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Concept
Weather front
Summary
A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.
Cold fronts generally move from west to east, whereas warm fronts move poleward, although any direction is possible. Occluded fronts are a hybrid merge of the two, and stationary fronts are stalled in their motion. Cold fronts and cold occlusions move faster than warm fronts and warm occlusions because the dense air behind them can lift as well as push the warmer air. Mountains and bodies of water can affect the movement and properties of fronts, other than atmospheric conditions. When the density contrast has diminished between the air masses, for instance after flowing out over a uniformly warm ocean, the front can degenerate into a mere line which separates regions of differing wind velocity known as a shear line. This is most common over the open ocean.
Air mass
The Bergeron classification is the most widely accepted form of air mass classification. Air mass classifications are indicated by three letters: Fronts separate air masses of different types or origins, and are located along troughs of lower pressure.
The first letter describes its moisture properties, with
c used for ontinental air masses (dry) and
m used for aritime air masses (moist).
The second letter describes the thermal characteristic of its source region:
T for ropical,
P for olar,
A for rctic or ntarctic,
M for onsoon,
E for quatorial, and
S for uperior air (dry air formed by significant upward lift in the atmosphere).
Official source
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