Prof. Daniel Rosenfeld - Olivier Fitoussi - August 2011
Prof. Daniel Rosenfeld of Hebrew University. Photo by Olivier Fitoussi
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Hurricane Irene's high profile is due to the path it took, hugging the East Coast of the United States. Moving northeast while remaining above the ocean, it strengthened, but it was still close enough to the mainland to create damage all along the path from North Carolina to Virginia, Maryland, New York and Massachusetts.

This is the most destructive path anyone could have planned. Most of the damage it has caused is linked to the seawater that floods low-lying areas, and this is the reason these areas were evacuated. The tides, which create flash floods, are created by the low pressure in the center of the storm that sucks the water upward, but mostly because of the wind that pushes the water toward the shore. Storms like this hurricane also rain down huge quantities of water, up to 300 millimeters in the areas the storm passes. This adds to the damage.

Can you explain how a hurricane develops?

A hurricane is a storm that forms over a warm tropical ocean and is activated by the energy of the hot water at the end of the summer. It feeds the rain clouds and air rises, forming the shape of a cyclone in which the wind circles around the center of the storm. When the winds rise above 120 kilometers per hour, the storm is called a hurricane, or a typhoon as it is known in the Pacific Ocean.

What we're seeing above the Atlantic Ocean is a hurricane moving northwest above the Bahamas, where it reached Category 3 force with winds up to 200 kilometers per hour. The storm continued northwest to the East Coast of the United States while weakening somewhat, since it passed over cooler water while moving north. But it also passed over the Gulf Stream, which supplied it with energy. The Gulf Stream is a current of the Atlantic carrying hot water from the Gulf of Mexico to the northeast; it crosses the Atlantic toward western Europe. When the storm reaches dry land it is immediately cut off from its energy sources and subsides.

How is the swirling wind created?

It's the same dynamic that occurs when you remove a plug from a sink or bathtub. The water converges in the center where it drains, and while doing so it swirls. The speed creates a centripetal force that prevents the water from entering the center; this creates the hole in the whirlpool, or by analogy, the eye of the storm.

But while in a sink the water drains downward, air that has heated up - due to the warmth it has received from the ocean water - becomes lighter, rises and drains toward the upper atmosphere. There it produces a whirlpool that sucks air up and creates the phenomenon of a hole in the center of the storm.

Why do some seasons produce more hurricanes?

In addition to the seawater temperature changing from season to season, there are other atmospheric factors that support hurricanes. For example, if there are winds from one direction at a low height, and from another much higher, this will suppress the creation of hurricanes. In areas characterized by low-lying western winds and eastern winds at great heights, such as India in monsoon season, the wind prevents the creation of hurricanes .... So cyclones hit areas of India, the Bengal and Burma only at the beginning and end of the season, around the main monsoon season in the middle of the summer.

There have been changes over the years connected to fluctuations in the distribution of seawater temperature. There have also been changes in winds linked to this phenomenon that cause hurricane seasons to vary. This is what enables us to make certain predictions about the chance of hurricanes during the next season.

As with other extreme weather phenomena, people have begun wondering about the influence of global warning.

In principle, the hotter the water, the more we expect stronger hurricanes, so there's an expectation that we're seeing a trend of stronger hurricanes. But observation has not proved that this trend is taking place, according to statistics. If there is a sense that the phenomenon is increasing, it stems from the fact that the world is more connected and communicative. In addition, along with economic development, the sensitive shore areas are built up, so we make ourselves more and more vulnerable to the damage caused by hurricanes. So a hurricane of the same force that took place a few decades ago did much less damage compared to one today.

Why don't we have hurricanes over the Mediterranean Sea?

We will never see a hurricane over the Mediterranean in the summer. In the winter there are storms created by a similar mechanism but they can't turn into hurricanes. Basically, a storm forms because the air contains a lot of energy in the form of steam that it receives from hot seawater. This air can rise as high as 15 kilometers into the atmosphere while condensing and releasing it in precipitation. The release of energy causes a storm.

In the summer the air above the Mediterranean is hot and humid but can't rise high enough to create the phenomena we discussed, because it is blocked by the hot desert air that sinks down here. This is the reason our summers are dry, and storms, in particular hurricanes, can't form. But we can sum this up in a few simple words: The Mediterranean Sea is too small for a storm to get organized, and it is surrounded by a barrenness that causes the air that arrives here to be too dry to support a storm.