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Air masses are
parcels of air that bring distinctive weather features to the
country. An air mass is a body or 'mass'of air in which the
horizontal gradients or changes in temperature and humidity are
relatively slight. That is to say that the air making up the mass is
very uniform in temperature and humidity.
An air mass is
separated from an adjacent body of air by a transition that may be
more sharply defined. This transition zone or boundary is called a
front. An air mass may cover several millions of square kilometres
and extend vertically throughout the troposphere.
the Passing of the Front
with the Front
Passing of the Front
and getting colder|
||Levelling off then increasing
||Variable and gusty
rain or snow, hail sometimes
and cirrostratus changing later to cumulus and
The temperature of an
air mass will depend largely on its point of origin, and its
subsequent journey over the land or sea. This might lead to warming
or cooling by the prolonged contact with a warm or cool surface. The
processes that warm or cool the air mass take place only slowly, for
example it may take a week or more for an air mass to warm up by 10
°C right through the troposphere. For this to take place, an air
mass must lie virtually in a stagnant state over the influencing
region. Hence, those parts of the Earth's surface where air masses
can stagnate and gradually attain the properties of the underlying
surface are called source regions.
The main source
regions are the high pressure belts in the subtropics, which produce
tropical air masses, and around the poles, that are the source of
polar air masses.
Polar and tropical source
regions: The blue and red arrows show the polar and tropical regions
modification of air
As we have seen, it is in the source regions that
the air mass acquires distinctive properties that are the
characteristics of the underlying surface. The air mass may be cool
or warm, or dry or moist. The stability of the air within the mass
can also be deducted. Tropical air is unstable because it is heated
from below, while polar air is stable because it is cooled from
As an air mass moves away from its source region
towards the British Isles, the air is further modified due to
variations in the type or nature of the surface over which it
passes. Two processes act independently, or together, to modify an
An air mass that has a maritime track, i.e. a
track predominantly over the sea, will increase its moisture
content, particularly in its lower layers. This happens through
evaporation of water from the sea surface. An air mass with a long
land or continental track will remain dry.
Modification of air mass by land and ocean surfaces
A cold air mass
flowing away from its source region over a warmer surface will be
warmed from below making the air more unstable in the lowest layers.
A warm air mass moving over a cooler surface is cooled from below
and becomes stable in the lowest layers.
Fig 3: Modification of air mass
due to surface temperature
If we look at the
temperature profiles of the previous example, the effects of warming
and cooling on the respective air masses are very different.
Fig 4: Modified vertical temperature profiles (----- line) typical
of: a) tropical air cooled from below and b) polar air heated from
below on its way south. Note that where the air is heated from below
the effect is spread to a greater depth of the atmosphere.
weather in an air
Five basic types of air masses determine the
weather. They can bring anything from scorching heat to
bone-chilling cold depending on the type of air mass.
Long sea track
Very warm or
Moist in lowest
moist as Pm)
Fairly moist (not as
moist as Pm)
|Change of lapse
Clear, occasional thundery
Rain or snow
Often poor with coastal
Several fronts and
semipermanent high and low pressure systems characterize the Arctic.
The "polar front" marks the boundary between cold polar air masses
and warm tropical air masses. The polar front is intermittent rather
than continuous around the globe. The strength of the polar front
depends on the magnitude of the horizontal temperature gradient
across the front. Where the temperature gradient is steep, the front
is strong and is a potential site for cyclone or low pressure system
development. Where temperature contrast is small, the polar front is
Like the polar front,
the "arctic front" is discontinuous and depends on the temperature
contrast between two air masses. The arctic front is the boundary
between polar and arctic air masses and lies to the north of the
polar front. The arctic front can be as strong as the polar front.
It is particularly prominent during summer in northern
Semipermanent high and
low pressure systems ("highs" and "lows") are identified with
particular regions and have seasonal characteristics. In winter, the
Icelandic Low extends from near Iceland north into the Barents Sea,
and is associated with frequent cyclone activity. The Aleutian Low
is present in the Gulf of Alaska. The Beaufort-Chukchi Sea region is
dominated by a ridge of high pressure linking the Siberian High and
high pressure over the Yukon of Canada. In April and May arctic
pressure gradients decrease. The Icelandic and Aleutian lows weaken.
The Siberian High disappears, and is replaced by a wide but shallow
low. The Arctic High is centred over the Canadian Arctic
Archipelago. In summer, pressure gradients are generally weak.
Intermittently, however, cyclones enter the Arctic from northern
Eurasia and the north Atlantic, and tend to persist over the
Canadian Basin. By October the pattern has almost returned to the
winter configuration. The Icelandic and Aleutian lows strengthen, as
does the Siberian High.
Semi permanent Highs and
The Arctic is
characterized by "semi permanent" patterns of high and low pressure.
These patterns are semi permanent because they appear in charts of
long-term average surface pressure. They can be considered to
largely represent the statistical signature of where transitory high
and low systems that appear on synoptic charts tend to be most
This semi permanent
low pressure centre is located near the Aleutian Islands. Most
intense in winter, the Aleutian Low is characterized by many
strong cyclones. Travelling cyclones formed in the sub polar
latitudes in the North Pacific usually slow down and reach maximum
intensity in the area of the Aleutian Low.
This low pressure
centre is located near Iceland, usually between Iceland and
southern Greenland. Most intense during winter, in summer, it
weakens and splits into two centres, one near Davis Strait and the
other west of Iceland. Like its counterpart the Aleutian Low, it
reflects the high frequency of cyclones and the tendency for these
systems to be strong. In general, migratory lows slow down and
intensify in the vicinity of the Icelandic Low.
The Siberian High is
an intense, cold anticyclone that forms over eastern Siberia in
winter. Prevailing from late November to early March, it is
associated with frequent cold air outbreaks over east
The Beaufort High is
a high pressure centre or ridge over the Beaufort Sea present
mainly in winter.
The North American
High is a relatively weak area of high pressure that covers most
of North America during winter. This pressure system tends to be
centred over the Yukon, but is not as well-defined as its
continental counterpart, the Siberian High.
forming over open sea during the cold season within polar or
arctic air masses are called "polar lows." Typically several
hundred kilometers in diameter, and often possessing strong winds,
polar lows tend to form beneath cold upper-level troughs or lows
when frigid arctic air flows southward over a warm body of
Polar lows last on
average only a day or two. They can develop rapidly, reaching
maximum strength within 12 to 24 hours of the time of formation.
They often dissipate just as quickly, especially upon making
landfall. In some instances several may exist in a region at the
same time or develop in rapid succession.
In satellite imagery
polar lows show characteristic spiral or comma shaped patterns of
deep clouds, sometimes with an inner "eye" similar to those seen
in tropical cyclones. Convective cloud bands occupy the
surroundings (see figure below). Analysis of aircraft and
radiosonde data collected during field experiments reveals that
polar lows may possess warm cores. This finding, coupled with
their appearance in satellite imagery, has prompted some
investigators to refer to polar lows as "arctic hurricanes,"
although they seldom, if ever, possess hurricane strength
Polar lows are
difficult to predict even with current high resolution and high
performing operational numerical models, because they usually
occur in remote oceanic regions where data are too sparse to
define the model initial state on a sufficiently fine scale.
However, present-day models can depict synoptic-scale patterns
favourable to the development of the smaller scale systems,
allowing forecasters to use the predictions in conjunction with
satellite imagery and conventional observations to make subjective
forecasts of their occurrence.
A NOAA-9 polar orbiter satellite
image (visible band) of a polar low over the Barents Sea on 27
February 1987. The southern tip of Spitsbergen is visible at the
top of the image. The polar low is centred just north of the
Norwegian coast. Image contributed by S. Businger, Department of
Meteorology, University of
The polar vortex is a
persistent large-scale cyclonic circulation pattern in the middle
and upper troposphere and the stratosphere, centred generally in the
polar regions of each hemisphere. In the Arctic, the vortex is
asymmetric and typically features a trough (an elongated area of low
pressure) over eastern North America. It is important to note that
the polar vortex is not a surface pattern. It tends to be well
expressed at upper levels of the atmosphere (that is, above about
A front is defined as the transition
zone between two air masses of different density. Fronts extend not
only in the horizontal direction, but in the vertical as well.
Therefore, when referring to the frontal surface (or frontal zone),
we referring to both the horizontal and vertical components of the
A cold front is
that part (or parts) of a frontal system along which cold air is
advancing and is coloured blue on the weather map.
A warm front is that part (or
parts) of a frontal system along which cold air is retreating and is
coloured red on the weather map.
types of front
A warm front is defined as the transition
zone where a warm air mass is replacing a cold air mass. Warm fronts
generally move from southwest to northeast and the air behind a warm
front is warmer and more moist than the air ahead of it. When a warm
front passes through, the air becomes noticeably warmer and more
humid than it was before.
Symbolically, a warm
front is represented by a solid line with semicircles pointing
towards the colder air and in the direction of movement. On coloured
weather maps, a warm front is drawn with a solid red line.
There is typically a
noticeable temperature change from one side of the warm front to the
other. In the map of surface temperatures below, the station north
of the front reported a temperature of 53 degrees Fahrenheit while a
short distance behind the front, the temperature increased to 71
degrees. An abrupt temperature change over a short distance is a
good indication that a front is located somewhere in between.
If warmer air is
replacing colder air, then the front should be analyzed as a warm
front. If colder air is replacing warmer air, then the front should
be analyzed as a cold front. Common characteristics associated with
warm fronts have been listed in the table below.
||cool-cold, slow warming
||warmer, then steady|
||slight rise, followed by
||in this order: Ci, Cs, As, Ns, St, and
fog; occasionally Cb in summer
||clearing with scattered Sc; occasionally
Cb in summer|
||light-to-moderate rain, snow, sleet, or
||drizzle or none
||usually none, sometimes light rain or
||poor, but improving
||fair in haze|
As a mass of warm air
advances on a retreating mass of cold air, the warm air, being
lighter, ascends over the cold air in a long gentle slope. As a
result, the cloud formation associated with the warm frontal system
may extend for 500 or more nautical miles in advance of it. Warm
fronts usually move at relatively slow speeds and therefore affect a
vast area for a considerable length of time.
If the warm air is
moist and stable, stratiform clouds develop in a distinctive
sequence. The first signs of an approaching warm front are high
cirrus clouds which thicken to cirrostratus and altostratus as the
warm front approaches. The ceiling gradually falls and there follows
a long belt of steady rain falling from heavy nimbostratus cloud.
Precipitation may lead the frontal surface by as much as 250
If the warm air is
moist and somewhat unstable, cumulonimbus and thunderstorms may be
embedded in the stratiform layers. Heavy showers in advance of the
surface front can then be expected.
Very low stratus
clouds and fog throughout the frontal zone are typical
characteristics of warm fronts.
The passing of the
warm front is marked by a rise of temperature, due to the entry of
the warm air, and the sky becomes relatively clear.
cold front is defined as the transition zone where a cold air mass
is replacing a warmer air mass. Cold fronts generally move from
northwest to southeast. The air behind a cold front is noticeably
colder and drier than the air ahead of it. When a cold front passes
through, temperatures can drop more than 15 degrees within the first
cold front is represented by a solid line with triangles along the
front pointing towards the warmer air and in the direction of
movement. On coloured weather maps, a cold front is drawn with a
solid blue line.
There is typically a noticeable temperature
change from one side of a cold front to the other. In the map of
surface temperatures below, the station east of the front reported a
temperature of 55 degrees Fahrenheit while a short distance behind
the front, the temperature decreased to 38 degrees. An abrupt
temperature change over a short distance is a good indicator that a
front is located somewhere in between.
colder air is replacing warmer air, then the front should be
analyzed as a cold front. On the other hand, if warmer air is
replacing cold air, then the front should be analyzed as a warm
front. Common characteristics associated with cold fronts have been
listed in the table below.
||minimum, then sharp rise
||increasing: Ci, Cs and Cb
||short period of showers
||heavy rains, sometimes with hail,
thunder and lightning
||showers then clearing|
||fair to poor in haze
||poor, followed by
||good, except in showers|
||high; remains steady
When a mass of cold air
overtakes a mass of warm air, the cold air being denser, stays
on the surface and undercuts the warm air violently. Surface
friction tends to slowdown the surface air while a sharp fall
in temperature, a rise in pressure and rapid clearing usually
occur with the passage of the cold front.
advancing cold front will be relatively slow moving. Because
it does not undercut the warm air so violently, a rather broad
band of clouds develops extending a fair distance behind the
frontal surface. If the warm air is stable, these clouds will
be stratiform; if the warm air is unstable, they are
cumuliform and possibly thunderstorms. With passage of the
frontal surface, clearing is more
generally some part of a front along which the colder air is
neither advancing nor retreating. There is no motion to cause
the front to move because the opposing air masses are of equal
pressure. The surface wind tends to blow parallel to the front
and the weather conditions are similar to those associated
with a warm front although generally less intense and not so
extensive. Usually a stationary front will weaken and
eventually dissipate. Sometimes, however, after several days,
it will begin to move and then it becomes either a warm front
or a cold front.
A noticeable temperature change and/or
shift in wind direction is commonly observed when crossing
from one side of a stationary front to the other.
In the map above, temperatures south of
the stationary front were in the 50's and 60's with winds
generally from the southeast. However, north of the stationary
front, temperatures were in the 40's while the winds had
shifted around to the northeast. Cyclones migrating along a
stationary front can dump heavy amounts of precipitation,
resulting in significant flooding along the
progress of time as a depression advances, the cold front
gradually overtakes the warm front and lifts the warm sector
entirely from the ground. It is simply a case of the cold air
catching up with itself as it flows around the depression.
Thus only one front remains, which is called an occluded front
or occlusion. An occluded depression soon commences to fill up
and die away.
cyclone typically has a preceding warm front (the leading edge
of a warm moist air mass) and a faster moving cold front (the
leading edge of a colder drier air mass wrapping around the
storm). North of the warm front is a mass of cooler air that
was in place before the storm even entered the
As the storm
intensifies, the cold front rotates around the storm and
catches the warm front. This forms an occluded front, which is
the boundary that separates the new cold air mass (to the
west) from the older cool air mass already in place north of
the warm front. Symbolically, an occluded front is represented
by a solid line with alternating triangles and circles
pointing the direction the front is moving. On coloured
weather maps, an occluded front is drawn with a solid purple
temperature, dew point temperature, and wind direction can
occur with the passage of an occluded front. In the map below,
temperatures ahead (east of) the front were reported in the
low 40's while temperatures behind (west of) the front were in
the 20's and 30's. The lower dew point temperatures behind the
front indicate the presence of drier air.
wind shift also occurred across the occluded front. East of
the front, winds were reported from the east-southeast while
behind the front, winds were from the west-southwest. Common
characteristics associated with occluded fronts have been
listed in the table below.
||in order: Ci, Cs, As,
||Ns, sometimes Tcu and Cb
||Ns, As or
||light, moderate or heavy
||light, moderate or heavy continuous
precipitation or showers
followed by general clearing|
||poor in precipitation
||poor in precipitation
||usually slight drop, especially if
slight drop, although may rise a bit
The cold air, in
the distance it has travelled, may have undergone considerable
change. Therefore it may not be as cold as the air it is
overtaking. In this case (cool air advancing on colder air),
the front is known as an occluded warm front or a warm
occlusion and has the characteristics of a warm front, with
low cloud and continuous rain and drizzle. It the warm air is
unstable, heavy cumulus or cumulonimbus cloud may be embedded
in the stratiform cloud bank.
It the cold air
is colder than the air it is overtaking (cold air advancing on
cool air), the front is known as an occluded cold front or a
cold occlusion. A cold occlusion has much the same
characteristics as a warm front, with low cloud and continuous
rain. If the warm air is unstable, cumulonimbus and
thunderstorms are likely to occur, with the violent
turbulence, lightning and icing conditions associated with
It will be noted that in the
case of either a warm or cold occlusion, three air masses are
present, a cool air mass advancing on a cold air mass, or a
cold air mass advancing on a cool air mass, with, in either
case, a warm air mass lying wedge shaped over the colder air.
This wedge shaped mass of warm air is known as a trowel in
Canada. (In some other countries, such as the US, it is called
an upper front.)
In Canada, the
term upper front refers to a non-occlusion situation.
Sometimes, cold air advancing across the country may encounter
a shallow layer of colder air resting on the surface or
trapped in a topographical depression. The advancing cold air
rides up over the colder, heavier air. The cold front which is
the leading edge of the advancing cold air, therefore, leaves
the ground and moves along the top of the colder air. It is
then known as an upper cold front.
structure of the advancing cold front is such that the cold
air forms a shallow layer for some distance along the ground
in advance of the main body of cold air. The frontal surface
If the main mass of cold air, in this situation, will usually
be very steep. The line along which the frontal surface
steepens is also known as an upper cold front.
On occasion, an
advancing warm front rides up over a pool or layer of cold air
trapped on the ground. A station on the ground does not
experience a change of air mass because the front passes
overhead. This is known as an upper warm front.
surface of the cold air that is retreating ahead of an
advancing warm front is almost flat for some distance ahead of
the surface front and then steepens abruptly. The line along
which the surface of the retreating cold air steepens sharply
is also called an upper warm front.
The theory of
the polar front, which for the sake of simplicity has been
described in the form of its original conception, might leave
the impression that depressions form only along some well
defined line lying somewhere midway between the poles and the
equator. Air masses are in a constant state of formation over
all the land and water areas of the world. Once formed, they
tend to move away from the source regions over which they
form. The same frontal processes and phenomena occur whenever
a mass of warm air and a mass of cold air come in
There is a
widespread impression among pilots that fronts always bring
bad weather and that all bad weather is frontal. Actually some
fronts have little or no weather associated with them. A
slight change of temperature and a windshift may be the only
evidence that the front has gone through. And, of course, bad
weather can develop without the passage of a front. Fog, for
example, generally occurs when no fronts are present and
severe thunderstorms may develop in an air mass, which has no
misconception is that the front is a thin wall of weather.
This false idea is perhaps occasioned by the line that
indicates a front on a weather map. The line on the map only
shows the surface location at which the pressure change,
windshift and temperature change occur. The actual weather
associated with the front may extend over an area many miles
in width, both well ahead and also for many miles behind the
actual line on the weather map.
A front itself
is actually a transition zone between two large air masses
with different properties of temperature and moisture. Each
individual air mass may extend over hundreds of thousands of
square miles. Everywhere along the boundary of an air mass,
where it overrides or undercuts the air mass upon which it is
advancing and for a considerable height upward from the
surface as well, there is a frontal zone. The frontal weather
associated with the front, therefore, can be expected to
extend for hundreds of miles along the boundary of the air
Frontogenesis means a front, which is
increasing in intensity.
Frontolysis means a front, which is decreasing in
If you examine
the diagrams showing fronts on a weather map, you will notice
that all fronts lie in regions of lower pressure. The isobars
are bent sharply at a front. These two factors are
characteristic of all fronts.
at the cold front
Cold fronts are
not all the same. The weather associated with a cold front may
vary from a minor windshift to severe thunderstorms, low
ceilings, restricted visibility and violent gusty winds. The
severity of the weather is determined by the moisture content
and stability of the warm air mass that the cold air mass is
undercutting and the speed of the advancing cold
Fast moving cold
fronts may travel across the country with a speed of 30 knots
or more. If the warm air that is being undercut by the
cold air mass is very moist and unstable, towering cumulus
clouds and thunderstorms are likely to develop. Heavy rain or
hail may be associated with the front. A slower moving cold
front advancing on more stable and drier air in the warm
sector will produce less severe weather conditions, stratus or
altocumulus clouds with light or no precipitation.
A long line of
cumulus clouds on the western horizon is usually an indication
of an approaching cold front. Sometimes a deck of altocumulus
cloud or decks of stratus and stratocumulus extending ahead of
the front will mask the main frontal cloud from the view of
the high flying or low flying pilot respectively.
Wind: The wind
direction will always veer as the front passes. Gustiness may
be associated with the windshift.
Temperature: On the ground, the temperature may
drop sharply as the front passes, but usually it drops
gradually. The air immediately behind the front has been
warmed in passing over the warm ground. Therefore, it may be
several hours before the temperature drops to the true value
of the cold air mass. In passing through a cold front, there
will be a noticeable temperature change when passing through
the frontal surface.
Visibility: Visibility usually improves after
passage of a cold front. If the front is moving fairly
rapidly, the width of frontal weather generally is less than
Pressure: The approach of a cold front is
accompanied by a decrease in pressure. A marked rise will be
noticed when the front has passed.
Turbulence: Turbulence may be associated with the
cold front if it is active, although thunderstorms are not
always present. Even in cases where there are no clouds,
turbulence may be a problem.
Precipitation: The frontal rain or snow is usually
narrow, especially if it is showery in character.
A long line of
squalls and thunderstorms which sometimes accompanies the
passage of a cold front is called a line squall (or squall
line). It is usually associated with a fast moving cold front
that is undercutting an unstable warm air mass. It may form
anywhere from 50 to 300 nautical miles in advance of the front
itself. The line squall is a long line of low black, roller
like cloud, which often stretches in a straight line for
several hundred miles, and from which heavy rain or hail falls
for a short time. Thunder and lightning frequently occur. The
squall is also accompanied by a sudden wind change from
southerly or south-westerly to north or north-westerly,
together with a sudden drop in temperature and a rise in
barometric pressure. The actual wind squall lasts only for a
few minutes but is often extremely violent, constituting a
serious menace both to shipping and to airplanes. The signs
indicating the approach of a line squall are unmistakable.
at the warm front
changes are usually less pronounced than cold front changes.
The change is also generally very gradual. However, the
weather at a warm front is usually more extensive and may
cover thousands of square miles. A wide variety of weather
characterizes warm fronts. The weather may even vary along a
The degree of
overrunning and the moisture content and stability of the
overrunning warm air determine the seventy of the weather. If
the warm air is very moist, the cloud deck forming in the
overrunning air may extend for hundreds of miles up the slope
of the retreating cold air. It the warm air is unstable,
thunderstorms may be embedded in the cloud deck.
cloud is the first sign of the approach of an active warm
front. Cirrostratus soon follows (the high thin cloud which
causes a halo around the sun or moon). The cloud gradually
thickens and the base lowers until a solid deck of
altostratus/altocumulus covers the area. Low nimbostratus
moves in, merging with the altostratus. With the result that a
solid deck of cloud extending from near the surface to 25,000
feet or more covers the whole area. Precipitation is usually
Windshift: With the passage of a warm front, the
wind will veer, but the change will be much more gradual than
in the case of a cold front.
Temperature: The warm front brings a gradual rise
in temperature. A pilot flying through the frontal surface
will notice a more abrupt temperature rise.
Visibility: Low ceilings and restricted
visibility are associated with warm fronts and, because warm
fronts usually move quite slowly, these conditions persist for
When rain falls
from the overrunning warm air, masses of irregular cloud with
very low bases form in the cold air. Fog is frequently a
condition 50 nautical miles ahead of an advancing warm
Turbulence: Cumulonimbus clouds are frequently
embedded in the main cloud deck and these storms are
responsible for the most severe turbulence associated with a
warm front. However, these storms and the turbulence they
occasion are less severe than those associated with cold
fronts. The principal problem with these storms is that they
cannot be located by sight since they are embedded in the main
Precipitation: The first precipitation begins in the
region where the altostratus layer of cloud is from 8000 to
12,000 feet above the ground. As the front approaches, the
precipitation becomes heavier. Occasional very heavy
precipitation is an indication of the presence of
In winter, when
temperatures in the cold air are below freezing and
temperatures in the lower levels of the warm air are above
freezing, snow and freezing rain can be expected.
Snow falls from
that part of the warm air cloud that is high and therefore
below freezing in temperature. From the lower cloud, where
temperatures are above freezing, rain falls. However, as the
rain falls through the cold air (of the cold air mass that the
warm air is overrunning), it becomes supercooled and will
freeze on contact with any cold object. This is known as
freezing rain (ZR).
In the area
ahead of the freezing rain, there is a region where the rain
falling through the cold air becomes sufficiently supercooled
to freeze and falls to the ground as ice pellets (IP).
weather at trowals
and upper fronts
The weather that occurs with a trowal is a
combination of cold and warm front conditions. The cloud
pattern ahead of the approaching trowel is similar to that of
a warm front. Cold front cloud formations will exist behind
it. Cumulus build-ups and thunderstorms are likely to be
interspersed with stratiform clouds, continuous precipitation
and widespread low ceilings. In winter months, freezing rain
and severe icing conditions are likely hazards as the rain
aloft in the occluded warm air falls through the freezing
temperatures of the ground based cold sectors. The maximum
precipitation, convective activity and icing conditions
usually occur in the northeast sector of the low and extend
some 50 to 100 miles ahead of the occluded front.