[Ref Understanding Weather & Climate, Books a la Carte Edition (6th Edition) #ISBN-13: 978-0321773227 #ISBN-10: 0321773225]
Tropical storms have different names depending on where they occur
- Extreme Western Pacific – Typhoons
- Indian Ocean and Australia – Cyclones
- Atlantic and Eastern Pacific – Hurricanes
(Hurricanes are refer to any storm, regardless of location)
In structure, these 3 kinds of tropical storms are same
Features of hurricanes
By definition, hurricanes have sustained wind speeds of 65KT (120 km/hr) or greater
- Average peak winds are in a range from 80KT (150 km/hr) to 190KT (350 km/hr)
- Sea level pressure near the center of a typical hurricanes is around 950mb (and the pressure could be as low as 870mb)
- typically 324 NM (600 km) wide
- lifetime can be in a rage from several days to even a week or more
- Hurricanes are the most powerful of all storms
Important concept –
Tropical storm and hurricane is not common between 0° and 5° latitude; In contrast, it is more common between 5° and 20° latitude – for more, read below!
Hurricanes and Season
Hurricanes obtain most of their energy from the latent heat released by condensation, so they usually appear in a deep layer of warm water. This also explains why hurricanes (tropical cyclones and typhoon) common in the Northern Hemisphere between August and September and it appear in Southern Hemisphere between January and March.
Structure of Hurricane
Hurricanes contain a large number of thunderstorms arranged in a pinwheel (windmill) formation, with bands of thick clouds and heavy thundershowers spiraling counterclockwise (in Northern Hemisphere).
Those bands of convective clouds are separated by areas of weaker uplift and even descending air (where have less intense precipitation).
In the center of the system is called eye, average about 15 NM (30km) in diameter, where wind speed and intensity of precipitation also increase. The change in the size of an eye gives some indication of whether the hurricane is intensifying or weakening.
Generally speaking, a shrinking eye indicates an intensifying hurricane.
Eye wall is the area around the eye in a range of 5NM – 10 NM (10-20km), where strongest winds and intense precipitation bring windshear and downpour.
On the ground, when one side of the eye wall passes and the eye approaches the island, the could is clear and showing blue skies with clam wind condition – however, don’t be faked by this “brief lull”. Once the opposite side of eye wall covers the island, storm conditions resume.
Formation of “True hurricane” – 4 steps
- Tropical disturbances
- Tropical depression
- Tropical Storm
- True hurricane
small clusters of disorganized thunderstorms in the eastern portions of the oceans. These thunderstorms have weak pressure gradients and little or no rotation.
When a tropical disturbances develops to the point where there is at least one closed isobar on a weather map
If the tropical depression intensifies and maintains (sustains) wind speeds 30 KT (60km/hr)
The Tropical storm intensifies further and maintains (sustains) wind speeds 60 KT (120km/hr)
Conditions required for hurricane formation
- release of latent heat by evaporation
- high moisture in the air – over deep water surface (e.g ocean)
- temperature above 27°C
- coriolis force
- There is no coriolis force along equator, the hurricane formation is prohibited between 0° and 5° latitude; In contrast, tropical storm and true hurricane appear between 5° and 20° latitude
- The warmer the ocean, the stronger the tropical storm and hurricane
Indeed, hurricane is started from small clusters of disorganized thunderstorms. Their formation share some similar requirements.
What’s “warm-core cyclones“?
Hurricanes (differ from mid-latitude cyclones), which are “warm-core”.
Since the air flows inward toward the center of low pressure system. Firstly, the heat of the warm ocean surface transfer from outer to the center of the low pressure system. Secondly, the low pressure feature of the eye, air rise and adiabatic expansion cause the temperature to further increase. Overall, there is no significant temperature difference across the base of the storm, but much energy is added and reflects in temperature aloft – temperature significantly increase near the eye aloft.
In other words, flying above the eye of a tropical storm would decease aircraft’s performance!