Hazards which originate above the surface of the earth (in the atmosphere) are called exogenous hazards.

1. EXOGENOUS HAZARDS
• Hazards which originate above the surface of the
earth (in the atmosphere) are called exogenous
hazards.
• E.g. Drought, Rainfall, Snowfall, Winds, Hailstorm

2. CLASSIFICATIONS
The exogenous disasters are classified into 3
ways:
• Atmospheric Disasters
• Hydrospheric Disasters
• Lithospheric Disasters

3. ATMOSPHERIC DISASTERS
• Atmospheric Disasters that originate in the
atmosphere of the earth are called atmospheric
hazards. These include cyclones, tornadoes,
droughts, thunderstorms etc. Drought, Rainfall,
Snowfall, Winds, Hailstorm

4. CYCLONE
• A cyclone is a large scale air mass that rotates
around a strong center of low atmospheric
pressure. Cyclones are powerful and violent
windstorms, in which wind moves very fast in a
circular direction around a low pressure area.
The wind blows in anti clockwise circle in the
northern hemisphere and clockwise in the
southern hemisphere.

6. NOMENCLATURE
Nomenclature Region of the world
• Typhoons China Sea, Pacific Ocean
• Hurricanes Caribbean Sea, Atlantic Ocean
• Tornados Southern part of USA
• Willy – willies Australia
• Tropical Cyclones Indian Ocean

7. TYPES OF CYCLONES
Generally cyclones are two types
Warm-core cyclones: These type of
cyclones are warm at the centre and cold
near the edges.
Cold-core cyclones: These type of cyclones
are coldest at the centre than near the edges.

8. TYPES OF CYCLONES
• Based on structure on the area of origin
 Tropical cyclones: Cyclones that form between the
tropics of cancer and Capricorn are called tropical
cyclones.
 Temperature cyclones: Cyclones develop over
temperature zones and high latitude regions are
called temperature cyclones. They are also known
as mid-latitude cyclone, frontal cyclone and extra
tropical cyclone.

9. • Cyclones have been categorized by the Indian
Meteorological Department into the following
types of disturbances based on the wind speed.
Surface wind speed (Km/h) Type of disturbance
Less than 31 Low Pressure
31-49 Depression
49-61 Deep depression
61-88 Cyclone storm
88-117 Severe cyclonic storm
117-220 Very severe cyclonic storm
>221 Super cyclone

10. TROPICAL CYCLONE
• A tropical cyclone is an intense low pressure area or a
whirl in the atmosphere over tropical or sub-tropical
waters, with organised convection (i.e. thunderstorm
activity) and winds at low levels, circulating either
anti-clockwise (in the northern hemisphere) or
clockwise (in the southern hemisphere). From the
centre of a cyclonic storm, pressure increases
outwards. The amount of the pressure drop in the
centre and the rate at which it increases outwards gives
the intensity of the cyclones and the strength of winds.

11. FORMATION OF CYCLONES
Tropical cyclones require certain conditions for their
formation. These are
• A source of warm, moist air derived from tropical
oceans with sea surface temperature normally near to or
in excess of 27 °C
• Winds near the ocean surface blowing from different
directions converging and causing air to rise and storm
clouds to form

12. FORMATION OF CYCLONES
• Winds which do not vary greatly with height - known
as low wind shear. This allows the storm clouds to
rise vertically to high levels;
• Coriolis force or spin induced by the rotation of the
Earth. The formation mechanisms vary across the
world, but once a cluster of storm clouds starts to
rotate, it becomes a tropical depression. If it
continues to develop it becomes a tropical storm, and
later a cyclone/ super cyclone will be formed .

14. STRUCTURE OF TROPICAL
CYCLONES
• The main parts of a tropical cyclone are the
eye, the eyewall and the rainbands. Air
spirals in toward the center in a counter-
clockwise pattern in the northern hemisphere
clockwise direction in the southern
hemisphere.

15. EYE
• The hurricane's center is a relatively calm,
generally clear area of sinking air and light winds
that usually do not exceed 24 km/h and is typically
32-64 km across. An eye will usually develop when
the maximum sustained wind speeds go above 74 -
119km/h and is the calmest part of the storm.

16. EYE
• It probably has to do with the combination of "the
conservation of angular momentum" and
centrifugal force. The conservation of angular
momentum means is objects will spin faster as they
move toward the center of circulation. So air
increases it speed as it heads toward the center of
the tropical cyclone.

18. EYEWALL
• Where the strong wind gets as close as it can is
the eyewall. The eyewall consists of a ring of
tall thunderstorms that produce heavy rains and
usually the strongest winds. Changes in the
structure of the eye and eyewall can cause
changes in the wind speed, which is an
indicator of the storm's intensity. The eye can
grow or shrink in size, and double (concentric)
eyewall can form.

20. RAIN BANDS
• Curved bands of clouds and thunderstorms
that trail away from the eye wall in a spiral
fashion. These bands are capable of
producing heavy bursts of rain and wind, as
well as tornadoes. There are sometimes gaps
in between spiral rain bands where no rain or
wind is found.

21. RAIN BANDS
• In fact, if one were to travel between the
outer edge of a hurricane to its center, one
would normally progress from light rain and
wind, to dry and weak breeze, then back to
increasingly heavier rainfall and stronger
wind, over and over again with each period
of rainfall and wind being more intense and
lasting longer.

23. EFFECTS OF CYCLONES
• Cyclones like other natural disasters, cause tremendous
loss to the lives and property wherever they strike. The
coastal areas all over the globe are significantly
affected.
• Some of the adverse impacts of cyclone are as given
below.
• The building collapse and houses get blown away.
• Flooding of low – lying coastal areas

24. Contd…
• Road and Rail transport get blocked or damaged by
flood water.
• Power supply failure/power outages.
• Contamination of water from dead animals and
routing food.
• Outbreak of epidemic like gastroenteritis and
mosquito-borne illnesses.
• Disruption of international shipping activities.

25. Contd…
• Damage of installations, dwelling and
communication system.
• The winds formed during cyclones can have
destructive effect, causing soil erosion and
defoliation of forests.
• Heavy rainfall from the cyclones can cause
flooding over extensive areas, inundating land,
isolating communities and destroying
infrastructure.

26. DISTRIBUTION OF CYCLONES
• The tropical cyclones are distributed in six regions
across the globe:
 West Indies, Gulf of Mexico and Caribbean Sea
 West North Pacific, Philippines Islands, China Sea and
Japanese Islands
 Arabian Sea and Bay of Bengal
 South Indian Ocean off Madagascar
 Eastern Pacific Coastal region of Mexico & central
America
 Western South Pacific, East coast of Australia

27. Measures to mitigate the cyclones
• Cyclones are violent tropical storms or winds in which
the air moves very fast in a circular direction. The
mitigation measures to prevent cyclones are:
 Afforestation of land should be done, as forests act as
buffering area against strong winds and flash floods. It
also prevents soil erosion.
 Trees prevent the entry of cyclones in land, thus acting
as a barrier to cyclones
 Public should be alerted about the cyclones in
advance, so as to evacuate the areas prone to it.

28. Contd…
 Buildings constructed should be resistant to wind and
water.
 Construction of cyclone shelters help to minimize loss
of human lives.
 Means of communication should be through
underground, as it could lead to communication break
during cyclones.
 Cyclonic storms leads to flooding, hence measures
should be taken to prevent floods.

29. Contd…
 Construction of embankments helps to minimize
floods.
 Coastal shelter belt plantations helps to break
severe winds.
 Plantation along the coastal areas prevents the loss
of soil, hence fields can be protected.
 Public settlements should be avoided in coastal
areas in the range of 5kms from the sea.