A hurricane may weaken or decay for several reasons. These reasons include high wind shear; moving over cooler ocean water; having its moist, rising air stopped or dried out; and moving over land. Let’s consider each of these reasons in turn.
Suppose that you are measuring wind speed and direction close to the ground. Then suppose you could continue taking measurements on a ladder rising straight up into the atmosphere. If the wind speed or direction changes as you climb, you are measuring wind shear. Wind shear is a change in wind speed and/or direction at different heights in the atmosphere. The amount of wind shear is critical to the development or destruction of a hurricane. Hurricanes need some wind shear to form storm convections and intensify, but too much shear can rip the storm apart.
Low wind shear allows a hurricane to form and grow stronger. Warm, moist air can rise to form clouds and convection can develop. High wind shear can tear a hurricane apart. Surface winds may move the lower part of a hurricane in one direction while higher-level winds blow the top part in a different direction.
Hurricanes also weaken when they move over cooler water. Cool ocean water doesn’t evaporate as much as warm ocean water does. Less evaporation means that less energy is transferred from the ocean to the atmosphere. With less energy, a hurricane weakens or decays.
A hurricane can move over cool water in several ways. Prevailing winds can steer a hurricane away from the band of warm tropical waters where it forms. This happens frequently along the Atlantic coast of the United States. A hurricane also could move into an area where a cold ocean current flows. The cold ocean current off the coast of California helps prevent Pacific typhoons from reaching that state’s coastline. Finally, a hurricane may pass over cold water that was stirred up by an earlier storm. The stirred water is cooler because cool water from beneath the surface mixes with the warm water near the surface.
Jason Dunion recently discovered that another atmospheric event seems to impact hurricane development. His analysis of old satellite images shows that a layer of hot, dry, and dusty air from Africa might prevent or weaken some Atlantic Ocean hurricanes. His current research is to determine whether such an air mass can truly suppress hurricane development. Jason is using NOAA’s hurricane hunter airplanes to study this more closely.
The Saharan Air Layer carries dust from Africa long distances. African dust forms a layer of red mud at the bottom of large parts of the Atlantic Ocean. It can also blow all the way to the southeast coast of the United States.
This air mass, called the Saharan Air Layer, forms over North Africa during the summer. At times, the Saharan air blows out over the moist air layer that is just above the water of the Atlantic Ocean. When it moves westward over the Atlantic, the Saharan Air Layer interrupts the updrafts that help strengthen a hurricane.
For the most part, temperatures in the atmosphere decrease with height. The Saharan Air Layer, however, causes a temperature inversion when it spreads over the Atlantic Ocean. When a
temperature inversion occurs, a layer of warmer, less dense air lies above a layer of relatively cooler, more dense air. With this layered structure, air that has been warmed by the sun stays near the ocean's surface and the water vapor made through evaporation cannot rise through the "cap" of the warmer, less dense Saharan air. If moist surface air cannot rise to higher altitudes the energy is not carried up into the atmosphere; towering storm clouds cannot develop; and a hurricane cannot form.
An encounter with the Saharan Air Layer can also affect a hurricane in other ways. If a hurricane approaches such an air layer, some of the dry air can mix into the storm. This robs the storm of moisture and decreases the transfer of heat energy to the atmosphere. Yet another way that the Saharan Air Layer can weaken a hurricane is by wind shear. High wind speeds in a Saharan Air Layer can produce strong wind shear that can tear a hurricane apart.
Landfall can also weaken a hurricane rapidly by robbing it of moisture and heat when it comes ashore. When a hurricane makes landfall, its source of energy—warm ocean water—is cut off. The storm quickly begins to decay. Its winds slow, and its structure becomes less organized. Although weakened, it can still cause damage. High winds may persist for a couple of days. Heavy rains may fall far inland. The hurricane can affect weather patterns over a large region, even as it travels over land and becomes a less severe storm.