Phailin cyclone: How severe?

Source: University of Wisconsin
Source: University of Wisconsin

Both London-based Tropical Storm and the US Navy’s Joint Typhoon Warning Centre have forecast winds reaching up to 315 km per hour on landfall, which would make Phailin a category 5 storm—the most powerful. India Meteorological Department has, however, were daying till Friday that the cyclone would not be so severe. Phailin originated over east-central Bay of Bengal and has since intensified while moving north-westwards, 800 km southeast of Paradip (Orissa) and 870 km east-southeast of Visakhapatnam (Andhra Pradesh).

Phailin rapidly intensified on Thursday from a tropical storm with 65 mph winds to a top-end category 5 storm in just 24 hours. After reaching peak intensity near 8 pm Thursday, Phailin (meaning sapphire in Thai) began an eyewall replacement cycle.

Eye of storm

Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 kmph (115 mph), or major hurricanes (category 3 or above). When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small, some of the outer rain bands may strengthen and organize into a ring of thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a cyclone’s eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is “choked” by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.

Meteorologist Jeff Masters’s blog reads that in case of Phailin, the eyewall collapsed, and a new, larger-diameter eyewall formed from an outer spiral band. This process typically weakens the top winds of a tropical cyclone by 5-15 mph, and satellite estimates of Phailin’s central pressure increased from 910 mb to 934 mb during the eyewall replacement cycle, on Friday. However, satellite images show that Phailin has completed its eyewall replacement cycle and is now re-intensifying, with the cloud tops of the very intense thunderstorms in the eyewall expanding and cooling, as updrafts in the eyewall grow stronger and push the clouds higher into the atmosphere.

The latest satellite estimate of Phailin’s central pressure had dropped to 920 mb as on 13 UTC (9 am) on Friday. Radar out of Visakhatpanamin Andhra Pradesh shows that heavy rains from the outer bands of Phailin are already affecting the coast, and these bands were bringing rainfall rates of over an inch per hour, as estimated by microwave data from 10:55 UTC Friday. Phailin is over ocean waters that have warmed since Thursday, and are now 29 – 30°C. These warm waters extend to a lesser depth than before, and ocean heat content has dropped to a moderate 20 – 40 kJ/cm^2. Wind shear remains low, 5 – 10 knots.

In simpler terms, Phailin is now even bigger than Hurricane Katrina and half of the size of the Indian Sub-continent.

Tropical cyclone category system

CATEGORY 1 (tropical cyclone)

Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.

A Category 1 cyclone’s strongest winds are GALES with typical gusts over open flat land of 90 – 125 km/h.

These winds correspond to Beaufort 8 and 9 (gales and strong gales).

CATEGORY 2 (tropical cyclone)

Minor house damage. Significant damage to signages, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.

A Category 2 cyclone’s strongest winds are DESTRUCTIVE winds with typical gusts over open flat land of 125 – 164 km/h. These winds correspond to Beaufort 10 and 11 (Storm and violent storm).

CATEGORY 3 (severe tropical cyclone)

Some roof and structural damage. Some caravans destroyed. Power failures likely.

A Category 3 cyclone’s strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of 165 – 224 km/h.

These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

CATEGORY 4 (severe tropical cyclone)

Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.

A Category 4 cyclone’s strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of 225 – 279 km/h.

These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

CATEGORY 5 (severe tropical cyclone)

Extremely dangerous with widespread destruction.

A Category 5 cyclone’s strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of more than 280 km/h.

These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).


The Beaufort Scale
Average wind speed
Average wind speed
Estimating speed over land Estimating speed over water
0 Calm Less than 1 less than 1 Calm, smoke rises vertically. Sea like mirror
1 Light Air 1 – 3 1 – 5 Direction of wind shown by smoke drift, but not by wind vanes. Ripples with the appearance of scales are formed, but without foam crests
2 Light breeze 4 – 6 6 – 11 Wind felt on face; leaves rustle; ordinary wind vane moved by wind. Small wavelets, still short, but more pronounced; crests have a glassy appearance and do not break
3 Gentle breeze 7 – 10 12 – 19 Leaves and small twigs in constant motion; wind extends light flag. Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses
4 Moderate breeze 11 – 16 20 – 28 Raises dust and loose paper; small branches moved. Small waves, becoming longer; fairly frequent white horses
5 Fresh breeze 17 – 21 29 – 38 Small trees in leaf begin to sway; crested wavelets form on inland waters. Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray)
6 Strong breeze 22 – 27 39 – 49 Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray)
7 Near gale 28 – 33 50 – 61 Whole trees in motion; inconvenience felt when walking against the wind. Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
8 Gale 1 34 – 40 62 – 74 Breaks twigs off trees; generally impedes progress. Moderately high waves of greater length; edges of crests begin to break into the spindrift; the foam is blown in well-marked streaks along the direction of the wind
9 Strong gale 1 41 – 47 75 – 88 Slight structural damage occurs (chimney pots and slates removed). High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
10 Storm 2 48 – 55 89 – 102 Seldom experienced inland; trees uprooted; considerable structural damage occurs. Very high waves with long overhanging crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected
11 Violent storm 2 56 – 63 103 – 117 Very rarely experienced; accompanied by widespread damage. Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected
12 Hurricane 3,4,5 64 and over 118 and over Severe and extensive damage. The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected


Global tropical cyclone terminology

Tropical cyclones can be defined in different ways elsewhere in the world. Often news reports from the United States or Asia will refer to hurricanes or typhoons. These are all tropical cyclones, but with different names. While the category definitions are not identical, the following provides an approximate guide for comparison

Australian name Australian category US* US Saffir-Sim-
pson cate-
gory scale*
Arabian Sea /
Bay of
(East of
Tropical low Tropical depre-
Tropical depre-
Depression or severe depression Tropical depre-
Tropical depression
Tropical cyclone 1 Tropical storm Tropical storm Cyclonic storm Moderate tropical storm Tropical cyclone (Gale)
Tropical cyclone 2 Tropical storm Severe tropical storm Severe cyclonic storm Severe tropical storm Tropical cyclone (Storm)
Severe tropical Cyclone 3 Hurricane 1 Typhoon Very severe cyclonic storm Tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone 4 Hurricane 2 – 3 Typhoon Very severe cyclonic storm Intense tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone 5 Hurricane 4 – 5 Typhoon Super cyclonic storm Very intense tropical cyclone Tropical cyclone (Hurricane)

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