Surface fronts mark the boundaries between airmasses. Changes in wind, temperature, humidity, visibility, and other meteorological variables are frequently found at fronts. Along frontal zones are often low clouds, low pressure, and precipitation. Fronts rotate counterclockwise around the lows in the Northern Hemisphere as the lows move and evolve.
Fronts strengthen or weaken due to advection by the wind (kinematics), external diabatic heating (thermodynamics), and ageostrophic cross-frontal circulations (dynamics). One measure of frontal strength is the temperature change across it. The spread of cold air behind a front is constrained by Coriolis force, as winds adjust toward geostrophic.
Airmasses can form when air resides over a surface sufficiently long. This usually happens in highpressure centers of light wind. Airmasses over cold surfaces take longer to form than those over warmer surfaces because turbulence is weaker in the statically stable air over the cold surface. As airmasses move from their source regions, they are modified by the surfaces and terrain over which they flow.
Upper-tropospheric fronts can form as folds in the tropopause. Drylines form over sloping terrain as a boundary between dry air to the west and moist air to the east; however, drylines do not usually possess all the characteristics of fronts.