850mb, the 700mb, the 500mb, the 300mb, the 250mb, and the 200mb maps. The heights of a constant pressure surface depend mainly on temperatures. Except for near the ground, the colder the air is, the lower the pressure surfaces are. This is because a column of air will shrink when chilled. (Click on View the photo below)
View this photo This figure is a vertical cross section. P0, P1, P2, etc are pressure surfaces where P0> P1> P2, ETC. Cold air is to the left.
On the 850mb chart, above freezing isotherms are in solid orange or red. Freezing or below isotherms are in solid or dash blue. Note those areas where the isotherms are close together forming isotherm ribbons. Those isotherm ribbons are frontal zones or baroclinic zones. Where the isotherms are farther apart are areas that are less baroclinic and probably no front exists. The front at the ground or at sea level will be along or near the warm air side of the 850mb isotherm ribbon. Note that the 850mb chart is no good for locating fronts in the Rockies. This is because the 850mb constant pressure surface averages about 1,500m above mean sea level or just under a mile while the Rockies average more than 1mile high. To find fronts in the Rockies, the 700mb and/or 500mb charts can be used. The 700mb surface averages about 3,000M or 10,000ft above mean sea level and the 500mb surface averages about 5,670M or near 18,000ft above mean sea level. This completes Fronts One. The next part will discuss ways to define and find fronts by ways other than using isotherm patterns.
Fredmet
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