Cold fronts indicate that we are entering a cooler stretch, usually becoming more frequent in spring and fall. They can often cause extreme weather conditions.
Cold fronts are connected to low-pressure systems. They can travel for many thousands of miles pulling the cooler (and often drier) air from northern latitudes (in the case of the Northern Hemisphere). Some cold fronts are stronger than others, and depending on the season and atmospheric pattern, some may develop severe storms.
A cold front is the leading edge of an advancing cooler, drier airmass that is replacing a warmer, moist airmass.
How do cold fronts move?
Low-pressure winds rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is why the coldest winds come from the north in the Northern Hemisphere and from the south in the Southern Hemisphere.
Cold air is denser than warm air.
We can think of cold fronts as heavy air masses that sweep warmer air out of their path. When this happens, the warmer air, being less dense (weighs less) than the cold air masses (which are naturally heavier), rises above the chilly air. This destabilizes the atmosphere, causing the air to condense as it rises, creating clouds and, eventually, thunderstorms if the instability persists. Typically, the most severe storms form when there is a significant temperature difference between cold and warm zones, and there is also a lot of humidity above the warm zone, which further destabilizes the atmosphere.
When there is high humidity and heat, and a cold front arrives with very dry air, strong to severe thunderstorms develop. These storms often produce destructive winds, hail, and even tornadoes.
A significant shift in temperatures after a cold front often comes accompanied by dry air. The dry air is stable, usually providing us with crisp, blue skies.