There are two different meanings used within meteorology to describe weather around a frontal zone. The term "
anafront" describes boundaries which show instability, meaning air rises rapidly along and over the boundary to cause significant weather changes and heavy
precipitation. A "
katafront" is weaker, bringing smaller changes in temperature and moisture, as well as limited rainfall.
Cold front A cold front is located along and on the bounds of the warm side of a tightly packed temperature gradient. On surface analysis charts, this temperature gradient is visible in
isotherms and can sometimes also be identified using
isobars since cold fronts often align with a surface
trough. On weather maps, the surface position of the cold front is marked by a blue line with triangles pointing in the direction where cold air travels and it is placed at the leading edge of the cooler air mass. as the upward motion is really part of a maintenance process for
geostrophic balance on the rotating Earth in response to
frontogenesis.
Warm front Warm fronts are at the leading edge of a homogeneous advancing warm air mass, which is located on the equatorward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. A warm front moves more slowly than the cold front which usually follows because cold air is denser and harder to lift from the Earth's surface. and usually forms around mature low-pressure areas, including cyclones. It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold. In a cold occlusion, the air mass overtaking the warm front is cooler than the cold air mass receding from the warm front and plows under both air masses. In a warm occlusion, the cold air mass overtaking the warm front is warmer than the cold air mass receding from the warm front and rides over the colder air while lifting the warm air. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. They usually move in waves but not persistently. There is normally a broad
temperature gradient behind the boundary with more widely spaced
isotherm packing. A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts either dissipate after several days or devolve into shear lines, but they can transform into a cold or warm front if the conditions aloft change. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing opposite to each other, indicating no significant movement. When stationary fronts become smaller in scale and stabilizes in temperature, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shearlines. A shearline is depicted as a line of red dots and dashes. A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in
moisture. When moisture pools along the boundary during the warm season, it can be the focus of diurnal
thunderstorms. The dry line may occur anywhere on earth in regions intermediate between
desert areas and warm seas. The southern plains west of the
Mississippi River in the
United States are a particularly favored location. The dry line normally moves eastward during the day and westward at night. A dry line is depicted on
National Weather Service (NWS) surface analyses as an orange line with scallops facing into the moist sector. Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion.
Squall line Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but can outrun actively existing cold fronts in a pattern where the upper level jet splits apart into two streams, with the resultant
Mesoscale Convective System (MCS) forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. Even weaker and less organized areas of thunderstorms lead to locally cooler air and higher pressures, and outflow boundaries exist ahead of this type of activity, which can act as foci for additional thunderstorm activity later in the day. These features are often depicted in the warm season across the United States on surface analyses and lie within surface troughs. If outflow boundaries or squall lines form over arid regions, a
haboob may result. Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or
squal line, while outflow boundaries are depicted as troughs with a label of
outflow boundary. ==Precipitation produced==