Extreme events like hurricanes cannot be predicted, and even the statistics are less certain because it is hard to define what constitutes a hurricane, a cyclone, or a typhoon. A useful definition is ACE (accumulated cyclone energy), an index which takes into account both the wind velocities and how long they persevere. The ACE value can be used to tell what is a hurricane and what is just a bad storm. Statistics are gathered for each region and year. Perhaps the most interest­ing are the data for the Atlantic region. In the 1970-1994 period, there were on average 8.6 tropical storms, 5 hurricanes, and 1.5 major hurricanes; and their average ACE value was only 70% of normal. By contrast, the period 1995-2004 had 13.6 tropical storms, 7.8 hurricanes, and 3.8 major hurricanes, with an aver­age ACE value 159% of normal [6]. In fact, only two years in that period, 1997 and 2002, had fewer hurricanes than normal, and those were El Nino years. It is well known that El Nino produces more severe storms in the Pacific but the oppo­site in the Atlantic.

Although these statistics show an increase in destructive storms, no direct cause-and-effect relation with global warming can be proved. Nonetheless, there are physical reasons why hurricanes arise, and these are being used in attempts to model hurricanes. When the sea surface temperature rises, more moisture is evaporated into the atmosphere. The water vapor has a greenhouse effect that increases the temperature further. The heated air rises, creating an upward flow of air. When the temperature reaches 26°C (79°F) locally, the air current is strong enough to create a hurricane. Whether this happens or not depends on the wind shear in the atmosphere. If the cross-winds are weak, the upward air currents become very strong in one place, seeded by some random fluctuation there. By Bernoulli’s Law, a flowing fluid has less pressure than one that is not moving. This is the same effect that causes a baseball to curve if given a spin such that the air flows on opposite sides of the ball are not equal. The incipient hurricane then has less pressure, and air flows into the column from all sides. The Coriolis force then causes the column to spin and develop into a cyclonic vortex. We described the Coriolis force briefly in Footnote 8. How this force causes winds and spins is interesting and often misunderstood, so we have added a detailed explanation in Box 1.2.

Tropical storms have a cooling effect on surface temperature. Evaporation of seawater cools the surface just as the evaporation of sweat cools our skin. Eventually, the moisture in the atmosphere condenses into rain, reversing the process and carrying the heat back into the ocean; and there is no net cooling. Storms, however, stir up the atmosphere so that this heat is carried up to higher altitudes, where it can be radiated into space before it comes back to earth. This may be a way for nature to stabilize the ocean’s temperature. Lightning-lit forest fires renew our forests by burning the undergrowth and allowing new trees to grow. Hurricanes and forest fires may be natural mechanisms that stabilize the present conditions on the planet. Both are catastrophic for mankind, but humans are only a minuscule part of life on earth.

Box 1.2 Why Do Northern Hurricanes Rotate Counter-Clockwise?

Hurricanes have been observed to rotate clockwise in the Southern Hemisphere and counter-clockwise in the Northern Hemisphere, and this has been attributed to the Coriolis force, illustrated in Fig. 1.23. The earth is shown rotating from west to east, causing the sun to rise in the east and set in the west. Several latitude lines are shown. Since these circles are smaller at higher latitudes, the ground speed of the rotation is highest at the equator and diminishes as one moves toward the poles. The atmosphere is dragged by the ground, and therefore the air has a different speed at each latitude, as shown by the lengths of the orange arrows at the left. Nothing happens until the air masses move north-south. Looking at the northern hemisphere in the left diagram, we see that if the air mass at the equator, say, moves northward from A to B, the large velocity of the air at A is brought into a region where the normal velocity is smaller. This motion is indi­cated by the wiggly blue arrows. The difference between the velocities is shown by the thick blue arrow. The people at latitude B, therefore, feel a wind blowing from west to east. The same happens in the Southern Hemisphere if the air moves south out of the tropics. Now suppose the air flow is toward the tropics, south­ward in the north and northward in the south. This is shown in the right diagram. Then the air masses move into regions where the normal velocity is larger. This slowing down of the normal speed appears as a wind going in the opposite direction, namely westward. This is shown by the thick blue arrows in the right diagram. The Coriolis force is the imaginary force that causes that wind.

Whether air moves north or south depends on other conditions, such as temperature or barometric pressure differences at different latitudes. It turns out that for latitudes between 30° and 60° N the motion is northward, as at B,

Fig. 1.23 Illustration of Coriolis force causing westerly (left) and easterly (right) winds