1. Cyclones and Hurricanes
Cyclones refer to a system of winds that rotate inwards towards an
area of low pressure. In temperate latitude, such a weather pattern
is called a depression and in tropical area, a hurricane. Cyclones are
known by different names in different parts of the world but they are
all essentially the same. They are called:
Hurricanes in the Caribbean
Cyclones off Africa
Typhoons in the Far East
Willy-willies in Australia
Hurricanes are violent tropical storms with wind speed of at least
120 km/hour. Wind speed may rise as high as 250 km/hour and
hurricanes are usually according to their speed from force 1-5.
Certain conditions must be met for hurricanes to be formed. They
normally occur July and October since they form over tropical oceans
where temperature must meet at least 27˚c. Only in the warmer
summer months in the northern hemisphere is this possible.
Northern Australia is the exception where they are called willy-
Characteristics of Hurricanes
In the centre of the hurricane, air pressure is very low. It is usually
below 985mb (millibars), and sometimes as low as 860-890mb.
There are strong updraughts over most of the weather system.
Around the centre of the low pressure, there is a strong revolving
wind system. In the northern hemisphere, they rotate in an anti-
Wind speeds are very high. Near the centre of the storm, they
may reach 360km/hr (100m/sec).
2. There is a spiral pattern of enormous cumulonimbus clouds. Some
of the clouds tower up to a height of 16km. Where the clouds are
thickest, there is very little light even at midday.
At a high level, there is a canopy of cirrus clouds. Above 9km, the
winds spiral outwards from the centre of the hurricane.
In some hurricanes, there is very heavy rain. During one hurricane
in 1909, more than 2,400 mm of rain was recorded over a four day
period at Silver Hill in Jamaica.
Right at the centre of the storm there is an eye, 20-50 km across.
In the eye, conditions are very different. There are light winds,
blowing at perhaps 10-20 km/hr and there may be no rain falling.
Around the eye there is a wall of dark clouds.
Within the eye, there is a strong downwards current of air.
Because of this, the air there is very warm, and there are no low-
Immediately around the eye is the vortex. In this area moist air is
3. Underwater landslides and Tsunami
Underwater/ undersea landslide is a form tsunami. Tsunami (also
known as tidal waves), is a destructive wave caused by the
disturbance on the ocean bed. These can be:
Under-sea volcanic eruptions
Under-sea hurricanes and
Other large storms at sea
Whatever the cause, large amounts of rock or sediment are moved
around the seabed. This under-sea activity displaces a huge amount
of water which forms the tsunami. One disturbance usually
generates several tidal waves. These can cross ocean very quickly.
Tsunami can reach speeds of 750 km per hour. In the open ocean,
tsunamis are typically 800 metres long and only a few metres high.
When they run onto a shelving shore, they reach heights of 12
metres or more and cause massive destruction. A tsunami may form
a wall of water more than 100 feet high when it approaches shallow
water near shore.
Tsunamis caused by undersea earthquakes are called seismic sea
waves. Scientists using seismographs can accurately predict when a
tsunami will arrive at a given seacoast. When a seaquake occurs,
seismograph picks up vibrations, called seismic waves that travel
outward in all directions from the quake’s point of origin.
4. Under-sea landslides
The technical term for a tsunami caused by landslides, is
megatsunami. Undersea landslides have considerably higher
amplitude than typical tsunamis because the amount of water
displacement increases the wave size more than a submarine
Kick-'em-Jenny is a submarine volcano located 6 miles (10 km) north of the
island of Grenada. It is the southern-most active volcano in the Lesser Antilles
and the only active submarine volcano in the arc. Kick-'em-Jenny has erupted
10 times since 1939 with the most recent eruption in 1990. The 1939 eruption
sent a black cloud up to 885 feet (270 m) above sea level. During the 1965
eruption, earthquakes of intensity V were felt on Isla de Ronde.
During the 1974 eruption, the sea above the volcano was boiling turbulently
and spouting steam. Most of the eruptions were detected by submarine
hydrophones. The hydrophones detect shock waves from explosive eruptions
as they travel through the water. Using data from several hydrophones,
volcanologists can determine where an active submarine volcano is located
and when it started erupting.
There is no indication that eruptions at Kick-'em-Jenny are hazardous. Kick-
'em-Jenny has a basal diameter of about 3 miles (5 km) and rises about 4,300
feet (1,300 m) above the sea floor.