In Fronts 1 and 2, methods of locating and defining fronts by use of temperature fields and jet stream positions were discussed. In this discussion locating and defining fronts by use of sea level isobar and isallobar pattern will be discussed. Before proceeding, some definitions must be made. If you understand the reduction of station pressure to sea level pressure skip the next paragraph.
An isobar is a line of constant pressure. When analyzing an isobaric pattern on a sea level weather map, sea level reduced pressures are used. This is done because of altitude differences between Stations such as Denver, Colorado will always have much lower pressures than Houston, Texas or New York, New York. The actual pressure at Denver or any other station is called the station pressure. At Denver the station pressure is usually near 840MB while at Houston, it averages near 1016MB. To reduce station pressure to sea level pressure, one adds the previous 24hr high temperature to the 24hr low temperature and divides by two. Then using the station pressure and the average between the high and low temperatures, a factitious atmosphere with an average temperature is assumed to exist below ground level. The actual hydro-static equation used is complicated. However, using graphs or special slide rules based on the hydrostatic equation, a sea level pressure is found. Errors made in sea level reduction can be large for stations higher than 1 mile above sea level. However, for stations 1 mile or less, the method usually works fine.
Isobar analysis using the reduced sea level pressures reveal the existence of low pressure areas, high pressure areas, low pressure troughs, and high pressure ridges. A low pressure area is an area with lower pressures than the surrounding areas. A high pressure area is an area with higher pressures than the surrounding areas. A low pressure trough is a belt or zone where pressures are lower than on either side. A high pressure ridge is a belt or zone where pressures are higher than on either side. Ridges never contain fronts. Most fronts are found in troughs, but not all troughs contain fronts. Most low pressure areas will have fronts extending from them and fronts are found in low pressure troughs that extend from the lows.
Isallobars are lines of constant pressure changes. Most analysis use three hour pressure changes. Rising pressures often indicate cold frontal passage. Falling pressures are associated with warm fronts. The isallobar pattern is useful in locating fronts in mountainous regions. Zones separating rapidly falling pressures from areas of more slowly falling pressures are likely to be warm frontal zones. Likewise, zones separating areas with rapidly rising pressure from areas with more slowly rising pressures or falling pressures are likely to be cold frontal zones.
To determine if a trough contains a front, methods described in FRONTS ONE and FRONTS TWO are used. The locations of troughs as well as isallobar patterns are used to refine the actual frontal positions. In FRONTS FOUR, the use of satellite images to locate fronts will be described.
Fredmet
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