Geo Factsheet
January 1997
Number 5
An Idiot's Guide To Air Masses
Air Masses are often a source of great confusion even for otherwise strong candidates. This Factsheet will describe in simple
terms the origins and nature of the air masses off the British Isles. Future Factsheets will cover what happens when different air
masses meet.
What Is An Air Mass?
Fig 2. Source Regions Of Three Air Masses Types
An Air Mass is a large body of air in which temperature and humidity
are almost uniform at its surface. Temperature and humidity change
with height but at any horizontal level almost uniform conditions of
temperature and humidity occur (See Fig 1.)
Pc
Pc
TM
Tc
Tm
Tc
Tm
Tm
TM
Fig 1. An Air Mass
j
100's of Km across
How Are Air Masses Classified?
1. One air mass has a different surface
temperature or humidity from another.
So air masses are classified by their surface
temperatures and humidities.
a) Tropical air masses are warm at their
surface.
b) Polar air masses are cool at their surface.
c) Maritime air masses are moist at their
surface.
d) Continental air masses are dry at their
surface.
Note: polar air masses do not form at the
poles. Cold air masses that originate near
the poles are called Arctic.
2. Surface temperature and surface humidity
characteristics are then combined to
identifiy an air mass type. For example one
type of air mass could be tropical maritime,
another could be tropical continental.
Note: Tropical continental is the same as
continental tropical, tropical maritime is the
same as maritime tropical and so on.
i
Tc=Tropical Continental
Tm
Tm=Tropical maritime
Surface winds blow outwards from surface high pressure areas so air masses
move from their source regions. As an air mass moves it is modified by the
nature of the surface over which it moves. For example, when tropical
maritime air moves polewards, its surface layer is cooled.
Polar continental air which moves eastwards from Canada, or eastwards
from northern Asia, moves over large ocean
areas (the Atlantic and Pacific respectively) so
Classification of Selected air Masses
its surface layers become more moist and
warmer. Such a large surface area is modified
that the air is identified as a distinct air mass
Surface Humidity
Surface Temperature
called polar maritime. Its source regions are not,
Moist
Dry
usually, areas of high pressure or light winds.
Warm
Tropical
Maritime
(Tm)
Tropical
Continental
(Tc)
Cool
Polar
Maritime
(Pm)
Polar
Continental
(Pc)
The directions from which these types of air
mass often reach the British Isles are shown in
Fig 3.
Fig 3. Air Masses Affecting the
British Isles
Where do air masses originate?
The area where an air mass forms is called
The Source Region. The surface air
temperature and humidity of each type of air
mass is related to it's source region. For example,
low latitude ocean areas result in tropical
maritime air. The source regions of Tm, Tc, and
Pc air masses all occur in areas where surface
atmospheric pressure is high and where wind
speeds are unusually light. Slow moving air has
time to become almost uniform in it's horizontal
air temperatures and humidities, and thus
becomes an air mass.
1
Pm
k
Tm
Pc
Tc
Tm
Pm
Pc
k
AIR
Either ocean or continent
January
Pc=Polar Continental
Tm
Tm
l
but these are different from those of other levels
k
temperature
and
humidity
change
with height
k
{
almost uniform temperature and humidity at any
one level
Pc
Tm
Tc
Polar Continental
Tropical Continental
Tropical Maritime
Polar Maritime
Geo Factsheet
An Idiot's Guide To Air Masses
This Factsheet discusses only the weather that
occurs when a particular air mass reaches a
British coast. The weather at high altitudes or
in inland areas will differ in detail and will be
the subject of future Factsheets.
Polar Maritime Air Mass
1) Winter air temperatures
are cool, but not cold. Typical daily
maximum temperatures are eight degrees
in January. Pm is cool because it originated
in high latitudes, but not cold because its
surface layers have been warmed by its long
passage over the Atlantic ocean (which
stores heat in winter). Pm has also moved
over the North Atlantic Drift (a warm ocean
current).
2) Summer air temperatures
are warm but not hot. Typical daily
maximum temperatures are sixteen degrees.
Temperatures are, at best, only warm
because Pm air arrives after a long trawl
over the Atlantic ocean which, in summer,
is cooler than continents in the same
latitude.
3) In both winter and summer
showers occur. In coastal areas showers are
more frequent in winter than in summer
because the sea is a store of warmth in
winter; warm seas encourage convection.
Some reasons for showers
a) Convection causes localised uplift of Pm
air
b) Pm is unstable. This means that once some
Pm air is set in vertical motion, the air soon
reaches a height beyond which the air will
continue to rise of its own accord. This
happens when the air is warmer and so less
dense than the surrounding non-rising air.
Some reasons why Pm is unstable
1). When Pm air is NOT in vertical motion,
temperatures decrease rapidly with
height.
2). When part of the Pm air rises and when
it has cooled sufficiently for water vapour
to condense, the rate of cooling of the
rising air becomes slower than when the
air was dry.
3). Thereafter, at any given height, the rising
air is warmer than the non-rising Pm air.
4). The warmer rising air is less dense than
the non-rising air so the rising air
continues to rise. [Note to examination
candidates: these ideas can be expressed
as lapse rates. But you should use lapse
rates only when you can clearly explain
them].
[The rising air will continue to ascend until
its temperature is the same as the
temperature of the surrounding air].
c) As Pm is unstable, rising Pm air can reach
heights where it is cold enough for ice to
form. When ice particles are sufficiently
large they can fall to the ground as a hail,
snow or (if the ice melts on the way down)
as a rain shower. Between showers, skies
are clear with daytime sunshine.
2) Visibility (between showers) is excellent
because Pm air rises so readily that solid
particles in the air are dispersed.
Tropical Maritime Air Mass
1) Winter air temperatures are warm, typical
daily maximum temperatures are eleven degrees
in January. The air mass is warm because the
air mass formed in low latitudes. Though surface
cooling occurs as the air moves polewards, Tm
air still reaches the British Isles as a warm air
mass.
2) Summer air temperatures are also warm;
typical daily maximum sixteen to eighteen
degrees in July depending on whether the sun
breaks through the low cloud. The reasons are
similar to those for winter temperature.
3) Sea fog is a common occurrence (especially
in early summer), because Tm air is very moist
and as the Tm air moves polewards its surface
layer cools. Cooling leads to the water vapour
in the air condensing as liquid water droplets
(or fog).
4) Sometimes drizzle occurs from the often
persistent low cloud, otherwise there is no
precipitation (rain, hail, snow). This is because
Tm is a stable air mass. A stable air mass will
rise only as long as some external force causes
it to rise: it will not rise of its own accord. [Tm
air is stable because it cools only slowly with
height, the reverse of Pm air - see earlier].
5) Visibility is poor because of any sea fog that
exists and partly because solid particles remain
in the air near the ground. The particles are not
dispersed because Tm does not rise readily.
Tropical Continental Air Mass
1) Winter air temperatures are warm with
typical daily maximum values of about thirteen
degrees in January. This is because the air mass
originated in North Africa, which even in
winter is warm. Some surface cooling occurs as
the air moves to the British Isles but it is
insufficient to make the air cool.
2) Summer air temperatures are hot, typical
daily maximum values in July are at least twenty
2
five degrees. Tc is hot because the air blows
over the hot French land surface.
3) Usually the weather is dry since temperature
slowly decreases with height. Occasionally, cool
air lies above the Tc air. The upper atmosphere
is accordingly unstable and thunderstorms may
then break out.
4) Visibility is only moderate. Even though fog
does not usually form (because the air is dry)
solid particles are not easily dispersed through
the air because Tc air does not rise readily.
Furthermore, dust carried from Spain or even
North Africa and anthropogenic pollutants in
Spain or France also reduce visibility. Hence,
whilst skies are usually clear, the atmosphere
is hazy.
Polar Continental Air Masses
1) Winter air temperatures are cold, daily
maximum temperature in January may barely
reach nought degrees, because the air mass
originated over the cold land mass of Eastern
Europe. The surface layers of Pc air are slightly
warmed as the air mass passes over the North
Sea but the North Sea is insufficiently wide for
the air to become cool, let alone warm.
2) Pc air does not reach the British Isles in
summer
3) On the east coast, snow showers occur in
winter. These happen because Pc is modified
as it passes over the North Sea. The surface
layers of Pc air are warmed and gain moisture.
So whilst Pc was stable over mainland Europe,
by the time the air reaches eastern England it
has become unstable. Once convection (from the
relatively warm North Sea) has caused some of
the Pc air to rise, it will soon reach heights where
the air will rise readily of it's own accord. Ice
particles will then grow in the turbulent air and
lead to snow showers. Between showers the sky
is clear with daytime sunshine.
4) Visibility (between any showers) is good as
solid particles are dispersed in the rising air.
Acknowledgements;
This Geo Factsheet was researched and
written by Anthony Silson.
Geo Press
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