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Weather front
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<p>A weather front is a boundary separating <a href="/facts/Air_mass/8Yrr84Ip">air masses</a> for which several characteristics differ, such as <a href="/facts/Density_of_air/7WqbHI6L">air density</a>, <a href="/facts/Wind/Nt0QIu5B">wind</a>, <a href="/facts/Temperature/QyGe4gmj">temperature</a>, and <a href="/facts/Humidity/Nwb28VlT">humidity</a>. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, <a href="/facts/Cold_front/hLASBjPL">cold fronts</a> can bring bands of <a href="/facts/Thunderstorm/F6hUzuAu">thunderstorms</a> and <a href="/facts/Cumulonimbus_cloud/DPWNqTDE">cumulonimbus</a> <a href="/facts/Precipitation/uq3nGQc8">precipitation</a> or be preceded by <a href="/facts/Squall_line/vn1lU2uY">squall lines</a>, while <a href="/facts/Warm_front/Q0Gq9wkI">warm fronts</a> are usually preceded by <a href="/facts/Stratus_cloud/xGz9XQNU">stratiform</a> <a href="/facts/Precipitation_(meteorology)/uq3nGQc8">precipitation</a> and <a href="/facts/Fog/mrIdyIc7">fog</a>. In summer, subtler humidity gradients known as <a href="/facts/Dry_line/Iwu6fJE2">dry lines</a> can trigger <a href="/facts/Severe_weather/9kD3FWWU">severe weather</a>. Some fronts produce no precipitation and little cloudiness, although there is invariably a <a href="/facts/Wind_direction/IjdaT59D">wind shift</a>. </p><p>Cold fronts generally move from west to east, whereas warm fronts move <a href="/facts/Geographical_pole/dTShPqll">poleward</a>, 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 <a href="/facts/Density_contrast/zFm3kYfn">density contrast</a> 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 <a href="/facts/Wind_velocity/QghYfL3e">wind velocity</a> known as a <a href="/facts/Shear_line_(meteorology)/9BsJAYXl">shear line</a>. This is most common over the open ocean. </p>