16. A cold front is a front along which a cold air mass is overtaking a warm air mass. A warm front is a front along which a warm air mass is overtaking a cold air mass. Weather in the mid-latitudes is dominated by an ongoing battle between cold air masses from the north (cP) and warm air masses from the south (mT). That is why the weather often changes so rapidly.

As any type of front approaches, air pressure falls. Cold fronts typically move more rapidly than warm fronts. As a result, cold fronts produce more intense rainfall and destructive forces (hail, strong winds, lightning and tornadoes).

Temperatures will often rapidly drop as a cold front approaches; whereas, they will slowly rise when a warm front passes through. Warm fronts usually are associated with one or more days of light rain and calmer winds. In either case, rainfall is more concentrated over the cold air mass at a front (#15, Fig. 1).

Most regional low pressure areas found outside the tropics (the so-called extratropical cyclones) have fronts running right through their centers -- they are, in fact, a product of lift along the front. Fronts also tend to skirt between high pressure areas (Fig. 1).

The mid-latitudes are well-known for their rapidly changing weather conditions. This can be attributed to the frequency of frontal activity. When our weather is a product of frontal activity, we have what is known as frontal weather. Otherwise, in areas of high pressure, we have air mass weather. Air mass weather tends to be stable, more predictable, and generally provides clearer skies than frontal weather.

Figure 1. Simplified weather map showing the presence of lows along frontal lines. Note the presence of high pressure areas to the northeast, northwest, and southeast (Atlantic) that help to steer the lows.