Clouds ahead of the warm front are mostly strati form, and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the strati form clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
Development
Air masses are large bodies of air with similar properties of temperature and humidity that form over source regions, and the warm air masses behind warm fronts are not only warmer but higher in humidity than the colder air preceding them. Because of a warm air mass’s higher temperature and thus lesser density, mixing between the two air masses is unlikely. Being lighter, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 130 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapor condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus. Once the clouds have thickened to 2,500 metres (8,200 ft) from the earth’s surface, rain can begin to fall from the heavy nimbostratus cloud.References: http://passporttoknowledge.com/scic/jetstreams/educators/fronts.pdf
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ReplyDeleteFirst (whoever you are), thanks for posting on this topic. It really broadened my knowledge.
Some definitions:
1. What is an isotherm gradient?
An isotherm refers to line drawn on a weather map or chart linking all points of equal or constant temperature. Thus, an isotherm gradient simply refers to the slope of an evenly heated mass of air that forms the warm front.
2. What are strati (plural) or stratus (singular) clouds?
a low cloud form extending over a large area at altitudes of usually 2000 to 7000 feet (600 to 2100 meters)
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I was quite interested to know that warm fronts tend to form stratus clouds most frequently, and that rainfall gradually increases as we go down the front.
Why do stratus clouds form? As mentioned above, warm fronts are broad masses of low pressure air that rise very slowly. Thus, because of the slowspeed, clouds of vertical height such as cumulo-nimbus clouds cannot be formed, and clouds that are flat and cover a wide area tend to form.
But why would precipitation increase as the front approaches? As mentioned above, warm fronts are large and have hugely varying pressures. Thus, we can conclude that as the front moves over 'overruns' the cold air, the top of the front would be relatively higher pressure than the bottom, which would be relatively lower pressure. Thus, at the top there would be less adiabatic cooling on contact with the cold air, thus producing lighter clouds such as cirrostratus that rarely produce precipitation.
At the bottom, there is a great mass of low pressure air, and thus, fat heavy clouds would form that cause precipitation.
Thanks!
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