A cold front is the leading edge of a colder air mass, replacing a warmer air mass at ground level, found within a fairly sharp low-pressure surface channel. It is formed as a consequence of an extratropical cyclone, at the forefront of its cold air advection pattern, which is also known as the cyclone's dry conveyor belt circulation. Temperature changes across the limit can exceed 30 ° C (54 ° F).
When there is enough humidity, it can rain along the boundary. If there is significant instability along the boundary, a narrow line of storms can form along the frontal zone. If the instability is less, a wide rain shield can move behind the front, increasing the temperature difference across the boundary. Cold fronts are strongest in the fall and spring transitional seasons and weakest during the summer. When a cold front reaches the previous warm front, the part of the boundary that does so is known as the occluded front.
Colder, denser air cracks under warmer, less dense air, rising. This upward movement causes lower pressure along the cold front and can cause a narrow line of showers and thunderstorms to form when there is sufficient humidity. On weather maps, the position of the cold front surface is marked with the symbol of a blue line of triangles / peaks (points)pointing in the direction of travel. The location of a cold front is at the leading edge of the temperature drop, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and is typically within a sharp surface channel. Cold fronts move faster than warm fronts and can produce more abrupt changes in the weather. Since cold air is denser than warm air, it quickly replaces the warm air that precedes the boundary.
In the northern hemisphere, a cold front generally causes a clockwise southwest to northwest wind change, also known as a turn, and in the southern hemisphere a northwest to southwest change (counterclockwise, reverse).
