Hurricanes are powerful cyclonic (rotating) storms with maximum sustained winds of 74 miles per hour (119 km per hour) or higher. In the Pacific, they are called typhoons while in the Indian ocean they are known as cyclones. Hurricanes develop over warm seas and oceans from atmospheric disturbances such as tropical waves. In order for a hurricane to develop, the surface temperature of the sea or ocean must be at least 27 degrees Celsius (80 degrees Fahrenheit).
If conditions are favourable, an atmospheric disturbance can strengthen into a tropical depression. A tropical depression is an area of low pressure (about 1005-1010 mb) with a closed circulation. This means that winds spiral in towards this area of low pressure. The maximum sustained wind speed in a tropical depression is 38 miles per hour (62 km per hour).
A tropical depression may develop into a tropical storm under the right conditions. Sustained winds must be at least 39 miles per hour (63 km per hour) in order for a system to be classified as a tropical storm. The atmospheric pressure in a tropical storm is usually between 990 mb and 1000 mb. When a system becomes a tropical storm, it is given a name from a predetermined list. Storms are named in alphabetical order. The first storm of the year is given a name which starts with “A”, the second one gets a name which starts with “B” and so on.
If conditions continue to be favourable, a tropical storm will continue to strengthen. It becomes a hurricane when sustained winds reach 74 miles per hour (119 km per hour). Hurricanes may continue to strengthen and achieve sustained wind speeds much higher than this. Atmospheric pressure within a hurricane is less than 990 mb. Atmospheric pressure drops as the hurricane strengthens. The lowest pressure ever recorded for an Atlantic hurricane is 882 mb. It was recorded during the peak intensity of Hurricane Wilma in October 2005. At this point, Hurricane Wilma was a category 5 hurricane with sustained winds near 175 miles per hour (281 km per hour). The video below shows how hurricanes form.
Classification of Hurricanes
Hurricanes are classified into five categories according to the Saffir – Simpson Hurricane Wind Scale. The classification is as follows:
Category 1: Hurricanes with sustained winds of 74-95 mph or 119 – 153 km per hour.
Category 2: Hurricanes with sustained winds of 96-110 mph or 154 – 177 km per hour.
Category 3: Hurricanes with sustained winds of 111-129 mph or 178 – 208 km per hour.
Category 4: Hurricanes with sustained winds of 130-156 mph or 209 – 251 km per hour.
Category 5: Hurricanes with sustained winds of 157 mph (252 km per hour) or higher.
For more information on the Saffir – Simpson Scale click here. Be sure to check out the animation near the bottom of the page.
Structure of a Hurricane
A mature hurricane is roughly circular in shape and may be hundreds of miles across. The entire feature rotates around a relatively calm center which is known as the eye of the hurricane. In the northern hemisphere, hurricanes rotate in an anti-clockwise direction. Rain bands containing massive cumulonimbus clouds spiral in toward the eye. These clouds form as a result of the strong updraughts (rising air currents) within the hurricane. The rain bands are capable of producing the very heavy rainfall associated with hurricanes.
Strong winds spiral in towards the eye. Wind speed increases toward the center of the hurricane. The strongest winds occur in a part of the hurricane which is known as the eyewall. The eyewall is the part of the hurricane immediately surrounding the eye. Winds spiral into the eyewall and then they spiral upward toward the top of the hurricane. At the top of the hurricane, winds spiral outward in a clockwise direction.
At the center of a hurricane is the eye. The eye of a hurricane is the “hole” in the middle of the hurricane. A typical hurricane has an eye which is about 20 – 40 miles (32 – 64km) across. Conditions within the eye are relatively calm. Winds are light and there is little or no rain. Within the eye, air is descending. The sky may be clear. As the eye of a hurricane passes over an area, people who don’t know better may think the hurricane has passed. The structure of a hurricane can be seen in the diagram below.
The Movement of a Hurricane
Generally speaking, hurricanes move toward the west at about 10 to 15 miles per hour. In the northern hemisphere, they tend to curve toward the north-west. They tend to curve toward the south-west in the southern hemisphere. However, some hurricanes do not follow this general pattern of movement. In 1999, Hurricane Lenny developed in the Caribbean Sea and headed east, crossing the Leeward Islands and heading out into the Atlantic. Lenny is remembered as a hurricane which spent its entire lifespan heading the wrong way.
Before a hurricane makes landfall, the weather may be calm. Humidity is high. Wind speed and cloud cover increase as the hurricane approaches. When a hurricane makes landfall, the area experiences very strong winds. Often, there is very heavy rainfall. Winds are strongest in the eyewall. As the eye of the hurricane passes over an area, the wind dies down. It may even stop raining. Once the eye has passed, strong winds start blowing again. This time the winds blow in the opposite direction. As the eye moves away from an area, the wind speed decreases.
Warm seas and oceans are the source of a hurricane’s energy. As long as a hurricane remains over a warm tropical sea or ocean it can continue to sustain itself by “sucking up” warm, moist air. However, when a hurricane makes landfall or moves over cold water, it is cut off from its energy source and it begins to weaken. The video below shows how Hurricane Katrina developed and then dissipated after making landfall in Louisiana.
While over a tropical sea or ocean, a hurricane may be weakened by what is known as “wind shear”. Wind shear occurs when high level winds blow in a different direction (or at a different speed) from winds near the surface.