Understanding Air Masses
An air mass is a large section of the atmosphere that has relatively uniform characteristics of temperature and humidity measured horizontally. By 'large' we mean an extent of several thousands of kilometres. Temperature and humidity in an air mass may change to some extent as one travels through the mass, but any changes are gradual, and the mean profile of temperature and humidity remains as constant as can be expected in something as fickle as the sea or air that surrounds us.
The moisture content of air masses in North America is described using the
terms Continental or Maritime. A continental air mass is generally dry while a
maritime air mass contains more moisture. The air masses are also categorized
by latitude to describe the temperature profile. Tropical air masses are
warmer while polar and arctic air masses decrease in mean temperature. Both
terms are used in conjunction to give a descriptive name to the air mass. Therefore
a Maritime Tropical air mass is warm and moist while a Continental Arctic air
mass is cold and dry.
The six air masses that have been categorized
are:
Continental Arctic cA dry and cold
Maritime Arctic mA moist and cold
Continental Polar cP dry and moderate
MaritimePolar mP moist and moderate
Continental Tropical cT dry and warmer
Maritime Tropical mT moist and warmer
Of these six air masses, the four usually associated
with the atmosphere in Canada are the Continental Arctic, Maritime Arctic,
Maritime Polar and Maritime Tropical.
Continental Arctic (cA) Air Mass
The
source of cA air over Canada are the large snow and ice surfaces of northern
Canada, Siberia and the Arctic Ocean. In these areas there is little
insolation (energy delivered by the sun) and the air tends to be very stable.
The air is very dry but the relative humidity can be very high at the surface.
While this may sound confusing know that cold air holds very little moisture
even when saturated. Therefore while the air mass may show a relative humidity
of 85-90% at the surface, there is no great moisture content in the air. cA
air generally shows a pronounced temperature inversion below 4000 feet and a
slow decrease in temperature with height above this inversion. As cA air moves
southward it's temperature profile will modify as it moves over a warmer
surface. Increased turbulence and mixing in the surface layer can cause a
weakening of the inversion. Changes in the cA air mass as a result of
convection as the air moves southward over a snow-free surface can be very
rapid. When cA air moves over open water, convection can create intense snow
squalls.
Maritime Arctic Air
Mass
Maritime Arctic (mA) air is formed when cA air moves
over the Gulf of Alaska and is heated rapidly from below by the much warmer
ocean surface. Heat and moisture is rapidly transported aloft from the low
levels resulting in very unstable air as the temperatures above 500 millibars
still resemble the original cA air. In summer, mA air in Canada is usually
fomred in the north where the surface is covered by many swamps and lakes. As
this air moves south it develops a steep lapse rate. When an mA air mass
reaches the land mass of North America it may be modified in one of two
ways.
1 - In middle latitudes it may be as warm or even warmer
as the surface below it and little convective warming usually takes place.
Radiation inversions may develop at night and some surface layer warming due
to subsidence may result as the airmass moves over the coastal mountain
ranges.
2 - Below 40deg N as the air mass moves over the coastal
mountains and rockies the air may be warmed by subsidence in the low and
middle levels. It will dry and develop a dry adiabatic lapse rate in the
lowest 3000 to 4000 feet.
Maritime Polar (mP)
Air Mass
Maritime Polar air is formed after moving over a
long path across the mid-Pacific Ocean with water temperatures between 10 and
12 deg C. Usually the air mass has reached equilibrium with the underlying
surface and is therefore more stable than mA air. Usually this air mass is
greatly modified when it crosses the coast and the mountain ranges.
Maritime Tropical (mT) Air Mass
In
winter air of this type form in the subtropical highs in the Atlantic and
Pacific Oceans between 30 and 35 N latitude. In these areas the winds are
generally light and the surface flat and with a uniform water temperature in
the 18-24 deg C range.
Pacific mT air crosses the coast in northern
Mexico or California from the west to northwest and is cooled from below. Air
that starts in the Pacific is generally somewhat cooler in the lower levels
than the same air mass if it had entered from the Atlantic. Pacific mT air is
very stable in low levels and may have a dry lapse rate due to heating from
the surface as the air mass moves onshore.In summer, the subtropical highs are
normally 5-10 deg further north and water temperatures are higher. This tends
to create a sharp ridge of high pressure off the west coast in a north/south
direction and precludes the mT air from moving directly into the western
states as in winter.
Due to subsidence around the subtropical high
pressure areas there will frequently be an inversion at 700 millibars with a
rapid decrease in moisture through the inversion. This inversion disappears as
the air moves northward and moisture becomes equally distributed throughout
the air mass.
Atlantic mT air is very warm and humid and enters the
continent on the west side of the subtropical high. As it moves northward it
tends to ascend and develop instability aloft. In the lower levels the air
mass is cooled from below and off the east coast of Canada may develop a sharp
inversion.
Up to 500 millibars the charts suggest that mT air
entering the southwestern USA is of Atlantic origins versus Pacific.
Air Mass Clouds and Weather
Air
mass clouds and weather are influenced by the moisture content of the air, the
cooling processes operating on the air mass and the stability of the air mass.
Some air masses are very dry and consequently develop little in the way of
clouds. mT air, on the other hand, has a very high moisture content so that
clouds, fog and precipitation are often present. Even if air is moist, it's
temperature must be lowered in order to bring about condensation and therefore
cloud and precipitiation. This can occur when the air mass is in contact with
a cooler surface, via radiation cooling, advection over a colder surface or by
expansion through lifting (convection).
An example of radiation
cooling would be a clear night in the summer. The surface radiates away heat
which is not returned by cloud. This cools the surface layer of the air mass
and low level cloud or fog may develop. Advection over a colder surface
happens when an air mass moving northward over cool water or land is cooled
from below. As in radiation cooling, this creates an inversion with the
possible formation of fog or low cloud. Convective cooling takes place when an
air mass is lifted by crossing a mountain range (orographic lift). As the air
rises it's density decreases and it expands. This expansion cools the air till
condensation occurs.
Understanding Fronts
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