Cold front
Encyclopedia
A cold front is defined as the leading edge of a cooler mass of air, replacing (at ground level) a warmer mass of air.
along the cold front and can cause the formation of a narrow line of shower
s and thunderstorm
s when enough moisture
is present. On weather map
s, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature
drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough
. Cold fronts can move up to twice as fast as warm front
s and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary. Cold fronts are usually associated with low-pressure areas.
In the northern hemisphere, a cold front usually causes a shift of wind from southeast to northwest, and in the southern hemisphere a shift from northeast to southwest. Common characteristics associated with cold fronts include:
*Provided there is sufficient moisture.
that follows along the leading edge of the cold front. These bands of precipitation are often very strong in nature, and can bring severe thunderstorms, hail
storms and/or tornado
es. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient
is tighter than normal. During the winter months, cold fronts sometimes come through an area with little or no precipitation. In the autumn months, cold fronts rarely bring severe thunderstorms, but are known for bringing heavy, and widespread rainstorms. These rainstorms sometimes bring flood
ing, and can move very slowly because cold fronts are more prone to slow movement in the fall. In the winter, cold fronts can bring severe cold spells, and heavy snowstorms. If moisture is not sufficient, cold fronts can pass without producing any precipitation at all, and the skies could be cloudless. Cold fronts do not produce the moisture, it will just condense against the cold air into cloud and rain droplets if there is enough water vapor in either airmass.
. Provided with sufficient moisture, the rising air would thus condense, creating storms, clouds, and/or rain. While this concept is used to generally describe frontal precipitation patterns, it is technically incorrect.
is the process of creating or tightening the temperature gradient of a front. During this process the atmosphere reacts in an attempt to restore balance, the consequence is a circular motion along the front where air is being lifted up, along the cold front and dropping downward, behind the frontal boundary. This is the actual force of upward motion along a front that is responsible for clouds and precipitation.
As the temperature gradient tightens during frontogenesis, the thermal wind
becomes imbalanced. To maintain balance, the geostrophic wind
aloft and below adjust, such that regions of divergence/convergence form. Mass continuity
would require a vertical transport of air along the cold front where there is divergence (lowered pressure
). Although this circulation is described by a series of processes, they are actually occurring at the same time, observable along the front as a thermally direct circulation. There are several factors that influence the final shape and tilt of the circulation around the front, ultimately determining the type and location of clouds and precipitation.
spells in the fall
(autumn) and winter. Very often, cold fronts are associated with deadly cold weather. Sometimes, though, cold fronts have no significant effect on the weather. The cold fronts in the late fall become more polar
in nature, and tend to bring very cold weather, and temperatures can drop by as much as 30°F. When cold fronts come through, there is usually a quick, yet strong gust of wind, that shows that the cold front is passing. The effects from a cold front can last only a few hours to several weeks, depending on when the next weather front comes through. The air behind the front is cooler than the air it is replacing and the warm air is forced to rise, so it cools. As the cooler air can not hold as much moisture as warm air, clouds form and rain occurs.
Cold fronts are very often associated with a warm front
, squall line, or other weather front. Very commonly, cold fronts have an adjacent warm front that is ahead of the cold front. This is known as an occluded front
. This forms an area where warm air is occurring and interacting with the cold front, an area known as a warm sector. In the warm sector, severe thunderstorms, tornadoes
, and hailstorms are a common occurrence, due to the sharp difference between the warm air that is associated with the warm front and the cold air that is associated with the cold front. A cold front is considered a warm front
if it retreats, and called a stationary front
if it stalls.
Development of cold front
The cooler and denser air wedges under the less-dense warmer air, lifting it. This upward motion causes lowered pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
along the cold front and can cause the formation of a narrow line of shower
Shower
A shower is an area in which one bathes underneath a spray of water.- History :...
s and thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s when enough moisture
Moisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...
is present. On weather map
Weather map
A weather map displays various meteorological features across a particular area at a particular point in time. Such maps have been in use since the mid-19th century and are used for research and weather forecasting purposes. Maps using isotherms show temperature gradients, which can help locate...
s, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough
Trough (meteorology)
A trough is an elongated region of relatively low atmospheric pressure, often associated with fronts.Unlike fronts, there is not a universal symbol for a trough on a weather chart. The weather charts in some countries or regions mark troughs by a line. In the United States, a trough may be marked...
. Cold fronts can move up to twice as fast as warm front
Warm front
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient...
s and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary. Cold fronts are usually associated with low-pressure areas.
In the northern hemisphere, a cold front usually causes a shift of wind from southeast to northwest, and in the southern hemisphere a shift from northeast to southwest. Common characteristics associated with cold fronts include:
| Weather phenomenon | Prior to the Passing of the Front Weather front A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front... |
While the Front is Passing | After the Passing of the Front |
|---|---|---|---|
| Temperature | Warm | Cooling suddenly | Steadily cooling |
| Atmospheric pressure Atmospheric pressure Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point... |
Decreasing steadily | Lowest, then sudden increase | Increasing steadily |
| Winds |
|
Gusty; shifting |
|
| Precipitation/conditions* | Light patchy rain can be produced by stratocumulus or stratus in the warm sector. | Prolonged rain (nimbostratus) or Thunderstorms (cumulonimbus): depends on conditions. | Clearing, followed by heavy showers. |
| Clouds* | Increasing: Usually Cirrocumulus and/or cirrostratus then altocumulus and/or altostratus then stratocumulus, which joins together into stratus Stratus -Weather:*Stratus cloud, a cloud type**Nimbostratus cloud, a cloud type**Stratocumulus cloud, a cloud type**Altostratus cloud, a cloud type**Altostratus undulatus cloud, a cloud type**Cirrostratus cloud, a cloud type-Music:... then thickens to nimbostratus. Often it only has some of these cloud types preceeding it, and in unstable conditions, a cold front may just take the form of a line of cumulonimbus with patchy altocumulus or stratocumulus or the usual cloud sequence ahead of it. |
Cumulonimbus or nimbostratus | Patchy altocumulus or cirrocumulus then cumulus Cumulus Cumulus is a type of cloud with the appearance of a lump of cotton wool.Cumulus may also refer to:*Cumulus Media, a radio broadcasting company*Cumulus , digital asset management software developed by Canto Software*Reinhard Cumulus, glider... and cumulonimbus |
| Visibility* | Fair to poor in haze Haze Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic... |
Poor, but improving | Good, except in showers |
| Dew Point | High; steady | Sudden drop | Falling |
Precipitation
A cold front commonly brings a narrow band of precipitationPrecipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
that follows along the leading edge of the cold front. These bands of precipitation are often very strong in nature, and can bring severe thunderstorms, hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
storms and/or tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient
Pressure gradient
In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...
is tighter than normal. During the winter months, cold fronts sometimes come through an area with little or no precipitation. In the autumn months, cold fronts rarely bring severe thunderstorms, but are known for bringing heavy, and widespread rainstorms. These rainstorms sometimes bring flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
ing, and can move very slowly because cold fronts are more prone to slow movement in the fall. In the winter, cold fronts can bring severe cold spells, and heavy snowstorms. If moisture is not sufficient, cold fronts can pass without producing any precipitation at all, and the skies could be cloudless. Cold fronts do not produce the moisture, it will just condense against the cold air into cloud and rain droplets if there is enough water vapor in either airmass.
Undercutting
The idea that cold air wedges, or undercuts, the warm air is often used to depict how advancing cold fronts force warm air to rise along the sloping cold air, much like a snow shovel scooping up snow, creating instabilityConvective instability
In meteorology, convective instability or stability of an airmass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear...
. Provided with sufficient moisture, the rising air would thus condense, creating storms, clouds, and/or rain. While this concept is used to generally describe frontal precipitation patterns, it is technically incorrect.
Frontogenetical Circulation
FrontogenesisFrontogenesis
Frontogenesis, in meteorology, refers to the formation or strengthening of an atmospheric front. During frontogenesis, the temperature gradient tightens and as a result the thermal wind becomes imbalanced. To maintain balance, the geostrophic wind aloft and below adjust, such that regions of...
is the process of creating or tightening the temperature gradient of a front. During this process the atmosphere reacts in an attempt to restore balance, the consequence is a circular motion along the front where air is being lifted up, along the cold front and dropping downward, behind the frontal boundary. This is the actual force of upward motion along a front that is responsible for clouds and precipitation.
As the temperature gradient tightens during frontogenesis, the thermal wind
Thermal wind
The thermal wind is a vertical shear in the geostrophic wind caused by a horizontal temperature gradient. Its name is a misnomer, because the thermal wind is not actually a wind, but rather a wind shear.- Physical Intuition :...
becomes imbalanced. To maintain balance, the geostrophic wind
Geostrophic wind
The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effect and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars . This balance seldom holds exactly in nature...
aloft and below adjust, such that regions of divergence/convergence form. Mass continuity
Continuity equation
A continuity equation in physics is a differential equation that describes the transport of a conserved quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described...
would require a vertical transport of air along the cold front where there is divergence (lowered pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
). Although this circulation is described by a series of processes, they are actually occurring at the same time, observable along the front as a thermally direct circulation. There are several factors that influence the final shape and tilt of the circulation around the front, ultimately determining the type and location of clouds and precipitation.
Temperature changes
Cold fronts are the leading edge of a frigid air mass, hence the name "cold front". They can bring severe coldCold
Cold describes the condition of low temperature.Cold may also refer to:*Common cold, a contagious viral infectious disease of the upper respiratory system*Chronic obstructive pulmonary disease...
spells in the fall
Autumn
Autumn is one of the four temperate seasons. Autumn marks the transition from summer into winter usually in September or March when the arrival of night becomes noticeably earlier....
(autumn) and winter. Very often, cold fronts are associated with deadly cold weather. Sometimes, though, cold fronts have no significant effect on the weather. The cold fronts in the late fall become more polar
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
in nature, and tend to bring very cold weather, and temperatures can drop by as much as 30°F. When cold fronts come through, there is usually a quick, yet strong gust of wind, that shows that the cold front is passing. The effects from a cold front can last only a few hours to several weeks, depending on when the next weather front comes through. The air behind the front is cooler than the air it is replacing and the warm air is forced to rise, so it cools. As the cooler air can not hold as much moisture as warm air, clouds form and rain occurs.
Association with warm fronts
Warm front
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient...
, squall line, or other weather front. Very commonly, cold fronts have an adjacent warm front that is ahead of the cold front. This is known as an occluded front
Occluded front
An occluded front is formed during the process of cyclogenesis when a cold front overtakes a warm front. When this occurs, the warm air is separated from the cyclone center at the Earth's surface...
. This forms an area where warm air is occurring and interacting with the cold front, an area known as a warm sector. In the warm sector, severe thunderstorms, tornadoes
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
, and hailstorms are a common occurrence, due to the sharp difference between the warm air that is associated with the warm front and the cold air that is associated with the cold front. A cold front is considered a warm front
Warm front
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient...
if it retreats, and called a stationary front
Stationary front
A stationary front is a boundary between two different air masses, neither of which is strong enough to replace the other. On a weather map, this is shown by an inter-playing series of blue spikes pointing one direction and red domes pointing the other. They tend to remain essentially in the same...
if it stalls.