On the other hand, if a cold air mass is retreating and warm air is advancing, a warm front exists. Otherwise, a stationary front is present if the cold air is neither advancing nor retreating from the warm air mass. In a cold front set-up, the boundary between the cold and warm air masses is relatively steep see below , typically causing the warm air in front of it to rise rapidly. This rising air creates energetic, billowing cumulonimbus clouds leading to showers and thunderstorms.
This is a simplified view of a cold front. Sometimes, fronts aloft above the surface can result in precipitation ahead of cold fronts. Warm fronts also are common, especially from Fall through Spring when larger temperature differences exist across the United States.
Relatively cool or cold air is present ahead of a warm front with warmer air behind the front, i. However, while cool air at the surface exists ahead of a warm front, relatively warmer air often is located above it as the warmer surface air behind the front rises up and over the cool air below. If enough moisture is present, this can result in precipitation along and ahead of the front. With a warm front, the cool air ahead of it must retreat before the warm air behind it can advance.
A vertical cross-section of a warm front dark red lines shows how surface warm air behind the warm front flows upward in a slantwise manner over top low-level cool or cold air ahead of the position of the surface warm front.
This causes clouds and precipitation ahead of the surface warm front, assuming enough upward motion of air parcels and available moisture. A stationary front is similar to a warm front, i. However, while a warm front shows movement, a stationary front shows little or no movement as the cooler, more dense air remains in tact and does not retreat.
Notice the winds from the northeast in the picture on the right above. Since these winds are blowing toward the front somewhat, this cooler air is not retreating, so the warmer air to the south of the front cannot lift northward. With a stationary front, a balance usually exists between the warmer and colder air masses on both sides of the front, so that neither air mass can advance on the other one.
Thus, the front remains nearly stationary. Here, we see what a typical low pressure system might look like on a surface weather map. A cold front extends to the south of the low pressure center, with a warm front to the east. Warm air is located ahead of the cold front and behind the warm front the so-called "warm sector" , while cool air exists ahead of the warm front and cold air is present behind the cold front.
Not all weather systems, however, are as simple as this model. This graphic shows how temperature " advection " occurs. To "advect" means to move from one place to another via the wind. At far left, warm air is being advected to the north, i. This is called " warm air advection " and occurs, for example, with warm fronts. Warm air advection can occur at the surface or aloft, and given enough lift and moisture, will result in precipitation.
Notice on the map that temperatures at ground level are cooler in front of the front than behind it. A stationary front is represented on a map by triangles pointing in one direction and semicircles pointed in the other direction.
A stationary front forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other, but neither is powerful enough to move the other.
Winds blowing parallel to the front instead of perpendicular can help it stay in place. A stationary front may stay put for days.
If the wind direction changes, the front will start moving again, becoming either a cold or warm front. Or the front may break apart. Because a stationary front marks the boundary between two air masses, there are often differences in air temperature and wind on opposite sides of it. The weather is often cloudy along a stationary front, and rain or snow often falls, especially if the front is in an area of low atmospheric pressure.
On a weather map, a stationary front is shown as alternating red semicircles and blue triangles like in the image at the left. Notice how the blue triangles point in one direction, and the red semicircles point in the opposite direction. An occluded front is represented on a weather map by a purple line with alternating triangles and semicircles.
Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a colder air mass the warm front , and then another cold air mass pushes into the warm air mass the cold front. Because cold fronts move faster, the cold front is likely to overtake the warm front. This is known as an occluded front. North of the warm front is a mass of cooler air that was in place before the storm even entered the region. As the storm intensifies, the cold front rotates around the storm and catches the warm front.
This forms an occluded front, which is the boundary that separates the new cold air mass to the west from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving.
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