El Aula en los colegios de la Élite

Polar continental (Pc) air

In winter this is a very cold stable airstream 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 charac - teristics of polar maritime air. For example, in winter, polar continental air from North America becomes polar maritime air during its passage east wards 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 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 encoun - tered. 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

*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.

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 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.)

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.

Air masses reaching the British Isles.

Some paths of typical air masses of the N

American continent and adjacent w

ater

s

1 Define the term air mass.

2 In which parts of the world are the principal source regions of air masses found?

3 Describe the characteristics of the following air mass types, at source only: Pm, Pc, Am, Ac, Tm, Tc, E.

4 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.

5 Describe the typical characteristics of Pm air on moving to lower latitudes.

6 Describe the path taken by a Pm air mass which approaches the British Isles from the Atlantic Ocean as a wPm air mass.

7 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.

Out of seven characteristic isobaric patterns there are only two types of weather systems which are fundamental – the depression and the anticyclone – the remainder being either outward extensions from one of these or a neutral area between them.

These seven distinctive isobaric forms are:

Depression
Anticyclone
Secondary depression
Trough
Ridge or wedge
Col
Straight isobars

Depressions, fronts and anticyclones are discussed in greater detail in later chapters.

Depression (or low)

An area of low barometric pressure surrounded by an area in which the pressure is relatively high. The isobars are roughly circular or oval in shape and, in accordance with Buys Ballot’s Law (see Chapter 9), the wind flows in an anticlockwise direction round the area of low pressure in the northern hemisphere (see Figure 12.1) and clockwise in the southern hemisphere.

It should be noted that in both hemispheres the surface wind flows slightly in towards the central area (see Convergence in Glosssary) where the worst weather is usually encountered.

Depressions are of greatly varying intensities and are usually associated with bad weather – ie much cloud and precipitation with strong or gale force winds,