Tropical cyclones form when warm ocean water evaporates, is
further warmed by the sun, and rises to create a high, thick
layer of humid air. This rising of warm, dense air creates an
area of low pressure, technically known as a depression, near
the ocean's surface. Surface winds converge and, due to the
earth's Coriolis force, display a clear cyclonic pattern.
The inward rush of peripheral surface winds toward the
central area of low pressure, the rise of warm humid air in the
center, and the subsequent outflow away from the system at high
altitude, combine to create a self-sustaining heat engine. The
warmer the water temperature, the faster the air in the center
of the system rises. The faster this air rises, the greater will
be the difference between the surface air pressures inside and
outside the vortex.
Air flows from areas of relative high pressure to relative
low pressure. The greater the difference between peripheral and
central pressures, the faster the inflow. When wind speeds reach
40 miles per hour, the depression reaches tropical storm status.
When wind speeds reach 74 miles per hour, the storm is
designated a hurricane or typhoon. The term
"super-typhoon" is used for tropical cyclones that
reach maximum sustained 1-minute surface winds of at least 130
knots, which is the equivalent of a strong Category 4 or
Category 5 hurricane in the Atlantic basin. Note that the terms
"hurricane" and "typhoon" are regionally
specific names for the same phenomenon. Severe tropical cyclones
that occur in the Atlantic and eastern Pacific are referred to
as hurricanes, and in the western Pacific as typhoons.
The intensity of tropical cyclones varies by ocean basin. Of
the approximately 80 tropical cyclones that form each year
throughout the world, more than half will develop into severe
storms with wind speeds greater than 74 miles per hour. In the
western North Pacific, however, this figure is significantly
higher. Over two-thirds of all tropical cyclones that form in
this region are likely to develop into typhoons. The primary
reason for this region's propensity to spawn intense storms is
the extremely large area of very warm water. It is not unusual
for water temperatures here to exceed 86 degrees Fahrenheit.
Another reason is the formation of seasonal monsoon troughs,
which are climatologically preferred for tropical cyclone
formation. In fact, the lack of a monsoon trough in the Atlantic
may be one reason why tropical cyclones form less frequently
there and are typically less intense.