After the mass evacuation of New Orleans, Hurricane Gustav was not quite the monster storm that was feared, but it was a narrow escape.

On Monday Gustav struck the Gulf Coast as a Category 2 storm with winds of 179km/h (111mph) – weaker than expected, but stronger than the storm of October 1987 in England. So why did Gustav weaken?

When the storm passed over Cuba, it slowed down somewhat and got slightly roughed up. Afterwards the storm swept over a current of warm water, which should have supercharged it into something far stronger. But winds high up above were blowing at different speeds and altitudes, and this wind shear damaged the storm. Gustav did not recover from that assault, and having passed over the warm water current it hit a cooler patch of sea and lost power again.

If the wind shear had not sapped its strength, Gustav would have been a far worse threat: Louisiana and New Orleans had a lucky escape.

But there is no time for complacency. There are currently four tropical storms swirling around the Atlantic. Gustav is now far weaker but creating havoc inland in the US by dropping phenomenal amounts of rain. Tropical Storm Hannah is growing stronger and is forecast to hit the East Coast of the US tomorrow, anywhere from Florida to South Carolina. Tropical Storm Ike is following in its wake and steering towards the Bahamas. Behind that is Tropical Storm Josephine far out in the Atlantic. Such a flurry of Atlantic storms at the same time is quite unusual, and highlights what an active hurricane season this is turning out to be.