Fronts

The most common definition of a front is a “boundary between dissimilar air masses.” While this boundary is typically manifested as a thermal discontinuity, in some cases a moisture discontinuity can also behave as a front. In the plains of the United States, this moisture discontinuity is commonly referred to as a dryline, separating continental tropical and maritime tropical air masses.

The classic definition of a front, which implies a sharp zero-order boundary, often leads to a misunderstanding of its concept and structure. A front is actually a human construct considered to represent the leading edge of a strong thermal gradient. As a result, it is very difficult to find consistency in its placement on weather maps, even by professional meteorologists. This is especially true with warm fronts.

Zero-order fronts are in fact rare, occurring most frequently during winter in the western high plains and prairies of North America as a result of the ebb and flow of chinook winds. Chinook winds are strong downslope westerly winds that force existing shallow arctic air masses to the east and then allow them to slosh back westward again when the winds weaken. The greatest temperature changes ever recorded in North America are in response to chinook-based zero-order fronts. For example, a 2-minute change of 27 °C (49 °F) was observed in Spearfish, South Dakota, on January 22, 1943, and a 24-hour increase of 56 °C (103 °F), from −47 to +9 °C (-54 to +49 °F) was noted at Loma, Montana, on January 15, 1972.

Compared with zero-order boundaries, most fronts are in reality a transition zone of significant horizontal planar extent, generally up to hundreds of kilometers in width with cross-sectional slopes and depths that vary with each individual air mass. For example, cold fronts generally exhibit slopes of between 30:1 and 50:1, while warm fronts are less steep, with ratios of 100:1 to 200:1. As a result, precipitation shields can extend more than 1,000 km ahead of surface warm fronts, while a cold frontal precipitation is generally of a much smaller horizontal extent and a shorter duration.

Another common misconception is that all fronts extend from the surface to the tropopause, when in fact this is found only with deep baroclinic cyclones. In many cases, upper-level and/or split fronts can pass through a region completely undetected by surface observation stations.

Split fronts exist in cases of differential advection, that is, when multiple air masses are moving through a region at different heights. For example, it is not uncommon for maritime tropical air to be moving north at the surface into the Great Plains of the United States from the Gulf of Mexico at the same time when a shallow layer of hot, dry [Page 1173]continental tropical air aloft is migrating northeastward from the mountains of Northern Mexico. In many cases, the shallow layer of hot, dry air arrives after the maritime tropical air at the surface, resulting in a split front structure. Split fronts are also common with winter storms in the Eastern United States, where the heaviest snowfall develops northwest of the surface cyclone center in response to frontogenesis at the 700-mb (millibar) level.

...