CAUSE
How typhoons form ?
There are main requirements for typhoon formation and development:
Sufficiently warm sea surface temperatures(SST), atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a pre-existing low level focus or disturbance, and low vertical wind shear.
Before discussing the process of typhoon formation, we need to first understand something about the basic principles.
Atmospheric Circulation
The troposphere undergoes circulation because of convection.
Convection is a mode of heat transfer wherin the heat moves with the material.
Warm air, becuase it is less dense than cooler air, rises and cold air sinks back toward the surface.
Convection in the atmosphere is mainly the result of the fact that more of the Sun's heat energy is received by parts of the Earth near the equator than at the poles.
Thus air at the equator is heated reducing its the density. Lower density causes the air to rise.
At the top of the troposphere this air spreads toward the poles.
If the Earth were not rotating, the atmospheric circulation will be applied with above shape. But, the Earth is in fact rotating.
As a result, atmospheric circulation patterns are much more complex.
The reason for this is the Coriolis Effect.
At the top of the troposphere this air spreads toward the poles.
If the Earth were not rotating, the atmospheric circulation will be applied with above shape. But, the Earth is in fact rotating.
As a result, atmospheric circulation patterns are much more complex.
The reason for this is the Coriolis Effect.
The Coriolis Effect
The Coriolis Effect causes any body that moves on a rotating planet to turn to the right (clockwise) in the Northern hemisphere and to the left (counterclockwise) in the Southern hemisphere. The effect is negligible at the equator and increases both north and south toward the poles.
So, the rotation of a typhoon is a product of the Coriolis force, a natural phenomenon because the Earth rotates out from under all moving bodies like water, air, and even airplanes.
Because of the Coriolis Effect, the pattern of atmospheric circulation is broken into belts as shown in the left side of Figure.
In the Northern Hemisphere, winds deflect to the right.
In the Southern Hemisphere, they deflect to the left.
This wind deflection gets storms spinning.
As a result, the typhoons in the Northern Hemisphere rotate counterclockwise and clockwise in the Southern Hemisphere.
The force also affects the actual path of the typhoon, bending them to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) if you're south of the equator.
In the Northern Hemisphere, winds deflect to the right.
In the Southern Hemisphere, they deflect to the left.
This wind deflection gets storms spinning.
As a result, the typhoons in the Northern Hemisphere rotate counterclockwise and clockwise in the Southern Hemisphere.
The force also affects the actual path of the typhoon, bending them to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) if you're south of the equator.
The Steps for Typhoon Formation
Typhoons are like giant engines that use warm and moist air as fuel. That is why they form only over warm ocean waters near the equator.
Due to evaporation and convection, the air above the ocean water becomes warm and moist. The warm air above the ocean will be rising to the upper atmosphere, which intensifies the release of laten heat by condensing. As the warm moist air rises, it becomes denser as it cools and begins to sink into the center of the storm. This center will become the eye of the hurricane if the storm progresses to that stage. The air continues to rise and sink as it spirals.
This causes wind. The Earth’s rotation causes the wind to begin and swirl as it rises.
As a result, the whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.
< The diagram of formation steps >
< The shape of typhoon>
If you could slice into a typhoon, it would look something like this.
The small red arrows show warm, moist air rising from the ocean’s surface, and forming clouds in bands around the eye.
The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds.
The large red arrows show the rotation of the rising bands of clouds.
The small red arrows show warm, moist air rising from the ocean’s surface, and forming clouds in bands around the eye.
The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds.
The large red arrows show the rotation of the rising bands of clouds.
To undergo these steps to form a typhoon, several environmental conditions must first be in place:
① Warm ocean waters (of at least 26°C) throughout about the upper 50 m of the tropical ocean must be present.
The heat in these warm waters is necessary to fuel the tropical cyclone.
② The atmosphere must cool fast enough with height, such that it is potentially unstable to moist convection.
It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated and used for tropical
cyclone development.
③ The mid-troposphere (5 km altitude), must contain enough moisture to sustain the thunderstorms.
Dry mid levels are not conducive to the continuing development of widespread thunderstorm activity.
④ The disturbance must occur at a minimum distance of at least 500 km from the equator.
For tropical cyclonic storms to occur, there is a requirement that the Coriolis force must be present.
Remember that the Coriolis effect is zero near the equator and increases to the north and south of the equator.
Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
⑤ There must be a pre-existing near-surface disturbance that shows convergence of moist air and is beginning to rotate.
Tropical cyclones cannot be generated spontaneously.
They require a weakly organized system that begins to spin and has low level inflow of moist air.
⑥ There must be low values (less than about 20 mph) of vertical wind shear between the surface and the upper troposphere.
Vertical wind shear is the rate of change of wind velocity with altitude.
Large values of vertical wind shear disrupt the incipient tropical cyclone by removing the rising moist air too quickly,
preventing the development of the tropical cyclone. Or if a tropical cyclone has already formed, large vertical shear can
weaken or destroy it by interfering with the organization around the cyclone center.