A front is a band of low-pressure systems and marks the transition from one weather regime to another. It is typically formed at the boundary of two distinct air masses. In most cases, fronts are associated with a type of large-scale weather system called a midlatitude cyclone, which has a low-pressure center and causes winds to blow cyclonically (that is, in a counterclockwise direction). Midlatitude cyclones are the largest weather systems on Earth and generate most of the winter storms over the midlatitude continents. A front is a part of the midlatitude cyclone system, which trails a band of low-pressure air extending outward from the low-pressure center of the cyclone. Therefore, various weather systems, such as thunderstorms, heavy precipitation, snowstorms, and tornadoes, are also formed along the frontal band.

Different air masses are characterized by different physical properties of atmosphere, such as density, temperature, pressure, winds, and moisture. Because a front is a line that separates two different air masses, the atmosphere exhibits different physical properties on either side of a front. For example, air can change from warm to cold or from cold to warm, winds can blow from northerly to southerly or from westerly to easterly, and air can vary from dry to moist or from moist to dry across the frontal zone. Based on the movement of the frontal band and the temperature and humidity differentials, fronts can be classified as cold fronts, warm fronts, stationary fronts, and occluded fronts.

A cold front is formed when a cold and dry air mass advances and replaces a warm and moist air mass. In this case, the cold and dry air pushes and undercuts the warm and moist air ahead of it. The temperature will generally decrease in an area where a cold front is passing through. Because the cold front will lift the warm and moist air, clouds and precipitation can form at or behind the cold front.

A warm front is formed when a warm and moist air mass advances and replaces a cold and dry air mass. In this case, the warm and moist air pushes and overrides the cold and dry air, and the cold and dry air retreats. The temperature will generally rise in an area where a warm front is passing through. Because of the overriding of warm and moist air over the cold and dry air ahead of it, clouds and precipitation typically form ahead of a warm front.

An occluded front forms when a cold front catches up to and overtakes a warm front. In this case, a warm and moist air sector between the cold and warm fronts disappears, causing a complete convection of warm air in the storm center. This stage marks the full maturity of a midlatitude cyclone. Further dynamic and thermodynamic supports for the storm no longer exist, and the storm will dissipate from this time on.

A stationary front can form when a cold and a warm front move in opposite directions. When they meet, they can be locked in location. The cold and warm air mix together, so that there is no dominant overtake and apparent movement from either warm or cold air. This kind of situation often arises when fronts interact with the surface topography beneath them.

Fronts are important weather systems affecting people's daily lives. They mainly occur in middle latitudes, where large landmasses and dense human populations are located. A midlatitude cyclone, fronts, an upper-level jet stream, and specific storm tracks are all related from one to another and constitute a complete synoptic weather system (a weather system that can be analyzed on a weather map). Thus, a change in one part of the atmospheric environment will result in a change of the entire weather system. Studies show that global warming tends to widen the tropics and extend the troposphere vertically. There are many consequences of these changes. One of them is a poleward shifting of future jet streams. This shift would cause climatologic locations for midlatitude cyclones, fronts, and storm tracks to change accordingly.

The current global warming trend may also suggest a decrease of surface temperature gradient, since many observations and atmospheric model simulations indicate that a larger warming tends to occur in the colder regions. Since the horizontal temperature gradient is the key mechanism for the development of midlatitude cyclones, global warming might decrease the occurrence and intensity of midlatitude cyclones and associated fronts. On the other hand, because global warming tends to increase water content in the atmosphere, midlatitude cyclones may derive more energy from latent heat release and become more violent. There are no definite answers so far for how midlatitude cyclones and fronts are affected by these competing mechanisms.

Finally, frontal dynamics provides an important mechanism to cause convection and to form clouds. Precipitation related to fronts is a major process removing water from the midlatitude atmosphere. A potential change in frontal climatology in a future warm climate, regardless of whether it is an increase or decrease, will result in redistribution of snow and rain, changing the distribution of Earth's hydrosphere, especially in middle and high latitudes.

Bibliography:

1) Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2008.

2) Archer, Cristina L., and Ken Caldeira. "Historical Trends in the Jet Streams." Geophysical Research Letters 35, no. 24 (2008).

3) Intergovernmental Panel on Climate Change. Climate Change, 2007--Synthesis Report: Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by the Core Writing Team, Rajendra K. Pachauri, and Andy Reisinger. Geneva, Switzerland: Author, 2008.

4) Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere. 10th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.

5) Yin, Jeffrey H. "A Consistent Poleward Shift of the Storm Tracks in Simulation of Twenty-first Century Climate." Geophysical Research Letters 32 (2005).

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