Warm fronts are usually not as well defined as cold fronts on upper air soundings. When the front is strong and little mixing has occurred, the front may show a well-marked inversion aloft. However, mixing usually occurs and the front may appear as a rather broad zone with only a slight change in temperature. Quite frequently there may be two inversionsone caused by the front and the other caused by turbulence. 

Isotherms are parallel to the front and show some form of packing ahead of the front. The stronger the packing, the more active the front. The packing is not as pronounced as with the cold front.

Warm fronts aloft seldom occur, but generally follow the same principles as cold fronts aloft. One case when they do occur is when the very cold air underneath a warm front is resistant to displacement and may force the warm air to move over a thinning wedge with a wave forming on the upper surface. This gives the effect of sec-ondary upper warm fronts and may cause parallel bands of precipitation at unusual distances ahead of the surface warm front. Warm air advection is more rapid and precipitation is heaviest where the steeper slope is encountered. Pressure falls rapidly in advance of the upper warm front and levels off underneath the horizontal portion of the front. When a warm front crosses a mountain range, colder air may occur to the east and may move along as a warm front aloft above the layer of cold air. This is common when a warm front crosses the Appalachian Mountains in winter.

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