The mechanics of constant-pressure analysis require that all levels interrelate; vertical consist-ency between levels is a must. The procedure used to determine vertical consistency is one of overlay-ing charts. Upper-level analyses are superimposed on lower-level analyses to assure a consistent slope to the various features with height. This permits visual determination of how systems stack. In this way, violations of internal consistency are prevented. For example, by overlaying a 1200 850-mb chart over the surface chart of the same day and time, youll see that some pressure systems stack vertically, while others have a distinct slope. Lows normally slope upward toward colder air (usually westward and pole-ward), and highs toward warmer air (usually westward and equatorward). Fronts slope upward toward colder air. These spatial relations are an absolute requirement for a proper three-dimensional representation of pressure systems and fronts.

Although computer-generated surface and constant-pressure charts do not depict fronts, fronts do intersect upper levels. A fast-moving cold front with a slope of 1:40 intersects the 850-mb level approximately 35 miles to the rear of the surface position. A slow-moving cold front (1:100) is found no more than 90 miles behind the surface position. These same cold fronts intersect the 700-mb level 70 and 175 miles respectively to the rear of the surface position. Warm fronts have a more gradual slope (1:150) and may not intersect the 700-mb level. At the 850-mb level, expect to find them no less than 135 miles ahead of their surface positions. If warm fronts do intersect the 700-mb level, do not look for them within 250 miles of their surface position.

A good policy to follow in locating fronts is to first locate them aloft, then use the surface data to determine their exact positions. When upper-air information is sufficiently complete, only fronts meeting the following two criteria are ordinarily significant enough to be carried: Fronts that are supported by the temperature and wind patterns at the 850-mb level (700-mb level over mountainous regions); and fronts that can be identified in upper-air soundings. Well discuss frontal analysis and temperature (isotherm) pattern relationships later in this unit.

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