Blowin' in the Wind

About Hurricane Formation

After crossing Florida, Hurricane Katrina headed into the Gulf of Mexico early on August 26, 2005 as a Category One hurricane. Just 55 hours later it was a Category Five, packing wind speeds up to 175 miles/hour, with the Gulf Coast in its sights. How could this happen so fast? To better understand these storms, meteorologists have made the intensification of hurricanes a major focus of their research.

Air from surrounding regions flows into a region of low pressure. In the northern hemisphere, the Coriolis force curves winds to the right, as shown, producing the characteristic counterclockwise rotation of a low pressure area.

Adapted from a drawing in "Getting Around the Coriolis Force" by Dave Van Domelen, Kansas State University

Hurricanes are born in a concentration of tropical thunderstorms that forms an area of low pressure, with winds blowing in from surrounding areas. The Coriolis force (see sidebar), a so-called “fictitious” force that is a consequence of Earth’s rotation, curves these winds into a rotating system that spins counterclockwise in the northern hemisphere, as shown in the drawing, and clockwise in the southern hemisphere.

Hurricanes get their energy from the latent heat of water, the hefty 2500 kilojoules/kilogram released upon condensation of water vapor (the release of the energy needed to evaporate the water). The image shows Katrina over the warm waters of the Gulf of Mexico, which, as the colors indicate, was warmer than the Atlantic at this time.

A powerful feedback process can contribute to rapid hurricane intensification. As the spinning low pressure area moves over warm water, seawater evaporates and the warm, humid air rises, expands, and cools, condensing water vapor and releasing its heat of vaporization, making the air rise further. This rising air lowers the pressure and increases the surface winds, which in turn speeds up the evaporation from the sea surface. It is this cycle that enables the hurricane to gain strength so rapidly. To prevent tearing apart the developing storm as the warm air rises, wind shear—different horizontal wind speeds and directions at different altitudes—must be small.

Beyond the latent heat of water, mentioned above, its large specific heat—the amount of heat needed to raise a gram of water one degree centigrade—plays a role too. As the hurricane forms, the energy to evaporate the seawater comes out of the water itself. Since removing a given quantity of heat from a given mass of water produces a relatively small temperature decrease, the large specific heat minimizes the evaporative cooling.

Also, the winds in the hurricane have angular momentum, like a spinning ice skater. As these winds are sucked in toward the eyewall, decreasing their distance from the axis of rotation (see drawing), they speed up due to the conservation of angular momentum—like spinning ice skaters who draws in their arms to spin faster—thus intensifying the hurricane.

So putting it all together, hurricane development requires:

• Warm water (tropical latitudes, but see Coriolis force)
• Concentration of thunderstorm activity
• Humid air
• Small wind shear

Hurricane Katrina over the Gulf of Mexico: Colors correspond to three-day averages of sea surface temperatures; note that the Gulf was warmer than the Atlantic. (image courtesy of NASA/SVS)

Wind spiraling in towards the eyewall of the hurricane. The conservation of angular momentum requires that the wind speed increase as the wind’s distance from the center of the storm decreases. Also, note how the air spirals upward in the eyewall. (image courtesy of NOAA)