36
released energy is converted into mechanical energy; the faster winds and lower
pressure associated with them in turn cause increased surface evaporation and thus
even more condensation. Much of the released energy drives updrafts that increase
the height of the storm clouds, speeding up condensation. This positive feedback
loop continues for as long as conditions are favourable for tropical cyclone development. Factors such as a continued lack of equilibrium in air mass distribution would
also give supporting energy to the cyclone. The rotation of the Earth causes the
system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.
What primarily distinguishes tropical cyclones from other meteorological phenomena is deep convection as a driving force. Because convection is strongest in a
tropical climate, it defines the initial domain of the tropical cyclone. By contrast,
mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. To continue to drive its heat engine, a tropical
cyclone must remain over warm water, which provides the needed atmospheric
moisture to maintain the positive feedback loop running. When a tropical cyclone
passes over land, it is cut off from its heat source and its strength diminishes rapidly
as observed in case of AILA that passed across the mangrove ecosystem of lower
Gangetic delta during 23–26 May, 2009 (Fig. 2.2 and 2.3).
Types of Tropical Cyclones
Tropical cyclones are classified into three main groups, based on intensity: tropical
depressions, tropical storms, and a third group of more intense storms, whose name
depends on the region. For example, if a tropical storm in the Northwestern Pacific
reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon;
if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in
the Atlantic, it is called a hurricane. Neither “hurricane” nor “typhoon” are used in
either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to as simply “cyclones”. Comparison between different basins
become difficult as each basin uses a separate system of terminology (Table 2.2). In
the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the
International Date Line into the Northwest Pacific, becoming typhoons; on rare
occasions, the reverse may occur. It should also be noted that typhoons with sustained winds greater than 67 metres per second (130 kn) or 150 miles per hour
(240 km/h) are called Super Typhoons by the Joint Typhoon Warning Center.
A tropical depression is an organized system of clouds and thunderstorms with
a defined, closed surface circulation and maximum sustained winds of less than
17 metres per second (33 kn) or 39 miles per hour (63 km/h). It has no eye and does
not typically have the organization or the spiral shape of more powerful storms.
However, it is already a low-pressure system, hence the name “depression”. The
practice of the Philippines is to name tropical depressions from their own naming
convention when the depressions are within the Philippines’ area of responsibility.
2 Mangroves: A Shield Against Storms and Wave Actions
released energy is converted into mechanical energy; the faster winds and lower
pressure associated with them in turn cause increased surface evaporation and thus
even more condensation. Much of the released energy drives updrafts that increase
the height of the storm clouds, speeding up condensation. This positive feedback
loop continues for as long as conditions are favourable for tropical cyclone development. Factors such as a continued lack of equilibrium in air mass distribution would
also give supporting energy to the cyclone. The rotation of the Earth causes the
system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.
What primarily distinguishes tropical cyclones from other meteorological phenomena is deep convection as a driving force. Because convection is strongest in a
tropical climate, it defines the initial domain of the tropical cyclone. By contrast,
mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. To continue to drive its heat engine, a tropical
cyclone must remain over warm water, which provides the needed atmospheric
moisture to maintain the positive feedback loop running. When a tropical cyclone
passes over land, it is cut off from its heat source and its strength diminishes rapidly
as observed in case of AILA that passed across the mangrove ecosystem of lower
Gangetic delta during 23–26 May, 2009 (Fig. 2.2 and 2.3).
Types of Tropical Cyclones
Tropical cyclones are classified into three main groups, based on intensity: tropical
depressions, tropical storms, and a third group of more intense storms, whose name
depends on the region. For example, if a tropical storm in the Northwestern Pacific
reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon;
if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in
the Atlantic, it is called a hurricane. Neither “hurricane” nor “typhoon” are used in
either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to as simply “cyclones”. Comparison between different basins
become difficult as each basin uses a separate system of terminology (Table 2.2). In
the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the
International Date Line into the Northwest Pacific, becoming typhoons; on rare
occasions, the reverse may occur. It should also be noted that typhoons with sustained winds greater than 67 metres per second (130 kn) or 150 miles per hour
(240 km/h) are called Super Typhoons by the Joint Typhoon Warning Center.
A tropical depression is an organized system of clouds and thunderstorms with
a defined, closed surface circulation and maximum sustained winds of less than
17 metres per second (33 kn) or 39 miles per hour (63 km/h). It has no eye and does
not typically have the organization or the spiral shape of more powerful storms.
However, it is already a low-pressure system, hence the name “depression”. The
practice of the Philippines is to name tropical depressions from their own naming
convention when the depressions are within the Philippines’ area of responsibility.
2 Mangroves: A Shield Against Storms and Wave Actions