AIR MASSES AFFECTINGTHE BRITISH ISLES
The British Isles experiences varied weather, which is 
brought about by winds which drive air masses from 
their source region. 
At their source they have certain characteristics, e.g. 
wet or dry, hot or cold. 
On their journey (path) air masses are modified. 
When these air masses reach British Isles they 
influence local weather conditions.
The table below summarises the typical Air mass characteristics typical characteristics 
of the six main air masses which 
Tropical 
Continental 
(Tc) 
Polar 
Continental 
(Pc) 
Tropical 
Maritime 
(Tm) 
Polar 
Maritim 
e (Pm) 
Arctic 
Maritim 
e (Am) 
Returning 
Polar 
Maritime 
(rPm) 
Summer Winter Long 
Sea 
Track 
Short Sea 
Track 
Expose 
d 
Sheltere 
d 
Temp Very 
warm or 
hot 
Average Cold Very cold Near 
sea 
temp 
Warm Rather 
cold 
Cold 
(colder 
than 
Pm) 
Warm 
(warmer 
than Pm) 
Humidity Relatively 
dry 
Rather 
moist 
Moist 
in 
lowest 
layers 
Very dry Very 
moist 
Moist Moist Fairly 
moist 
(not as 
moist as 
Pm) 
Fairly moist 
(not as 
moist as 
Pm) 
Change 
of Lapse 
Rate 
Little 
change 
Cooled 
from 
below 
Heated 
from 
below 
Little 
change 
Cooled 
from 
below 
Warmed 
in 
summer 
Heated 
from 
below 
Heated 
from 
below 
Heated 
from below 
Stability Generally 
stable 
Stable Unstabl 
e 
Stable Stable Stable 
aloft 
Unstable Unstable Unstable 
Weather Clear, 
occasional 
thundery 
showers 
Clear Rain or 
snow 
shower 
s 
Clear Low 
cloud, 
drizzle 
Broken 
cloud, 
dry 
Variable 
cloud, 
showers 
Showers 
(mainly 
coastal) 
Showers 
(mainly 
coastal) 
Visibility Moderate 
or poor 
Moderate 
or poor 
Good Moderate 
or poor 
Often 
poor 
with 
coastal 
Moderat 
e 
Good Very 
good 
Very good 
affect the British Isles.
Air Masses Affecting the British Isles

Air Masses Affecting the British Isles

  • 1.
  • 2.
    The British Islesexperiences varied weather, which is brought about by winds which drive air masses from their source region. At their source they have certain characteristics, e.g. wet or dry, hot or cold. On their journey (path) air masses are modified. When these air masses reach British Isles they influence local weather conditions.
  • 11.
    The table belowsummarises the typical Air mass characteristics typical characteristics of the six main air masses which Tropical Continental (Tc) Polar Continental (Pc) Tropical Maritime (Tm) Polar Maritim e (Pm) Arctic Maritim e (Am) Returning Polar Maritime (rPm) Summer Winter Long Sea Track Short Sea Track Expose d Sheltere d Temp Very warm or hot Average Cold Very cold Near sea temp Warm Rather cold Cold (colder than Pm) Warm (warmer than Pm) Humidity Relatively dry Rather moist Moist in lowest layers Very dry Very moist Moist Moist Fairly moist (not as moist as Pm) Fairly moist (not as moist as Pm) Change of Lapse Rate Little change Cooled from below Heated from below Little change Cooled from below Warmed in summer Heated from below Heated from below Heated from below Stability Generally stable Stable Unstabl e Stable Stable Stable aloft Unstable Unstable Unstable Weather Clear, occasional thundery showers Clear Rain or snow shower s Clear Low cloud, drizzle Broken cloud, dry Variable cloud, showers Showers (mainly coastal) Showers (mainly coastal) Visibility Moderate or poor Moderate or poor Good Moderate or poor Often poor with coastal Moderat e Good Very good Very good affect the British Isles.