As a result of the observations of atmospheric conditions at the surface and aloft, the following types of fronts are identified: (1) cold front, (2) warm front, (3) occluded front, and (4) stationary front.
The frontal symbols used on weather maps. Here the structure and some of the salient features of the various. Frontal symbols used on weather maps, types of fronts have been discussed in detail.
It should be remembered that fronts are differentiated by determining whether the colder air mass is moving on the warmer air mass or vice versa. However, it is the aggressor air mass that, to a large extent, determines the type of weather that will be produced along a front.
(1) Cold front:
A cold front is defined as a front along which cold air is invading the warm- air zone. Since the colder air mass is denser, it remains at the ground and forcibly uplifts the warmer and lighter air mass.
In fact, when pressure distribution is such as to force the cold air to advance and the warm air to retreat, the zone of transition is called a cold front.
The vertical structure of warm air that is forced upward by an advancing wedge of cold air determines the type of weather produced along the cold front.
The effect of friction retards the air motion near the ground, while the free air aloft has a higher velocity. This causes the cold front to become much steeper than the warm front. The steepness of the front is closely related with its velocity.
Thus, higher velocity results in the steeper slope, while lower velocity makes the slope of the front rather gentle, sometimes were approaching that of a warm front. The slope of a cold front varies from 1 : 50 to 1 : 100.
In other words, the wedge of cold air has a slope of 1 km of rise for 50 or 100 km of distance. When a cold front moves rapidly, warm air can be forced to rise ahead of the front. Depending on the instability of the overrunning warm air, convective clouds or even thunderstorms may occur along the leading edge of the cold front.
This type of front slopes backward instead of forward, so there is no warning far in advance of an approaching cold front and no preceding cloudiness until the front is near. The cold front in general is associated with a narrow band of cloudiness and precipitation.
The cold front passes more rapidly. The sky becomes clear soon after the passage of the front. However, the weather produced along the cold front is more violent.
When the cold front moves over a rough terrain, the lower air is retarded by the effect of friction at the ground. This causes the air aloft to run ahead of the surface air and a bulge or squall head is formed.
The lapse rate steepens and the convective overturning takes place resulting in heavy showers in a line ahead of the surface position of the front.
In meteorology this is called the squall line. However, precipitation depends on the moisture and temperature conditions of the warm air lying above.
With the approach of the cold front there is some increase of wind in the warm sector. Cirrus and cirrostratus clouds appear in the sky. These cloud types are quickly followed by lower and denser altocumulus and altostratus clouds.
At the actual front, the clouds are of nimbostratus and cumulonimbus type which produce heavy showers. In certain cases precipitation falls ahead of the front, while on occasions it is behind the same.
However, there are departures from this typical condition depending on the physical characteristics of both the air masses.
If the cold air moves over a warm water surface, the lower layers absorb heat and moisture which results in heavy rain or snowfall. Such a precipitation occurs in the cold air.
This phenomenon is characteristic of late fall and early winter when there is a considerable difference between the temperatures of air and water. On occasions, secondary cold fronts develop some distance behind the rapidly moving cold front.
With the passage of a cold front, the sky becomes rapidly clear and the weather improves. There is a sudden drop in temperature. A wind shift from south to west or northwest generally accompanies the frontal passage. There is a marked decrease in the specific and relative humidity.
The weather after a cold front has passed is dominated by subsiding and relatively cold air mass. In winter, the passage of a cold front is followed by a cold wave which further reduces the surface temperature.
(2) Warm front:
A warm front is defined as a gently sloping frontal surface in which there is active movement of warm air over cold air. As the surface position of a warm front moves, the warm air occupies territory formerly covered by cooler air. Because of friction, advance of the surface position of the front is slowed down so that the front has a small slope.
The average slope of a warm front is from 1 : 100 to 1 : 200. As the warm air gradually ascends the gently sloping surface of the wedge of cold air lying ahead, it cools adiabatically. This cooling leads to the cloudy condensation and precipitation. Unlike the cold front, the changes in temperature and wind direction are gradual.
The sequence of clouds precedes a warm front. The appearance of cirrus clouds heralds the approach of a warm front. These high-altitude clouds form some 1000 kilometers or more ahead of the surface front indicating the ascent of overrunning warm air over the retreating wedge of cold air.
The cirrostratus clouds that form ahead of the front produce halos around the sun and the moon. In case there is instability in the overrunning warm air, mackerel sky is produced by cirrocumulus clouds.
As the front draws closer to the observer, the clouds become lower and thicker. About 300 kilometers in advance of the surface position of the front thicker stratus and nimbostratus clouds appear and precipitation starts falling from them.
Thus, as the warm front approaches, the clouds show a sequence of cirrus, cirrostratus, altostratus, stratus and nimbostratus, and finally nimbus. The thick cloud sheet overlying the surface position of the front gives steady precipitation extending over a long distance ahead of the front.
Warm fronts usually yield moderate to gentle precipitation over a relatively larger area for several hours. This is in conformity with the gentle slope of the front. Convective activity is generally absent along a warm front.
But there are occasions when cumulonimbus clouds and the associated thunderstorms are produced along the warm fronts. But this is exception rather than the rule.
Sometimes the rains falling through the cold air mass below evaporate. This causes the air below the cloud base to become saturated and stratus clouds form. These clouds make atmospheric visibility poor causing much hardship to pilots of the small aeroplanes.
During the winter, an inversion of temperature is produced along the warm front which is situated near the surface. The inversion layer may produce during the cold season freezing rain or sleet in the very cold air ahead of a warm front.
The passage of a warm front is marked by a rise in temperature and pressure. There is a small shift of about 45° in the direction of wind.
The specific humidity rises, and there is usually a change in the weather. The weather in the warm sector, of course, depends on physical properties of the air mass and the season.
(3) Occluded front:
An occluded front is defined as a front formed when a cold front overtakes a warm front. The cold front moves more rapidly than the warm front with the result that the warm sector is progressively reduced in size.
Ultimately the cold front overtakes the warm front and completely displaces the warm air at the ground. Ultimately the cold and warm fronts combine into one. Thus, a long and backward swinging occluded front comes into existence.
There are two types of occlusion: (a) cold front type occlusion and (b) warm front type occlusion.
(a) Cold front occlusion:
It occurs when the cold air which overtakes the warm air is colder than the retreating cold air. It is illustrated. In the initial stages of the cold front type occlusion, the weather system of the warm front still persists.
Later on, when the warm front has been pushed further upward it has little effect on weather conditions. At the later stages the weather conditions resemble those of the cold front. Cold front type occlusion is the common type.
(b) Warm front occlusion:
The warm front type occlusion occurs when the retreating cold air mass is colder than the advancing cold air mass. In this case the advancing cold air being relatively less dense overrides the retreating cold air mass.
This type of occlusion generally takes place when the retreating cold air becomes progressively colder by radiation, and the advancing cold air mass is of the maritime polar type.
Weather produced at an occluded front is usually very complex depending to a large extent on the nature of warm air being forced aloft.
As a matter of fact, in either type of occlusion it is the uplifting of the warm air mass which results in cooling, condensation and precipitation.
The weather that is produced along an occluded front is usually a combination of the cold front and the warm front weather.
(4) Stationary front:
There are situations in which the surface position of a front does not move. Therefore such a front is called a stationary front.
The wind motion on either side of such a boundary is nearly parallel to the position of the front. Whenever some overrunning of warm air occurs along a stationary front, warm front type precipitation is likely to be produced.