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.
2. 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.
3. The AGE of an air mass determines the height to which the surface
characteristics will penetrate.
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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
1. 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 4.
(c) The greater the temperature difference between the air mass and the
underlying surface the more vigorous will be the convection currents.
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(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 8.
(t) Poor visibility is favoured by a stable air mass.

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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
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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
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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.

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QUESTIONS
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.

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