2. cyclone The term ‘cyclone’ refers to large-scale rotating
weather systems which rotate with positive vorticity.
Two major types of cyclones are tropical cyclones and
extra- tropical cyclones. The latter are also known as
depressions. The mechanisms of formation and the
structures of these two forms are so different that they
should be considered separately.
3. Tropical cyclones are also known as
hurricanes and typhoons. They develop over
tropical oceans and can produce extremely
heavy rainfall and devastating winds with
sustained wind speeds sometimes in excess
of 100 metres per second (m s−1). Satellite
pictures (Fig. 1) reveal a striking circular
symmetry in tropical cyclones with a small
(50 km diameter) cloud-free ‘eye’. The
preferred regions for hurricane development
are oceans where the winds are light, the
humidity is high, and the surface water
temperature is high (usually over 26 °C) over
4. Since these conditions exist in some
places for only part of unfortunate that
they share the same nameThe conditions
required for tropical cyclones are also
suitable conditions for
thunderstorms, deep convective clouds
with strong updraughts. Thunderstorms
can be organized into a tropical cyclone in
the presence of low-level convergence.
The winds converging in one particular
region increase the large-scale rotation in
an anti clockwise sense in the northern
5. Because this rotation is an important factor
in the development of tropical cyclones,
and because no rotation is imparted at the
Equator, no development of tropical
cyclones is found within about 5 degrees of
the Equator. As the air converges, the
thunderstorms become more organized
and closer together. Huge amounts of
water evaporating from the warm ocean
surface are carried aloft in the bands of
thunderstorms. As the air rises it cools, and
the water condenses releasing latent heat.
6. The latent heat released greatly enhances the
buoyancy of the air, producing even stronger
updraughts, which in turn draw in more
converging air at the base. There is positive
feedback as more warm moist air being drawn
into the base of the clouds produces even
stronger updraughts. At the tropopause the air
spreads out in bands moving away from the
centre of the cyclone. When this divergence of air
at high levels exceeds the convergence of air at
low levels, the surface pressure drops, forming a
low-pressure centre around which the air
circulates and converges, bringing in even more
warm moist air to feed the cyclone. Extra-tropical
cyclones.
7. Extra-tropical cyclones are the middle
latitude tropospheric circulation systems
also known as depressions. The life
cycle of a depression is often described
by the polar front approach, in which the
depression is seen as a disturbance
which grows and modifies the front as it
develops. Another approach is used
here to illustrate the three-dimensional
structure and the development
mechanism by which an extra-tropical
cyclone develops.
8. Fig.2 shows a wave depression at its
most vigorous stage of development.
The surface weather map shows an
open warm sector with a deepening
low-pressure centre. The low-pressure
centre is situated below a region in the
upper troposphere where a wave or
trough of low pressure is lying slightly
behind the surface low. This is exactly
the configuration required to enable a
low-pressure centre to deepen. The
circulation of the wave depression is in
9. . (In the southern hemisphere the
circulation would be clockwise, but the
cold air would be to the south and so the
mechanism would be the same.) The
warm air at low levels in the warm sector
is lifted above the warm front. This applies
not only to air at the surface, but to all the
air through a substantial depth. At the
same time the cold air moving southwards
behind the low-pressure centre is losing
height. The net effect of warm air rising
and cold air sinking is to decrease the
‘centre of gravity’ of the system because
10. By lowering the centre of gravity, some
potential energy is removed from the system
and converted to kinetic energy—the energy
of motion. (When we release an object from a
height and let it fall, we are converting
potential energy into kinetic energy). In the
wave depression the kinetic energy is
manifested by the strength of the winds in the
circulating system. Because it is these winds
that are moving the warm air upwards and
the cold air downwards, the process
accelerates and feeds on itself. This unstable
situation, known as baroclinic instability,
continues until the warm air is lifted from the
surface into the upper troposphere.
11. By moving warm, less dense, air into the
column ahead of the upper trough the
pressure ahead of the upper trough is
reduced. Conversely, the cold air introduced
behind the trough increases the pressure: so
the trough minimum is moved from east to
west. This means that the region of
divergence is now no longer above the centre
of low pressure at the surface. In fact, there is
neither divergence nor convergence at upper
levels, but convergence remains at the
surface. The effect is thus to increase the
surface pressure and complete the last phase
12. Cyclone and Anticyclone
The terms cyclone and anticyclone are used to describe
areas of low and high atmospheric
pressure, respectively. Air flowing around one or the
other of these areas is said to be moving cyclonically in
the first case and anticyclonically in the second. In the
northern hemisphere, cyclonic winds travel in a
counterclockwise direction and anticyclonic winds, in a
clockwise direction. When a cyclone or anticyclone is
associated with a wave front, it is called a wave, a
frontal, or a mid-latitude cyclone or anticyclone.
13. Vertical air movements are
associated with both cyclones
and anticyclones. In the former
case, air close to the ground is
forced inward, toward the center
of a cyclone, where pressure is
lowest, and then begins to rise
upward. At some height, the rising
air begins to diverge outward
away from the cyclone center.
14. In an anticyclone, the situation is reversed.
Air at the center of an anticyclone is forced
away from the high pressure that occurs
there and is replaced by a downward draft
of air from higher altitudes. That air is
replaced, in turn, by a convergence of air
from higher altitudes moving into the upper
region of the anticyclone.
Distinctive weather patterns tend to be associated with
both cyclones and anticyclones. Cyclones and low
pressure systems are generally harbingers of rain,
clouds, and other forms of bad weather, while
anticyclones and high pressure systems are predictors
of fair weather.
15. One factor in the formation of cyclones
and anticyclones may be the
development of irregularities in a jet
stream. When streams of air in the
upper atmosphere begin to meander
back and forth along an east-west axis,
they may add to cyclonic or
anticyclonic systems that already exist
in the lower troposphere. As a result,
relatively stable cyclones (or
anticyclones) or families of cyclones (or
anticyclones) may develop and travel
16. On relatively rare occasions, such
storms may pick up enough
energy to be destructive of
property and human life.
Tornadoes and possibly
hurricanes are examples of such
extreme conditions.
