Weather front - Wikipedia
A front is a boundary between air masses that leads to a low pressure system as the atmosphere attempts to even out the temperature contrast. If there is enough . Encyclopedic entry. A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than. Weather Fronts. Principle. Fronts are zones of transition between two different air masses. The zone may be 20 miles across or it may be miles across, but.
High and Low Pressure
At right, we see a "ridge" of high pressure aloft. A surface high pressure system and often fair weather typically are located ahead east of an upper ridge axis with lower surface pressure behind west of the ridge. Troughs and ridges can be weak or quite strong. In looking at upper air data, we also closely examine the "jet stream" which is a relatively high speed ribbon of air between 25, and 40, feet above the ground. The jet stream is strongest in the winter when the greatest temperature differences exist across the Northern Hemisphere.
In a general sense, warmest air masses are located south of the jet with cold air to its north left picture. Changes in the jet stream pattern and different wind speeds within the jet play a major role in surface temperature and precipitation patterns.
At right is a typical upper air chart or mb, i. In addition, wind vectors and colored isotachs lines of equal wind speed show the location of the jet stream green colors. Yellow, red, and pink colored isotachs represent a ribbon of higher speed winds within the jet.
Notice that within the jet stream, wind speeds usually differ. It is these differences in wind speeds within the jet stream that can cause significant weather. In this example, the highest winds are located along the East Coast.
This is called a "jet streak" within the overall jet stream. This picture is a satellite image showing clouds associated with a major low pressure system. Satellite pictures are taken by weather satellites located about 22, miles above the earth.
The surface low center in this case likely would be located in western Iowa with a cold front extending south through Missouri into eastern Texas along the cloud band.
Occluded Front: when a cold front overtakes a warm front
Rain showers may be occurring within this band. Meanwhile, across Nebraska, South Dakata, Minnesota, and northern Wisconsin, steadier rain or snow may be occurring to the north and west of the surface low and warm front which probably extends northeast from the low across central Wisconsin. Now we will look at different precipitation types.
The red line above represents a vertical profile of temperature in the atmosphere. Notice that temperatures the red line remain colder than 0 deg C 32 deg F throughout the atmosphere. Thus, snow will form aloft and remain as snow as it falls to the ground. Now we see a different temperature profile. The air is colder than 0 deg C aloft where snow forms, but then warms to just above freezing in a small layer.
In this layer, the falling snow partially melts so that the precipitation is now partly snow and partly water. As a result, a warm front usually moves more slowly than a cold front. Its inclination, or slope, is much less than that of cold fronts. Warm-front precipitation is generally much more uniform and widespread than that associated with cold fronts.
Sometimes in winter, if the warm air overrides cold air at subfreezing temperatures, severe ice storms may develop more than km 62 miles ahead of the surface position of the warm front. If, as often happens, a warm front is overtaken by a cold front moving around a low-pressure centre, the end result is an occluded front. Cyclonic storms in high and middle latitudes often start out as an undulation, or wave, on a frontal boundary between warm and cold air masses.
If the wave intensifies, the surface atmospheric pressure at the centre of the cyclone falls. Eventually, the advancing cold air behind the cold front catches up with the slower-moving cold air under the warm front. The intervening tongue of warm air is pushed aloft, and the cyclone is said to have occluded. Cold fronts often bring heavy thunderstormsrain, and hail.
Cold fronts can produce sharper changes in weather and move up to twice as quickly as warm fronts, since cold air is denser than warm air and rapidly replaces the warm air preceding the boundary. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangle-shaped pips pointing in the direction of travel, and it is placed at the leading edge of the cooler air mass.
The concept of colder, dense air "wedging" under the less dense warmer air is often used to depict how air is lifted along a frontal boundary.
The cold air wedging underneath warmer air creates the strongest winds just above the ground surface, a phenomenon often associated with property-damaging wind gusts. This lift would then form a narrow line of showers and thunderstorms if enough moisture were present.Cold Fronts and Warm Fronts
However, this concept isn't an accurate description of the physical processes;  upward motion is not produced because of warm air "ramping up" cold, dense air, rather, frontogenetical circulation is behind the upward forcing. Warm front Warm fronts are at the leading edge of a homogeneous 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 remove from the Earth's surface. Clouds ahead of the warm front are mostly stratiformand rainfall gradually increases as the front approaches.
Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
Occluded front Occluded front depiction for the Northern Hemisphere An occluded front is formed when a cold front overtakes a warm front,  and usually forms around mature low-pressure areas. In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front and plows under both air masses.
In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air.
Within the occlusion of the front, a circulation of air brings warm air upward and sends drafts of cold air downward, or vice versa depending on the occlusion the front is experiencing. Precipitations and clouds are associated with the trowalthe projection on the Earth's surface of the tongue of warm air aloft formed during the occlusion process of the depression. Stationary front A stationary front is a non-moving or stalled boundary between two air masses, neither of which is strong enough to replace the other.