The case study focuses on cold fronts. These are synoptic-scale swaths of cloud and precipitation associated with a significant horizontal temperature gradient. They occur in conjunction with westerly waves, depressions, or cut-off lows and should not be considered in isolation. Cold fronts occur most frequently in winter when the amplitude of westerly disturbances is greatest.
Cold fronts are part of larger systems referred to as extratropical cyclones. These storm systems range from a few hundred to a few thousand kilometers in size and occur in association with jet streams in the middle and high latitude regions of the globe both north and south of the equator (roughly 30° to 70° latitude).
Extratropical cyclones derive their energy from the potential energy in the pole-to-equator temperature gradient, which can become concentrated within zones called fronts. In these zones, temperature changes occur rapidly, with winds abruptly shifting direction. Depending on which airmass is replaced, a cold or warm front can occur. If warm air replaces cold air, a warm front forms. If cold air replaces warm air, a cold front forms. As a cold front advances, the trailing wedge of cold air pushes warm air upward where expansion cooling causes clouds to form.