Hawaii battered by Tropical Storm Darby

Darby is only the fifth storm to hit the islands since 1949.

Tropical Storm Darby hits Hawaii
Waves slam the coastline as Tropical Storm Darby bears down on Hawaii's Big Island [NBC]

Tropical Storm Darby has made landfall on the Big Island of Hawaii.

Winds are currently battering the islands with gusts of more than 80 kilometres per hour, but the storm is weakening rapidly.

The winds are smashing against the tall volcanic mountains of the Big Island, causing the storm to disintegrate.

The rain, however, is more of a problem. By the time the storm clears, up to 400mm of rain is expected to have fallen across parts of the islands.

Given the terrain of Hawaii, this much rain is likely to trigger life-threatening flash floods and landslides.

Hawaii is rarely struck by tropical storms or hurricanes. Despite being a tiny archipelago in the middle of the Pacific, unprotected by any land mass, Darby is only the fifth storm to make landfall since 1949.

Its ability to dodge storms is thanks to the region’s winds, which act as a protective shield.

A tropical storm needs the winds to be of the same strength and direction throughout the atmosphere. This is known as low “wind sheer”, and allows the storm to grow undisturbed.

However, near Hawaii, the winds high up in the atmosphere usually blow from the southwest at around 65 to 95km/h, whereas in the lower part of the atmosphere the winds generally tend to blow from the northeast.

This ensures that any storm that approaches the islands tends to be “blown over”, causing it to weaken or disintegrate altogether.

The islands are most vulnerable to hurricanes during an El Nino event, when the surface water of the Pacific Ocean is slightly warmer than usual.

El Nino causes the strong winds high up in the atmosphere to drift away northwards. Then, if the winds nearer the sea become southwesterly, they can become similar enough throughout the atmosphere for hurricanes or tropical storms to maintain their strength.

Although the recent El Nino has now eased and the waters of the Pacific Ocean have cooled, there is still likely to be a remnant influence of El Nino in the atmosphere.

This influence could well have contributed to the track of Tropical Storm Darby and explain why the storm is currently battering Hawaii.