General Aspects
What is a hurricane?
A hurricane is a type of tropical rotating storm. It is a generic term that is used for any meteorological phenomenon that has spiral winds and that moves across the earth’s surface.
It generally has a center of low atmospheric pressure and of temperatures higher than those that immediately surround it. It circulates in a closed form around a central point.
Tropical rotating storms are classified according to the speed of their winds: tropical depression (under 38 mph or 65 km/h), tropical storm (between 38 and 73 mph) or hurricane (above 73 mph or 110 km/h).
How does a hurricane originate?
A hurricane works similar to a simple steam machine, with hot and humid air providing its fuel.
When the rays of the sun heat the ocean’s waters, the humid air is warmed up, it expands and it begins to rise just as a hot-air balloon does. More humid air replaces the air that rose, and the same process starts again.
Certain elements must be present in order for a hurricane to form: |
1. Temperatures greater than 80 F: |
At this temperature, the ocean’s water is evaporating at an accelerated rate required for the system to form. It is the process of evaporation and condensation of the water vapor into clouds that releases the energy which gives force to the system in order to generate the strong winds and rain. In tropical zones temperatures are normally high, which gives rise to the second necessary element. |
2. Humidity: |
There needs to be high levels of humidity because a hurricane uses the evaporation energy as its fuel. Conditions are optimal on the ocean where a hurricane advances and increases in energy with ease. It is weakened however, when it reaches the mainland. |
3. Wind: |
The presence of warm wind near the surface of the ocean, creates the conditions for evaporation. The evaporated water begins to rise forming negative pressure that drags the air in a spiral form inwards and up. This in turn, allows the evaporation process to continue. Winds need to be weak in the higher levels of the atmosphere so that the structure stays intact and the cycle is not interrupted. |
4. Revolution o "Spin": |
The Earth’s rotation eventually gives movement in a circular form to this system: The system begins to turn and to move just like a gigantic spinning top. This spinning occurs counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. |
What is the size of hurricanes?
A hurricane is normally between 8 and 10 kilometers high and 100 to 500 km wide. However, its size can vary considerably.
The smallest hurricanes can measure only 40 km in diameter and the largest between 600 and 800 km. The most gigantic hurricanes form in the Pacific Ocean and can measure up to 1,700 km in diameter.
The eye of a hurricane usually measures between 25 and 35 km, although this can vary drastically. The eye of hurricanes in the Pacific, where cyclones have more water to cross before reaching the mainland, tend to be the largest in the world, with an approximate diameter of 80 km.
FREQUENCY |
In a normal year approximately 60 hurricanes occur, with much more frequency in the Northwest Pacific (Philippines and Japan). |
SPEED |
The speed of displacement of a hurricane is approximately 20 km/h, but this can vary considerably and change abruptly. A human being walks at a speed of 4 to 5 km/h. |
Where do hurricanes come from?
As the ocean temperatures need to be more than 80 F, hurricanes form in different areas in different months of the year, generally during the hottest period. Hurricanes happen in all tropical oceanic areas with the exception of the South Atlantic and the South Pacific.
Remember that a hurricane needs a large amount of ocean to generate strength and to nourish itself, and it moves with the Earth’s rotation towards the west. This means that it moves to areas where it can “run” without being interrupted or weakened by land. Tropical systems are formed constantly, but they do not all have the appropriate conditions and space to generate strength.
The Structure of a hurricane
This steam engine has a center that is warmer than the air around it. It receives its energy from the condensation of water vapor.
The vapor (generated by the evaporation of the sea) begins to expand and to ascend quickly. When it reaches the high atmospheric zones, where the temperature is cooler, the vapor re-condenses freeing large amounts of energy and creating enormous clouds (up to 15,000 m high) and abundant rain. These phenomena can be clearly distinguished on weather forecast satellite images on TV.
In the lower regions of hurricanes (up to 3,000 m) the air is sucked towards the center. In the middle levels there is cyclonal circulation of ascending air (air rotates around the center). In the higher regions of the hurricane (above 6,000 m) the air moves outwards.
The famous eye of the hurricane
The eye is an area of relative calm in the center of a hurricane that extends from sea level to the highest part. It is surrounded by a wall of thick rain clouds. Inside the eye, due to the high temperatures and the presence of hot wind, the evaporated water is dragged quickly upwards, creating dry air that cannot be condensed and is therefore, cloud free. This is what is most noticeable when observing the hurricane from a satellite.
Even though the hurricane is larger, its eye is most clearly visible, unless the hurricane has formed very high clouds, which prevent its visibility.
The wall of the eye is an area with two opposing forces: the force of the air that moves towards the center and the centrifugal force that moves outwards. In the wall of the eye the most intense winds can be found and this is where tornados are formed.
The presence of the eye differentiates hurricanes from tropical storms (where there is no eye as well as lower wind speeds).
The size of the eye is not always proportional to the magnitude of the hurricane, although the largest eyes have been seen in category 4 hurricanes.
Information taken from:
Temporada de Huracanes.Net
http://www.temporadadehuracanes.net/index.html
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