Boundary separating two masses of air of different densities

Weather front

Weather front

A weather front is a boundary separating <a href="/en/Air_mass" title="Air mass" class="wl">air masses</a> for which several characteristics differ, such as <a href="/en/Density_of_air" title="Density of air" class="wl">air density</a>, <a href="/en/Wind" title="Wind" class="wl">wind</a>, <a href="/en/Temperature" title="Temperature" class="wl">temperature</a>, and <a href="/en/Humidity" title="Humidity" class="wl">humidity</a>. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, <a href="/en/Cold_front" title="Cold front" class="wl">cold fronts</a> can bring bands of <a href="/en/Thunderstorm" title="Thunderstorm" class="wl">thunderstorms</a> and <a href="/en/Cumulonimbus_cloud" title="Cumulonimbus cloud" class="wl">cumulonimbus</a> <a href="/en/Precipitation" title="Precipitation" class="wl">precipitation</a> or be preceded by <a href="/en/Squall_line" title="Squall line" class="wl">squall lines</a>, while <a href="/en/Warm_front" title="Warm front" class="wl">warm fronts</a> are usually preceded by <a href="/en/Stratus_cloud" title="Stratus cloud" class="wl">stratiform</a> <a href="/en/Precipitation_(meteorology)" title="Precipitation (meteorology)" class="mw-redirect wl">precipitation</a> and <a href="/en/Fog" title="Fog" class="wl">fog</a>. In summer, subtler humidity gradients are known as <a href="/en/Dry_line" title="Dry line" class="wl">dry lines</a> can trigger <a href="/en/Severe_weather" title="Severe weather" class="wl">severe weather</a>. Some fronts produce no precipitation and little cloudiness, although there is invariably always a <a href="/en/Wind_direction" title="Wind direction" class="wl">wind shift</a>.<a href="#cite_note-Miller-2000-11-10-1" style="counter-reset: mw-Ref 1;">[1]</a>

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Boundary separating two masses of air of different densities</div>

<figure class="mw-halign-right wiki-thumb"><a href="/en/File:Unusually_well_defined_warm_front.jpg" class="mw-file-description image media-thumb" data-hash="File:Unusually_well_defined_warm_front.jpg"><img loading="lazy" src="//upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Unusually_well_defined_warm_front.jpg/200px-Unusually_well_defined_warm_front.jpg" decoding="async" data-file-width="1536" data-file-height="2048" data-file-type="bitmap" height="267" width="200" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Unusually_well_defined_warm_front.jpg/300px-Unusually_well_defined_warm_front.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Unusually_well_defined_warm_front.jpg/400px-Unusually_well_defined_warm_front.jpg 2x" alt="Unusually_well_defined_warm_front.jpg"></a><figcaption>Approaching weather fronts are often visible from the ground, but are not always as well defined as this.</figcaption></figure>
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Cold fronts generally move from west to east, whereas warm fronts move <a href="/en/Geographical_pole" title="Geographical pole" class="wl">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.<a href="#cite_note-Roth-2013-11-21-2" style="counter-reset: mw-Ref 2;">[2]</a> When the <a href="/en/Density_contrast" title="Density contrast" class="wl">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="/en/Wind_velocity" title="Wind velocity" class="mw-redirect wl">wind velocity</a> known as a <a href="/en/Shear_line_(meteorology)" title="Shear line (meteorology)" class="mw-redirect wl">shear line</a>. This is most common over the open ocean.

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 are known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably always 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. 