weather fronts

weather fronts Weather fronts are the boundaries between two different air masses. An air mass is a very large body of air which has approximately uniform properties, with little variation in temperature, humidity, visibility, or cloud type. Air masses form over regions of the Earth where air is slow-moving and takes on some of the characteristics of the surface. The main characteristics are the temperature and humidity. These are used to categorize air masses, using the letters c (continental) and m (maritime) to indicate dry and moist air, and T (tropical) and P (polar) to indicate warm and cold air, so that cT (continental tropical) air masses would be warm and dry, forming over regions such as the southern USA or north Africa, while mP (maritime polar) air masses are cold and moist and form over regions such as the northern Atlantic or Pacific oceans. To indicate extremely cold air, usually originating over snow-covered regions, the designation A (arctic) is sometimes used.

Our basic model of the structure of fronts was developed by the ‘Norwegian school’ of meteorologists based in Bergen, who were very productive in many areas of meteorology after the First World War. It is from that war that the term ‘front’ comes to indicate a boundary between two very different entities. Although this model is very useful in describing the formation and typical structure of fronts, it is very rare that an individual front looks exactly like this idealized model. Factors such as mountain ranges, coastal regions, the season of the year, or even the time of day can significantly alter the appearance or behaviour of individual fronts.

Fronts are drawn on weather maps as lines with symbols indicating whether they are warm fronts (semicircles) or cold fronts (triangles) (Fig. 1). When a warm front passes a particular location the air mass will change from cold to warm, while the opposite is true of a cold front. The designation of a front therefore depends on the motion of the front. If, as occasionally happens, a warm front slows down and stops and then moves in the opposite direction, it becomes a cold front. Fronts can also slow down and become stationary fronts. Since the front is not moving it is then neither a cold front nor a warm front. A stationary front is denoted by semicircles and triangles on alternate sides of the front. On a weather map, symbols are shown on the leading edge of a moving front. For stationary fronts the warm front symbols are, however, drawn on the side that would be the leading edge, if the front was moving as a warm front. The cold front symbols would be placed on the opposite side to indicate that if that were the leading edge the front would be a cold front. Occluded fronts are formed from a combination of both warm and cold fronts, and both symbols are accordingly used together.

The lines that represent fronts on weather maps indicate only part of the front. Fronts have a vertical as well as a horizontal structure. They are sloping surfaces, and the line on the weather map indicates where the frontal surface meets the Earth's surface. Fronts always slope so that the colder air, being more dense, is below and the warmer air mass is above. The frontal surface has the remarkable property that it acts almost like a solid, impermeable surface. Air from one air mass does not pass through the frontal surface to the other air mass but rises over the frontal surface or moves along it while remaining within the same air mass.

The polar front is a huge semi-permanent quasi-stationary front which separates warm air in the tropics from cold air in polar regions. Mid-latitude cyclones or depressions form on this polar front. As the depressions develop, their circulation bends the polar front and pushes it in such a way that a warm front and a cold front are formed. As the depression develops the cold front moves faster than the warm front and eventually catches up with it, producing an occluded front. The motion of the air around the depression is complicated by the fronts across which the air does not move. The resulting upward motion at fronts causes cooling of the air, condensation of water vapour to produce clouds, and eventually precipitation. The detailed characteristics of each type of front are described below.

Warm front

The warm front (Fig. 2a, right-hand side) has a slope of about 1 in 100. The uppermost part of the front is typically about 10 km above and 1000 km ahead of the region where the frontal surface meets the Earth's surface. Over all of this surface the air in the warm air mass is forced to rise over the frontal surface, and as the air cools water vapour condenses to form clouds. At the highest levels the clouds are composed almost exclusively of ice crystals; at middle levels they contain a mixture of ice crystals and water droplets. The clouds are deepest in the area just ahead of the surface front, and this is where precipitation falls as small raindrops. Embedded within this deep cloud there are commonly regions of more vigorous updraughts that produce heavier bands of rain. The rainfall area associated with warm fronts extends about 300 km ahead of the surface front. As the rain is about 700 km behind the leading cloud at the uppermost edge of the front, and the front moves with speeds in the range 20 to 100km/hr, it is possible to have several hours' warning of an approaching warm front simply by recognizing the sequence of clouds. This will start with high uniform ice crystal cloud (cirrus, Ci, and cirrostratus, Cs) followed by a lowering cloud base and darker, thicker cloud (altostratus, As), eventually becoming rain clouds (nimbostratus, Ns). At the same time as the upper level clouds are becoming darker, low-level clouds will become smaller or disappear altogether. The rain precedes the surface front, and so the increase in temperature associated with passage into the warm air mass is coincident with the cessation of rain and usually with the break-up of the dense cloud mass. The wind direction will also veer (i.e. come from a point further clockwise round the compass) as the surface front passes.

Cold front

At a cold front (Fig. 2a, left-hand side) the cold air undercuts the warm air and produces a sloping frontal surface with a steeper gradient than that of a warm front. The gradient at a cold front is typically about 1 in 75 but it is even steeper close to the surface front. The cold air moving behind the front is moving faster than the warm air ahead of the front, and this forces the warm air to rise rapidly at the frontal surface. Deep convective clouds (cumulonimbus) are produced at the cold front which produce heavy precipitation and sometimes hail. Since the clouds form behind the surface front, there is no early warning, as in the case of the warm front, although the towering heads of the cumulonimbus clouds can sometimes be seen from a considerable distance. The precipitation arrives at the same time as the temperature drops and the wind veers. There is often a significant increase in wind speed as well as a change in direction. When the rain stops the clouds usually clear rapidly, leaving the sky almost cloud-free and with excellent visibility. The cold air behind the front is, however, often unstable and showers frequently form in this cold airstream a few hours after the passage of the cold front.

Occluded front

Occluded fronts are formed when cold fronts meet warm fronts. Two different types of occlusion can form, according to whether the air ahead of the warm front or behind the cold front is the colder. The colder air undercuts the less cold air, and the warm air, which has been in contact with the Earth's surface, is now lifted above the surface. The occlusions are described as warm occlusions (Fig. 2b) or cold occlusions to indicate their similarity to warm or cold fronts. In a warm occlusion the air behind the cold front is relatively warmer than the air ahead of the warm front. This means that the air in the warm sector and the air behind the cold front rise above the coldest air; the precipitation then falls ahead of the surface occlusion, as would be the case at a warm front. In a cold occlusion, the reverse occurs and the occlusion resembles a cold front. In general, occluded fronts are not as vigorous as warm or cold fronts, and the changes in wind speed and direction are not so marked.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co.
McIlveen, J. F. R. (1986) Basic meteorology. Van Nostrand Reinhold.