BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 11

Chapter Eleven CHAPTER ELEVEN Air Masses and Associated Weather An air mass may be described as a huge body of homogeneous air covering thousands of square miles; throughout the air mass temperature and humidity are more or less uniform in anyone horizontal plane An air mass could be broadly classified therefore as warm or cold and moist or dry; the terms are relative Classification of air masses is described later The lower levels of the atmosphere automatically assume the characteristics of the underlying surface Thus an air mass originating over very cold land in winter would be cold and dry whereas an air stream approaching the land after a long sea passage from warmer latitudes would be relatively warm and moist Source regions The area in which an air mass originates is called the source region The principal source regions are the large anticyclonic areas which lie to the north and south of the disturbed Westerlies Le the polar highs covering the polar caps, the oceanic sub-tropical highs and the continental highs; the latter being the anticyclones which develop over large land masses during winter months (See Figures 9.8 and 9.9) showing world pressure distribution for January and July respectively.) Other parts of the world may become source regions for short periods In all these areas the pressure gradient is generally slight and the horizontal movement of air is slow, thus allowing plenty of time for the surface characteristics (temperature and humidity) to penetrate upwards to considerable heights Characteristics The characteristics of an air mass are governed by three factors: 1.The ORIGIN which determines temperature and humidity The PATH which determines the modifications which take place at the surface As an air mass moves away from its source region it assumes the characteristics of the surface over which it is passing; thus warm dry air moving over a cold sea will pick up moisture and gradually become cooler in the layers near the surface The AGE of an air mass determines the height to which the surface characteristics will penetrate 96 Chapter Eleven Modifications to the surface temperature may alter the stability of the air mass General classification of air masses This is broadly based on the source regions, and the terms used to describe a particular type of air mass may seem a little confusing at first, For example, socalled Polar air does not originate from the polar caps but from subpolar regions Air coming from polar regions is called Arctic air or Antarctic air, as appropriate Similarly so called Tropical air does not flow from tropical latitudes but from the sub-tropical belts An air stream flowing from between the Trade Wind belts is classified as Equatorial air These main types are subclassified as maritime or continental; the former originating over the sea and being moist in character the latter flowing from dry land and generally fairly dry but it is important to remember that the history of an air mass can change its characteristics The table of Air Mass Classification given below is general Almost any area of the world can occasionally act as a source region: AIR MASS WEATHER Air mass characteristics are based on the following general principles Cold air moving over a warm surface (a) Becomes heated at the surface by contact (b) The warmed air rises, not bodily but in vertical columns (called convection currents), through the colder environment The heights to which convection currents will go depends on a number of factors which are explained in Chapter (c) The greater the temperature difference between the air mass and the underlying surface the more vigorous will be the convection currents 97 Chapter Eleven (d) If the rising air goes high enough and there is sufficient moisture present, cumuliform cloud will appear and, with further development, there may be some precipitation, characteristically in the form of isolated showers (See Figure 11.1) (e) This is called unstable air because vertical movement is stimulated, especially when the air is humid (t)An unstable air mass is favourable for good visibility, except in showers Warm air moving over a cold surface (a) Becomes cooled at the surface by contact (b) Surface friction causes turbulent mixing of the air at and near the surface This diffuses the cooling upwards from a few feet to a height of 500 m (1,500 ft) or more depending on the speed of the wind and the roughness of the surface (c) This colder, denser, heavier air forms a shallow layer on the surface and is said to be stable because it offers resistance to any vertical displacement (See Chapter 4) (d) If the air contains enough moisture a layer of cloud will form below the top of the turbulence layer (See Figure 11.2) If the air is dry or fairly dry, skies are likely to be clearer (e) Fog can form in light winds if the temperature of the surface is below the dew point temperature of the air Types of fog and their causes are discussed in Chapter (t) Poor visibility is favoured by a stable air mass 98 Chapter Eleven DETAILS OF SPECIFIC AIR MASS TYPES Figures 11.3 and ll.4 illustrate paths taken by some typical types of air masses They should be studied in conjunction with the following descriptions of weather generally associated with each type Bear in mind that the air mass characteristics described apply equally to both northern and southern hemispheres Polar maritime (Pm) air Cold air from higher latitudes moving over a relatively warm surface Cool unstable cumuliform cloud, possibly with isolated squally showers (particularly when the air is moving quickly across the isotherms), very good or excellent visibility except in showers For example a northerly westerly air stream reaching the British Isles after a long sea passage This same air stream when passing over cold land in winter, may become stable with low stratus cloud or fog Polar continental (Pc) air In winter this is a very cold stable air stream with a low moisture content Little change takes place during its passage over cold land Clear skies can generally be expected but when such an air mass moves over a relatively warm sea surface evaporation and warming take place: this results in instability and cumuliform cloud, wintry showers may occur It thus assumes the characteristics of polar maritime air For example, in winter, polar continental air from North America becomes polar maritime air during its passage eastwards across the Atlantic Ocean In summer the polar continental air mass will remain dry and cloudless as it moves over land which is warmer than at the source but, when subjected to surface heating over long distances, it becomes converted into a warm air mass 99 Chapter Eleven which, if it then moves over a cool sea, becomes stable, picks up moisture and fog or low stratiform cloud may form Arctic maritime (Am) air Originates over ice and snow surfaces and is thus very cold at all levels Because of its low temperature the moisture content is low, but moisture is picked up over the sea as warming takes place from below The weather then becomes similar to that of polar maritime air but much colder and more intense in character, because the difference between air and sea temperatures is more marked than in polar maritime air Cumulus or cumulonimbus clouds form and squally showers of rain or hail occur Arctic continental (Ac) air" This is similar in character to polar continental air Tropical maritime (Tm) air Warm and very moist air moving into higher latitudes passes over a sea surface which becomes progressively cooler A stable air mass in which very widespread advection fog, or low stratiform cloud or drizzle may be encountered Orographic rain at high coastlines is common Example: Widespread advection fog often encountered over the relatively cool waters of the north-east Pacific and the Grand Banks of Newfoundland area in the North Atlantic In summer, when moving over hot land it may become very unstable giving cumulus cloud with showers and possibly thunderstorms Tropical continental (Tc) air Very warm and dry at source Moving into higher latitudes it becomes cooled in the lower layers and remains dry whilst passing over land When moving over the sea its temperature is higher than that of the sea surface and, although some moisture is picked up, there is generally very little cloud or precipitation because convection is arrested at a low level in the stable air An air mass originating in desert regions may carry quantities of fine dust for thousands of miles, thus hazy conditions are not uncommon in a tropical continental air mass (See Figures 11.3 and 11.4) Warm polar maritime (wPm) air or returning polar maritime (rPm) air A polar maritime air mass, after moving into lower latitudes where it becomes warmed in the lower level, sometimes curves round and increases its latitude again It then undergoes cooling in the surface layers, becomes stable and 100 Chapter Eleven assumes the characteristics of tropical maritime air On such occasions it is called returning polar maritime air or warm polar maritime air (See Figures 11.3 and 11.4) * Air masses which originate over the snow covered areas of the Arctic have similar characteristics to polar continental air and, for this reason, are designated as such in the North American continent Equatorial (E) air masses Warm, moist and often very unstable, especially after surface heating over land when convection currents carry large quantities of moisture to high levels forming cumulus and cumulonimbus cloud and producing copious rainfall 101 Chapter Eleven 102 Chapter Eleven QUESTIONS Define the term air mass In which parts of the world are the principal source regions of air masses found? Describe the characteristics of the following air mass types, at source only: Pm, Pc, Am, AC,Tm,Tc,E Describe the weather generally associated with a Tm air mass in temperate latitudes when moving over: (a) The sea and increasing latitude (b) Hot land in summer (c) Cold land in winter Describe the typical characteristics of Pm air on moving to lower latitudes Describe the path taken by a Pm air mass which approaches the British Isles from the Atlantic Ocean as a wPm air mass Classify an air mass whose source region is Northern Canada in winter Describe its initial characteristics and the changes you would expect as it moves eastwards across the Atlantic Ocean 103