<P> The cooler and denser air wedges under the less - dense warmer air, lifting it . This upward motion causes lowered pressure along the cold front and can cause the formation of a narrow line of showers and thunderstorms when enough moisture is present . On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles / spikes (pips) pointing in the direction of travel . A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough . Cold fronts move faster than warm fronts and can produce sharper changes in weather . Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary . </P> <P> In the northern hemisphere, a cold front usually causes a shift of wind from southwest to northwest clockwise, also known as veering, and in the southern hemisphere a shift from northwest to southwest (anticlockwise, backing). Normally, cold fronts can be marked by these characteristics: </P> <Table> <Tr> <Th> Weather phenomenon </Th> <Th> Prior to the passing of the front </Th> <Th> While the front is passing </Th> <Th> After the passing of the front </Th> </Tr> <Tr> <Td> Temperature </Td> <Td> Warm </Td> <Td> Cooling suddenly </Td> <Td> Steadily cooling </Td> </Tr> <Tr> <Td> Atmospheric pressure </Td> <Td> Decreasing steadily </Td> <Td> Lowest, then sudden increase </Td> <Td> Increasing steadily </Td> </Tr> <Tr> <Td> Winds </Td> <Td> <Ul> <Li> Southwest to southeast (northern hemisphere) </Li> <Li> Northwest to northeast (southern hemisphere) </Li> </Ul> </Td> <Td> Gusty; shifting </Td> <Td> <Ul> <Li> North to west, usually northwest (northern hemisphere) </Li> <Li> South to west, usually southwest (southern hemisphere) </Li> </Ul> </Td> </Tr> <Tr> <Td> Precipitation / conditions * </Td> <Td> Light patchy rain can be produced by stratocumulus or stratus in the warm sector . In summer, sometimes thunderstorms if a preceding squall line is present . </Td> <Td> Prolonged rain (nimbostratus) or thunderstorms (cumulonimbus): depends on conditions . </Td> <Td> Showers, then clearing </Td> </Tr> <Tr> <Td> Clouds * </Td> <Td> Often preceded by cirrus, cirrostratus then altostratus like a warm front (but usually with smaller amounts of these clouds). Areas of cirrocumulus and altocumulus within cirrostratus and altostratus more commonly seen than at a warm front . Larger cumulus clouds under the higher cloud types than at a warm front, where stratocumulus and cumulus humilis usually occur . Some of these cumulus clouds may produce showers ahead of the front . </Td> <Td> Cumulonimbus and cumulus congestus producing frequent showers, with a sheet of upper altostratus, through which the sun can sometimes be seen . Less commonly nimbostratus occurs with continuous rain . </Td> <Td> Patchy altocumulus or stratocumulus and higher cirrus clouds along with fast moving stratus fractus then eventually scattered cumulus and sometimes cumulonimbus . </Td> </Tr> <Tr> <Td> Visibility * </Td> <Td> Fair to poor in haze </Td> <Td> Poor, but improving </Td> <Td> Good, except in showers </Td> </Tr> <Tr> <Td> Dew Point </Td> <Td> High, steady </Td> <Td> Sudden drop </Td> <Td> Falling </Td> </Tr> </Table> <Tr> <Th> Weather phenomenon </Th> <Th> Prior to the passing of the front </Th> <Th> While the front is passing </Th> <Th> After the passing of the front </Th> </Tr>

Where is a cold front occurring right now