This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Stormy weather is often associated with low pressure systems. A cold front is the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. A warm front is the leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms.
Fog is common in the cold air ahead of the front.
Although clearing usually occurs after passage, some conditions may produced fog in the warm air. Current surface maps are updated every hour. Forecast surface maps are updated once each day.