The eye of a majestic hurricane is a very warm place to be. The air inside the eye is sometimes 15 to 20 degrees Fahrenheit warmer than the storm's surroundings. Where does all this hot air come from, and how does it power the hurricane's strong winds?


The air in a hurricane's center or eye is always warm relative to its surroundings, and this is what makes hurricanes a unique type of atmospheric vortex in the tropics. Hurricanes are also referred to as tropical cyclones. Outside the tropics, in the midlatitudes, giant storms which bring everyday weather to the United States (called extratropical cyclones) contain cold air in their center, and typically don't feature anything that resembles a clear eye like the hurricane.

So where does the warm air in the hurricane's eye come from? The warmest air occurs at high levels, higher than five or six miles above the surface. Here, the temperature is commonly ten degrees warmer than surroundings, and can approach 15 or even 20 degrees in the most intense hurricanes. The warm air has its origins in the energy contained in the uppermost layer of seawater. The hot tropical sun warms the ocean surface, to values exceeding about 80 degrees Fahrenheit. The warm seawater readily evaporates. When liquid water evaporates, it turns into gasseous form. This change of phase requires an energy input, namely, the heat contained in the ocean. So water vapor extracts ocean heat, then rises and condenses inside giant thunderclouds surrounding the hurricane vortex. Condensation is the opposite of evaporation, so when the vapor turns back into liquid (in the form of cloud droplets and rain), the heat energy gets released into the air. The warm air high up in the clouds is retained inside the swirling vortex of winds, and this is how the core of the hurricane warms up. In addition, with air rising inside the eyewall clouds, there must be some compensation, in the form of air sinking inside the eye. When air descends from great heights, it compresses and warms. This process adds further warmth to the inside of the eye.

So, air sinking inside the eye and heat released by eyewall clouds during condensation creates a warm hurricane core. But how does warm ai in the eye power the hurricane? When you heat any material, it expands. Imagine a vertical column of air stretching from storm top to ocean surface and contained within the eye. When this air column is warmed from the aforementioned processes, it expands vertically. Since air pressure decreases with height, air with lower pressure essentially moves downward, closer to the ocean surface, from above. Thus, the surface pressure inside the eye decreases. This causes air to flow inward toward the center of the storm from the higher pressure surroundings. This inflow of wind gets deflected into a counterclockwise spiral (in the northern hemisphere) due to the Earth's rotation...and now you can see how the tremendous swirling winds of the hurricane are generated.

If this sounds complicated, here's a simple analogy. The hurricane behaves like a "heat engine" similar to the engine in an automobile. In a car engine, combustion occurs in cylinders inside the engine. The cylinders spin the engine. Gasoline is burned inside the cylinders, giving rise to a tremendous amount of heat, and this heat does the work of spinning the engine components. In a hurricane "engine", the cylinders are the giant clouds surrounding the eye. The "fuel" is water vapor, and instead of combustion, we have condensation. Heat is released inside the storm...warming the hurricane "engine". Through the mechanism described above, the heat energy in the eye lowers the surface pressure, which acts to accelerate the winds into a spinning vortex.

You can put your hand on the hood of a car, and by feeling how hot the hood is, you can get some sense for how fast the engine is running. But in hurricanes, the top of the "heat engine" is often obscured by clouds. We can't directly observe how "fast" the storm is operating from space...that is, how strongly the winds are blowing. But NASA has found a way to drop small, instrumented devices inside the eye of a hurricane, using a high altitude research aircraft that flies above the storm. These "dropsondes" allow us to take the temperature of the hurricane eye...and the warmer the air, the faster the winds beneath rotate. NOAA also has satellites which can detect the heat by penetrating through clouds, using microwave energy, and scientists are finding that a good correlation exists between the temperature inside the hurricane eye and the intensity of the storm.

This week's question is provided by Dr. Jeffrey Halverson. Dr. Halverson investigates severe storms at the NASA Goddard Space Flight Center, serves as the Education and Outreach Scientist for NASA's Tropical Rainfall Measurement Mission satellite, and teaches courses on meteorology at the University of Maryland Baltimore County. He holds a PhD in Environmental Sciences and writes a column on interesting weather phenomena in the bi-monthly publication Weatherwise. Dr. Halverson is also an avid amateur astronomer and enjoys hiking throughout the Mid Atlantic to better understand the region's complex geology.