Atmospheric Circulation
I. Importance of Air Movement:
The atmosphere system is a dynamic system characterised by ceaseless air motion on a great
variety of spatial and temporal scales.
Air motion may be resolved into two components: horizontal and vertical . Horizontal
movement, or wind, is by far the faster and consists of air movement parallel to the surface. Vertical
motions, on the other hand, involve sinking rising masses of air perpendicular to the surface and are
usually 100 - 1000 times slower than their horizontal counterparts.
1. Horizontal Movement:
Horizontal movement is an important climatic factor to understand for a number of
reasons.
First, wind action physically relocates warm and cold bodies of air, thereby modifying
the thermal characteristics bestowed upon places by their radiation regime. Such modification
may have a considerable effect on the temperature of a place, as can be seen from Figure
below:
Second, wind action transport water vapour, the source of life - giving precipitation.
In particular, moisture is brought from areas where it is abundant, such as over the oceans, to
areas where it is often deficient, such as over the interiors of continents. Below figure
illustrates the significance of a seasonal reversal of wind direction in rainfall amounts for
Bombay in India.
Third, air in rapid motion is, on occasion, a sever environmental hazard. On average,
more lives are lost each as a result of tropical storms than from the combined effects of fire,
lightning, floods, tidal waves and earthquakes.
2. Vertical Movement:
Vertical movements of air, although normally less rapid than their horizontal
counterparts, are no less important, since they strongly influence whether the climate and
weather will be cloudy and rainy or clear and dry.
3. Air motion and the Global Energy Budget:
The unequal heating of the earth's surface by the sun produces a latitudinal contrast in
energy budgets. Between about 40oN and 40oS. The amount of incoming radiation exceeds
that lost by the cooling of the earth-atmosphere system, whereas towards the poles the reverse
applies. Obviously, if such a situation persisted over very long time periods, it would cause
the low latitudes to be very much hotter than they are at present, and the high latitudes to be
very much more frigid. The fact that this is not so implies the existence of a mechanism
whereby heat is moved from the surplus areas to the deficit areas to compensate for the
shortfall in the energy budget of the latter. This vital role, without which only a small
proportion of the globe would be habitable, is fulfilled by a global system of wind circulation.
II. Forces that Control Atmospheric Motion:
Atmospheric motion is controlled by the interplay between four forces
a. the pressure-gradient force,
b. the coriolis force,
c. the centripetal force, and
d. friction.
1. Pressure-Gradient Force:
In moving about randomly, gas molecules collide with each other and with any
surface which confines them. The push exerted by this continuous bombardment is known as
pressure. Pressure can be considered as resulting from the weight of overlying air pressing
down on a given area.
A pressure gradient exists both vertically and horizontally. Pressure decreases
vertically, as we move upwards through the atmosphere, the weight of overlying air
diminishes. It varies laterally because of differences in the intensity of solar heating of the
atmosphere. Where solar radiation is intense the air warms up, expands and its density
declines. Air pressure falls. Where cooling occurs, the air contracts, its density increases and
air pressure becomes greater.
Pressure Gradient (vertical)
Pressure gradient (horizonal)
But the pattern of air pressure close to the surface is reversed in the upper atmosphere
because cold air contracts, the upward decline in pressure is rapid and at any constant height
above a zone of cool air the pressure is relatively low.
Conversely, warm air expands and rises, so that vertical pressure gradient is less steep.
Above areas of warm air, therefore, the pressure tends to be relatively high.
Air motion is initiated by a pressure gradient between places, with initial movement
occurring from high to low-pressure locations. The air ought to move at right angles to the
isobars.
Isobars are lines joining points that have identical air pressure, and may be used to
identify pressure features and pressure gradient.
The change in barometric pressure across a horizontal surface constitutes a pressure
gradient. Where a pressure gradient exists, air molecules tend to drift in the same direction as
that gradient. This tendency for mass movement of air is referred to as the pressure gradient
force. The magnitude of the force is directly proportional to the steepness of the gradient. The
larger pressure gradient, the faster wind speed, so it is the basic force of the atmospheric
circulation.
2. The Coriolis Force:
It is produced by earth rotation. The force tends to turn the flow of air. The direction
of action of the coriolis force is stated in Ferrel's Law: "Any object or fluid moving
horizontally in the northern hemisphere tends to be deflected to the right of its path of motion.
In the southern hemisphere a similar deflection is toward the left of the path of motion."
The degree of the deflecting force
varies with the speed of the moving air
with latitude. The faster the wind, the
more ground it covers in a given time and
the greater the effect of rotation can be.
Near to the equator where the earth's
surface is spinning round in a plane
almost parallel to the axis of rotation, the
coriolis force is very slight. In higher
latitudes, it has marked effects. Coriolis
force is greatest at the poles where the
earth's surface is at right angle to the axis
of rotation.
With the combined effects of the pressure gradient force and the coriolis force
(Pressure-gradient force is equal to Coriolis force with opposite direction), the resultant wind
will flow in a direction at
right angles to the pressure
gradient, i.e. parallel with
the isobars - Geostrophic
wind.
3. Centripetal Force:
This force applies when the isobaric pattern is markedly curved,
eg. within cyclonic systems or around high pressure centers.
When air is following a curved path, a
force is acting centripetally, pulling
the air inwards. Wind which is in
balance with these three forces is
known as the gradient winds.
a. Motion around a low pressure
area:
(Anti-clockwise in the northern hemisphere. Cyclonic condition. The result of the
centripetal effect is to make the coriolis force weaker than the pressure gradient force.
The wind is, therefore subgeostrophic)
b. The anticyclonic flow in the high pressure case:
(Clockwise. Supergeostrophic since the coriolis force exceeds the pressure gradient
force.)
4. Frictional Force:
In the lower parts of atmosphere, normally below 750 m, the frictional drag exerted
by the ground on the air flow above has an effect on the balance of the other wind forces, as
friction acts as a force pulling against the direction of flow.
Friction lessens the speed of the wind, and so weakens the coriolis force, the pressure
of gradient force asserts its greater strength by causing the air to flow more towards low
pressure. Therefore surface winds flow at a slight angle to the isobars.
III. Microscale Movement:
1. Winds above the Friction Layer:
Above the influence of surface frictional drag, air movement is controlled by two
forces: the horizontal pressure-gradient force and the Coriolis force. (Geostrophic Wind)
Figure shows how a parcel of air in the northern hemisphere would respond to the two
controlling forces. From its starting point, the
air would begin to move in response to the
pressure gradient from high to low pressure.
Once it begins to move, however, it becomes
subject to the Coriolis force, which displaces it
to the right of its trajectory (path). As the
parcel speeds up as a result of the continued
presence of the pressure-gradient force, the
Coriolis force also intensifies. As the parcel of
air accelerates, the magnitude of the deflection
grows, until the air parcel is moving parallel to the isobars .
2. Winds within the Friction Layer:
One consequence of the frictional drag is that it disrupts the balance between the
horizontal pressure-gradient force and the Coriolis force. In so doing, this friction in the
lower layers alters wind direction. By reducing the speed of moving air, friction also reduces
the magnitude of the Coriolis force which is dependent on wind speed. Since the
pressure-gradient force is unaffected by wind-speed changes, the contest between it and the
Coriolis force becomes unequal, and the pressure-gradient force becomes the dominant
partner. In response to this now unbalanced force, winds blow obliquely across the isobars in
the direction of low pressure
The angle at which the wind will cross the isobars will vary according to how much
frictional reduction of speed takes place. Over an ocean surface, for example, only a slight
reduction of the Coriolis force will occur, and the air will flow across the isobars at a
relatively small angle 10-20o. Over very rough surface, where the wind speed close to the
surface may be halved by friction, the angle will be greater: 45o or more.
VI. Mesoscale Movement:
1. Land and Sea Breeze:
Air pressure differences cause motion on a variety of scales.
Name of scale
Time-scale
Length scale
Example
Macroscale
- General circulation
Weeks-years
- Synoptic scale
Days-weeks
100-5000km
Mintes-days
1-100km
Mesoscale
1000-40000km Waves in westerlies
Cyclones, anticyclones, hurricanes
Land-sea breeze, thunderstorms,
tornadoes,
Microscale
Seconds-minutes
<1 km
Turbulence
Each scale is closely connected with the other ones in what is ultimately a unitary
system driven by some form of energy input. Atmospheric motions that extend for horizontal
distances of 1-100 km are often termed mesoscale. A good example of this type of motion is
provided by the land -sea-breeze circulation.
The land surface, warming quickly early in the day, warms the columns of air
overlying it. The sea surface, by contrast, warms much more sluggishly, producing a
difference in temperature at similar heights in the air above both surfaces. Warming of the air
column over the land causes its expansion and increases the vertical distance between
isobaric surfaces.
1.
Warm air over land rises
1.
Cool air over land sinks
2.
Sea Breeze moves inland
2.
Land Breeze moves out over water
3.
Cumuli develop aloft and move seaward
3.
Relatively warmer water heats air which then rise
4.
Upper level return land breeze
4.
Upper level return sea breeze
5.
Cool air aloft sinks over water
5.
Cool air over land sinks
6.
Sea Breeze (meso-cold) Front
The result of this is that, at similar levels aloft, air pressure over the sea is slightly
lower than over the land, and air begins to drift seawards aloft in response to this. The
removal of air from the land-based column and its addition to the sea-based column produces
a reduction in air pressure at the surface over the land and an increase over the sea surface.
At the surface, therefore, a pressure gradient now exists from the sea to the land, and
it is this which results in the onset of the onshore breeze so characteristic of coastal areas on
warm summer days. A convective cycle of air motion has been established with air moving
onshore at low elevations, being warmed and induced to rise, flowing offshore aloft where it
cools and sinks to begin the cycle again.
Removal of the temperature differential between the land and sea surfaces (for
example, as the land cools rapidly in the evening) will result in the cessation of the sea breeze.
Reversal of the temperature gradient, as occurs during the night when the sea is warmer than
the land, may produce an offshore flow at the surface, known as the land breeze.
2. Katabatic and Anabatic Wind:
During the night, colder, denser air at higher elevations drains gently downslope
under gravity at speeds of about 1 m/s. Upon reaching the valley floor, a movement towards
the lower ground along the
valley axis occurs. Such
downslope movements of air
known as katabatic winds
(mountain wind) and may be
marked over surfaces such as
glaciers, where intense
chilling of air occurs. In
relatively enclosed low-lying
areas, katabatically induced
chilling of the air overly the
surface renders such areas
more susceptible to fog and
frost.
Farmers therefore should try to avoid valley-floor location for frost-sensitive crops
and plant them just above the level at which the katabatic pond (frost pocket) forms.
During the day, a reversal of this circulation occurs. Air moves upslope and up along
the valley axis. This latter circulation is known as an anabatic wind (valley wind).
VII. Macroscale Movement:
1. Introduction:
In terms of the relationship between radiation energy received and lost, two very
different categories could be identified. Equatorwards of 40oN and 40oS, a zone of surplus
energy existed for the earth-atmosphere system as a whole, whereas polewards of these
latitudes a zone of energy deficit was apparent, one in each hemisphere.
A place losing more energy than it receives in the course of a year would suffer a fall
in its mean annual temperature, and vice versa. Over a long period of time, the contrast in
temperature between high and low latitudes on the earth would become more and more
pronounced unless some mechanism existed whereby the excess heat of the low latitudes
could be transported towards the poles to subsidize the heat-deficient areas of the higher
latitudes. Fortunately, such a mechanism does exist in the motion of the winds and ocean
currents. These are actived on a global scale by the latitudinal heat imbalance and the
pressure contrasts to which it gives rise. A planetary scale convective circulation is the
atmospheric response to the spatial inequality in energy budget.
2. Pattern of Atmospheric Circulation:
The energy required to drive the gigantic circulation of the earth's atmosphere is
proved by the temperature contrasts between cold polar air and warm tropical air.
Consequently the atmospheric circulation must operate to transport the excess of heat from
the tropics and subtropics toward either pole.
The surface
easterly winds in low
latitudes, and probably
in high latitudes as
well, are what might
be expected in a
simple convectional
system: the flow runs
obliquely from higher
latitudes to lower
latitudes. For the low
latitudes, a surface
flow moves from the
subtropics toward the
equator, and that there
is a reverse flow aloft.
Thus the air usually
rising in equatorial latitudes and generally subsiding in the subtropics.
The westerly flow in middle latitudes is opposite to a convectional system, for here the
movement is from warm to cold sources. This reversed type of atmospheric circulation in the
middle latitudes produces an extended line of convergence of zone of conflict between
polar and tropical-subtropical air masses within the middle latitudes.
Much of the meridional heat exchange is accomplished by sporadic thrusts of polar air
toward lower latitudes and thrusts of tropical-subtropical air poleward. Such irregular surges
of polar and subtropical air are both a cause and an effect of the travelling cyclones and
anticyclones embedded within the westerly winds of middle latitudes. The principal
exchanges of polar and tropical air occur through the corridors provided by the breaks in the
subtropical high-pressure ridges, or around the flanks of individual cells.
3. Upper Air Flow:
To complete the general circulation pattern, the upper air flow should not be
overlooked. There are two systems of upper air flow - the upper westerlies and easterlies.
a. The Upper Westerlies:
The existence and intensity of the upper westerly flow is determined by the
equator-to-pole temperature gradient. Figure below shows that pressure decreases more
slowly with height in a column of warm tropical air than in a column of cold polar air,
producing a pressure gradient from equator to pole. This gradient increases with increasing
height. Air movement along it is modified by the Coriolis force to produce a westerly
motion aloft, which characteristically exhibits a wavy pattern. Normally they travel from
west to east rather slowly.
This system of westerlies is blowing in a complete circuit about the earth the earth
from about latitude 25o almost to the poles. At high latitudes these westerlies constitute a
circumpolar whirl. Toward low latitudes the pressure rises steadily at a given altitude, to
form two high-pressure ridges at latitudes 15o to 20oN and S. In the high-pressure zones,
wind velocities are low.
Between the high-pressure ridge is a trough of weak low pressure, in which the
winds are easterly - equatorial easterlies.
b. Upper Air Waves and the Jet Stream:
The uniform flow of the upper-air westerlies is frequently disturbed by the
formation of large undulations, called upper- air waves/ Rossby waves. The waves develop
in a zone of contact between cold, polar air and warm, tropical air.
It is by means of the upper air waves that warm air of low latitudes is carried far
north at the same time that cold air of polar regions is brought equator ward.
Associated with the development of such upper air waves at altitudes of 10-12 km
are narrow zones in which wind streams attain velocity up to 200-250 knots (100-125 m/s)
-Jet Stream which consists of pulselike movements of air following a broad curving track.
Like the upper westerlies, the jet streams are also a product of the all-important
temperature gradient between the equator and the poles. It is concentrated into a few
narrow zones where cold and warm air masses come into contact. Such boundaries are
known as fronts and often exhibit quite large horizontal differences in temperature over a
relatively short distance on either side of the boundary. From the figure blow, it may be
deduced that this intensification of the thermal gradient will produce an intensification of
the induced pressure gradient aloft. Consequently, a marked strengthening of the upper
westerlies may be expected to occur at such locations. These enhanced westerlies
correspond to the jet streams. Like the westerlies, the jet stream will be best developed
during winter, when the equator-pole temperature gradient is greatest.
4. The Importance in Heat Transfer:
The pattern of energy transfer in the atmosphere is complex. The circulation between
the tropics consists of two cells. Air blows in towards the equatorial trough across the
subtropical seas. As it does so evaporation of water from the ocean utilizes vast quantities of
energy so that the sensible heat transfer to the atmosphere is often small.
In contrast, over the desert land masses very little evaporation occurs, energy loss is
limited, and the incoming radiation heats the ground surface which then heats the atmosphere.
Thus much more of the energy is in the form of sensible heat. During the night this energy is
re-radiated back to space for the dry air is unable to intercept much outgoing long-wave
radiation. The net surplus of radiation is fairly small.
Winds approaching the equator rise as they meet the equatorial trough. The ascent of
this air is not a continuous, widespread phenomenon, but occurs mainly in association with
localized, often intense and short-lived updraughts such as in thunderstorms. As the air rises
and cools, the water vapour condenses and releases latent heat. The increased height of the air
also represents an increased potential energy.
The equatorial air then diverges and flows polewards, so the potential energy is
exported to higher latitudes. The cycle is completed as radiation cooling causes subsidence of
the air. In the process the air dries and warms as the potential energy is converted to sensible
heat. It also checks the rise of convection currents in these subtropical areas, producing clear,
cloudless skies.
In the temperate and polar areas the processes of energy transfer are more complex.
There is no general, cellular circulation of air as in the tropics, but instead a complicated
pattern in which individual, rotating storms play an important part. Within these storms warm
air masses rise, releasing latent heat and gaining potential energy. They then become
intermixed with descending cold air and gain sensible heat. The rotating storms are moving,
so the position of this intermixing changes constantly, although there is a tendency for a
concentration in certain zones in the northern hemisphere.
All these transfers of energy through the atmosphere are highly variable, and major
differences in the intensity and character of transfer occur over time. Thus, the flows of
energy represent net increments, often produced by individual, temporary processes.
VIII. Global Pressure Patterns and Planetary Wind Systems
1. Global Distribution of Surface Pressure Systems: (Refers to the figure in worksheet 22)
Barometric readings must be reduced to sea level equivalents, using the standard rate
of pressure change with altitude - 100 hPa/km (1013 hPa is taken as standard sea-level
pressure).
Doldrums: In the equatorial zone is a belt of low pressure (1011 - 1008 hPa) which is
known as the equatorial trough.
Subtropical High Pressure Belts: Lying to the north and south of the equator, there
are the subtropical belts of high pressure - horse latitudes.
Temperate Low Pressure Belts: Poleward of the subtropical high pressure belts are
broad belts of low pressure.
Polar High Pressure Belts: Locate on the North and South Poles.
The pressure belts shift seasonally through several degree of latitude.
2. Earth's Surface Wind Systems:
a. Planetary Wind Systems:
Over the equatorial trough of low pressure, lying roughly between 5oS and 5oN
latitude, is the equatorial belts of variable winds and calms, or the doldrums. There are no
prevailing surface winds here.
North and south of the doldrums are the trade wind belts, covering the zones lying
between 5o and 30oN and S. The trades are a result of a pressure gradient from the
subtropical belt of high pressure to the equatorial trough of low pressure. Trade winds are
noted for their steadiness and directional persistence.
The system of doldrums and trades shifts seasonally north and south, through several
degrees of latitude. Because of the large land areas of the northern hemisphere, there is a
tendency for these belts to be shifted farther north in the northern hemisphere in summer
than they are shifted south in winter.
Between latitudes 25o and 35o are the subtropical belts of variable winds and calms,
or horse latitudes. The high pressure areas are concentrated into distinct anticyclones or cells,
located over the oceans. The cells have generally fair, clear weather, with a strong tendency
to dryness
(Reasons:
).
It is because the anticyclonic cells are centres of descending air, settling from higher levels of
the atmosphere and spreading out near the earth's surface. Descending air thus becomes
increasingly dry.
Between latitudes 35o and 60o, both hemispheres, is the belts of the Westerlies. In the
southern hemisphere, there is an almost unbroken of ocean. Here the Westerlies gain great
strength and persistence - 'the roaring forties', 'furious fifties' and the ' screaming sixties'.
The polar easterlies are characteristics of the polar zones.
b. Monsoon Winds in Asia
In summer, southern Asia develops a cyclone into which there is a considerable flow
of air. This may be a thermal low. From the Indian Ocean and the southwestern Pacific warm,
humid air moves northward and northwestward into Asia, passing over India, Indochina and
China. This air flow constitutes the summer monsoon, which is accompanied by heavy
rainfall in southeast Asia.
In winter, Asia is dominated by a strong centre of high pressure, from which there is
an outward flow of air reversing that of the summer monsoon. Blowing southward and
southeastward toward the equatorial oceans, this winter monsoon brings dry, clear weather
for a period of several months.
IX. Synoptic Scale Movement:
1. Air Masses and Fronts
a. Air Masses:
A body of air in which the physical properties (the upward gradients of temperature
and moisture) are fairly uniform over a large area is known as an air mass. A given air mass
is characterized by a distinctive combination of temperature, environmental lapse rate, and
specific humidity.
The properties of an air mass are derived in part from the regions over which it passes.
Air masses are classified according to two categories of generalized source regions:
- latitudinal position on the globe, which primarily determines thermal properties.
- underlying surface - continents or oceans - determining the moisture content.
With respect to latitudinal position, five types of air masses are as follows:
=============================================================
Air Mass
Symbol
Source Region
=============================================================
Arctic
A
Arctic ocean and fringing lands
Antarctic
AA
Antarctica
Polar
P
Continents and Oceans, 50o-60o N and S
Tropical
T
Continents and Oceans, 20-35
Equatorial
E
Oceans close to equator
=============================================================
With respect to type of underlying surface, two further subdivisions are imposed on
the preceding types as follows:
=============================================================
Air Mass
Symbol
Source Region
=============================================================
Maritime
m
Oceans
Continental
c
Continents
=============================================================
By combining types based on latitudinal position with those based on underlying
surface a list of six important air masses results; these are listed in the table below. The table
gives typical values of temperature and specific humidity at the surface, although a wide
range in these properties may be expected, depending on season.
Air Mass
Symbol
Properties
Temperature
oF
oC
Specific
Humidity
g/kg
Continental-arctic
cA
Very cold, very dry
-50
-46
0.1
Continental Polar
cP
Cold, dry (winter)
12
-11
1.4
Maritime polar
mP
Cool, moist (winter)
39
4
4.4
Continental tropical
cT
Warm, dry
75
24
11
Maritime tropical
mT
Warm, moist
75
24
17
Maritime equatorial
mE
Warm, very moist
80
27
19
Properties of Typical Air Masses
When an air mass moves out from its source region, although it tends to undergo
progressive modification, it brings its distinctive properties with it to influence weather at
distant locations. The climate of a place may thus be described in terms of the frequency with
which it experiences particular air masses.
The planetary wind circulation system makes interaction between air masses more
likely to occur in some areas. For example, in the figure below, a convergence of air in the
vicinity of the equator is indicated. Since the source regions from which these air streams
originate, however, are at similar latitudes in either hemisphere, both air masses will have
similar properties and will mix freely. this is in sharp contrast to the other main area where air
mass interaction is indicated - the zone where tropical and polar air masses collide at the
high-middle latitudes in each hemisphere. In this case, fundamentally different air masses are
coming into conflict along a boundary zone which has become known as the polar front.
b. Fronts:
A given air mass may have a rather sharply defined boundary between itself and a
neighbouring air mass. This discontinuity is termed a front.
Fronts may be nearly vertical, as in the case of air masses having little motion relative
to one another, or they may be inclined at an angle not far from the horizontal, in cases where
one air mass is sliding over another.
Atmospheric fronts are typically 100-200 km wide and are zones of steep horizontal
gradients. When the cold air is advancing and, because of its greater density, undercutting the
warm air mass ahead of it, the discontinuity between them is known as a cold front. Its
passage at the surface heralds the arrival there of the cold air mass. Typically, the cold front
has a gradient of about 2o. As the advancing cold air forces the warm in front of it to rise, this
produces frontal precipitation.
When the warm air is advancing and the cold air is weak, because it is less dense, it
tends to glide up over any wedge of colder air in its path, producing a warm front. Although
the gradient of the frontal surface in this case is often below 1o, the cooling of the warm air
mass as it ascends is frequently sufficient for cloud formation and precipitation.
2. Disturbances in the Mid-Latitude Circulation:
a. Depressions (p.193-p195)
i) Nature and Origin
The convergence of contrasting air masses along the polar front
encourages rising air motions, and this leads to a fall in surface air pressure. If
conditions in the upper atmosphere is favourable, this flow of ascending air
becomes organized into a spiral of upward and inward-moving air known as a
depression.
The anticlockwise flow around this vortex (clockwise in the southern
hemisphere) moves polar air further towards the equator on its western flank and
drives tropical air further polewards on its eastern flank, producing a bulge or
wave on the polar front. Further uplift of air produces more cooling and
condensation, resulting in the release of large amounts of latent heat energy. The
latter promotes further instability and the disturbance grows rapidly in extent and
intensity. Throughout the time this wave depression is developing in amplitude
into a mature cyclonic disturbance, it is moving from west to east under the
influence of the upper westerlies within which its circulation is embedded.
ii) Development of the Depression
The initial stage of a depression is a slight deformation of the polar front,
producing a wave. If the wave is a poleward bulge into the cold air which begins
to flow around the rear of the wave as shown in the figure 1. At this stage the
wind has a component blowing from cold to warm air behind the wave and from
warm to cold ahead of it (such poleward transfer of warm air is an important
energy transfer process in the general circulation of the atmosphere). The whole
system will tend to move in a direction parallel to the isobars within the warm
sector.
As the development of the system continues, more and more cold air is
replaced by warm air, where lower density contributes to a decrease in pressure
and the development of cyclonic flow around the region of low pressure at the top
of the warm sector (with divergence aloft).
Within 24 hours of the initial disturbance of the front, a well-defined
warm-sector depression will have developed with warm front, cold front, and a
cloud and weather disturbance as shown in figure below. The cold air forms a
wedge in cross-section, undercutting the warm-sector air owing to its greater
density, with the warm air gliding up the frontal surface at the warm front. Cloud
belts and precipitation result from the convergence and ascent at the two fronts as
shown.
Within the system the air behind the cold front moves faster than the air
receding ahead of the warm front. With time the cold front 'catches up' the warm
front and the warm air is gradually lifted from the surface and the frontal system
begins to occlude. The front that is formed by the merger of the cold front and
warm front is called an occluded front. It is bounded on both sides by cold air of
slightly different thermal properties. The process of lifting of the warm air is
called occlusion.
The different temperatures of the cold air behind the cold front and ahead
of the warm front are generally due to their different paths. There are two types of
occluded front: the cold occlusion, when the air behind the occluded front is
colder than the air ahead of it; and the warm occlusion, when the reverse is the
case.
In the final stage of development, the cold front overtakes the warm front
at increasing distances from the centre of the depression, the occlusion grows in
extent and the warm-sector air becomes almost totally eliminated at the surface.
The depression is transformed into a large, weak vortex, and gradually fill and
dissipate.
iii) Change of weather Associated with the Passage of a Warm and Cold Front
=============================================================
Element
In advance
At passage
In the rear
=============================================================
Warm front
Pressure
slow fall
Wind
Steady fall
Fall ceases
Little change or
Increases and sometimes Veers and sometimes Steady direction
backs
decreases
Temperature
Steady slow rise
Dew-point
Rises in the area of
temperature
precipitation
Relative humidity
Little change
precipitation
Cloud
Stratus types
Rise
Rise
Little change
Steady
Rises in the area of
May rise further
Stratus types
Stratus types
Weather
Continuous rain (or snow)
Cloudy, drizzle
stops
Precipitation almost
Visibility
Usually poor, mist
Good, except in rain
Poor, often misty
or light rain
may
persist
Cold front:
Pressure
Fall
Sudden rise
Rise continues
Wind
Increasing and backing, Sudden veer, perhaps Backing a little after
becoming squally
squally
squall then steady
or veering
slowly
Temperature
variable in
Steady, but falls in rain
Sudden fall
Little change,
showers
Dew-point
Little change
Relative humidity
high in Rapid fall as rain ceases
Sudden fall
Little change
May rise in precipitation
Remains
precipitation
Cloud
Weather
Cumulus types
rain
Rain, often heavy, with Heavy rain for
short
perhaps thunder and hail
period,
sometimes more
persistent, then fair
with
occasional showers
=============================================================
b. Anticyclones
Anticyclones are atmospheric system where relatively high air pressure is experienced
at the surface, due to a downward and outward spiral of air from aloft.
In the northern hemisphere, this takes the form of a clockwise circulation of air
around the centre of subsidence, which, since it is fed from above, normally involves no
surface air mass differences in the manner of a depression. Indeed, with their gentle breezes,
absence of precipitation and often cloudless skies, anticyclones are in many respects the
antithesis of depressions. Stable and slow-moving, they are associated with warm, fine
weather in summer and cold, frosty conditions in winter.
3. Disturbances in the Low-Latitude Circulation: Tropical Cyclones - Typhoons
a. Nature and Distribution:
About 10% of wave disturbances intensify into the more violent rotating storms in
many parts of the tropics. They are known by a variety of name - typhoons (western Pacific
Ocean), hurricanes (Atlantic and eastern Pacific Ocean), willy-willies (Australia).
They are maritime phenomena, originating over tropical oceans where sea-surface
temperatures are in excess of 27oC. Over colder ocean areas, and particularly over land, they
seem to dissipate rapidly. A constant supply of warm, humid air thus appears to be a primary
nutrient for typhoons.
A second prerequisite relates to latitude. There is a typhoon-free belt close to the
equator, where formation is inhibited. This relates to the very weak Coriolis force existing at
these latitudes. Since the tendency for displaced air to rotate increases polewards, typhoon
formation evidently requires a modicum of 'spin' forces to trigger off rotation. These two
necessities explain why typhoons form usually at 5-8oN & S, over the tropical oceans.
Small islands on the ocean will be excellent for typhoon development. It is because
they provide a intensive low pressure centre.
b. Mechanisms of Typhoon Formation
i) Youth Stage:
First, for some small-scale initial disturbance, warm, humid oceanic air is
induced to rise. If sufficiently vigorous, and if the location is far enough away from
the equator, the effects of the rotation of the earth give the rising air a 'twist' and the
whole system begins to revolve. The organization of clouds into spiral bands is a
critical stage in this process, transferring energy form individual rising packages of air
into a more coherent vortex.
Simultaneously, large amounts of latent heat energy are released through
condensation, further enhancing instability, drawing in more humid, oceanic air from
adjacent areas, and accentuating the scale of the system. Mature typhoons range in
diameter: 200-500 km is typical. Central pressure is usually 12 km high is evidence of
the intense convection activity fuelled by latent heat release. Up to 15 x 109
tonnes/day of water vapour passes through the system, half of which may fall as rain.
ii)
Maturity Stage:
At the heart of the cloud spiral is often a quiescent area, perhaps 30-50 km in
diameter, with clear skies and light winds. This is the 'eye', a small area where
subsidence of air from aloft occurs. Its importance is twofold.
First, it signifies divergence aloft, a vital aspect in removing the rapidly rising
air of the typhoon and permitting convection to proceed at the surface.
Second, the warming of the descending air in the eye itself induces instability
at the surface and stimulates the storm's intensity.
iii)
Decay Stage
The system is self-perpetuating as long as a supply of warm, moist air
nourishes it. Movement over a warm oceanic area at about 20 km/hr. is ideal. Landfall,
or passage over colder ocean, destroys the disturbances.
c. Change of weather associated with the passage of a typhoon.
- Before the tropical cyclone arrives the air becomes very still.
Temperature: The temperature and humidity are high.
Cloud: It may be sunny with high clouds (cirrus) approaching.
Pressure: Pressure begins to drop.
Rain: Occasional showers may also be experienced.
- When the front vortex arrives H.K.,
Pressure: pressure drops further.
Wind: Gusty followed by violent destructive winds are experienced.
Wind direction: north-east or north-west in direction.
Rain: Thick clouds (cumulonimbus) appear and heavy rain.
Visibility: greatly reduced.
At sea, violent winds produce heavy swells.
- When H.K. is within the typhoon eye.
Pressure: drops to the lowest.
Wind: wind speed reduces suddenly to gentle.
Rain: rain stops, the sky is clear.
Such calm period may last for one or a few hours.
- The arrival of the rear vortex.
Pressure: suddenly increases.
Wind: wind speed increases again.
Wind direction: south-east or south-west.
Rain: Dense clouds and heavy rain again.
- As the typhoon crossed land.
Pressure: rises gently and then to normal.
Wind: wind speed decreases.
Wind direction: south-east or south-west.
Rain: heavy rainfall may still continue to last several hours or 1 or 2 days.
Normally weather resumes after the cyclone has completely disappear.
X. Climatic Classification and Climatic Regions
It was proposed by Dr. Wladimir Koppen in 1918. This system was devised by
combining temperature and precipitation characteristics, and setting limits and boundaries
fitted into known vegetation and soil distribution.
In this system, each climate is defined according to fixed values of temperature and
precipitation, computed according to the averages of the year or of individual months. Thus,
it is easy to assign a given place to a particular climate group because both air temperature
and precipitation are the most easily obtainable surface weather data.
These temperature divisions were chosen because they appeared to correspond with
vegetation boundaries. Plants were assumed to provide a good overall indication of climatic
conditions since they respond not only to mean conditions of temperature and precipitation
but also to the variability and seasonality. From the combinations of temperature and
precipitation conditions, twelve major climatic types were designated.
The table below shows the codes designating the different climate groups.
Main Symbol
Meaning
Climatic Groups
A
Tropical
Hot all year with every month over
18oC average.
Equatorial & Tropical Climates
B
Dry all year. Evaporation exceeds
Tropical & Mid-latitude Desert
Dry Climate
precipitation in all months.
C
Mesothermal
Seasonal temperatures but winters
not severe (not below -3oC)
D
Microthermal
Seasonal temperatures but with
severe winters (below -3oC)
E
Tundra (ET)
Ice Cap (EF)
No true summers; no months over
10oC average
Minor Symbols
Dry Climate:
Meaning
Mid-latitude Coastal Climate
(Warm Temperate)
Higher Mid-latitude
Continental
(Cold)
High Latitude
Polar Lands
Example
BS (Steppe Climate)
BW (Desert Climate)
h
k
Precipitation:
f
s
w
m
mean annual temperature over 18oC
mean annual temperature under 18oC
Tropical Desert (BWh)
Temperate Desert (BWk)
Rain in all seasons
(for A, C & D climates only)
Equatorial hot-wet (Af)
Mild humid no dry season (Cf)
Snowy forest climate,
moist winter (Df)
Mediterranean (Cs)
Savanna (Aw)
Mild humid dry winter (Cw)
Snowy forest climate,
Summer drought
Winter drought
Short dry season - monsoon type
(for A Climates only)
dry winter (Dw)
Tropical Monsoon (Am)