Called the greatest storm on Earth, a hurricane can devastate coastal areas with sustained winds of 155 MPH or greater, storm surges as tall as a 4-story building, and can dump nearly 6 feet of rain in a 24 hour period. If those facts aren’t impressive enough, consider that a hurricane can expend as much energy during its life cycle as 10,000 nuclear bombs!
The term hurricane is derived from Huracan, a god of evil recognized by the ancient tribe from Central America called the Tainos. While hurricane is the term used for tropical cyclones in the Atlantic Ocean and eastern Pacific Ocean, tropical cyclones are called typhoons in the western North Pacific and China Sea area. In the western South Pacific and Indian Ocean they are known as cyclones or tropical cyclones. The scientific name for a hurricane, regardless of location, is tropical cyclone.
Hurricane seasons do vary from place to place but in general the United States can expect a hurricane season falling from June to November. Regardless of what they are called or where they are formed, there are certain favorable environmental conditions for a hurricane to form that must be in place.
• Warm ocean waters that are at least 80˚ F throughout a depth of about 150 feet
• An atmosphere that cools fast enough with height to be potentially unstable for moist convection
• Relatively moist air near the mid-level of the troposphere-about 16,000 feet
• At least a distance of 300 miles from the equator
• A pre-existing disturbance (stormy area)
• Low values of vertical wind shear (change in wind speed with height) between the surface and the upper troposphere-somewhere less than 23 mph
Warm water is one of the most important keys to the generation of hurricanes. As water vapor (water in the gaseous state) rises, it cools and causes the water vapor to condense into a liquid we see as clouds. In the process of condensation, heat is released which then warms the atmosphere making the air lighter still which then continues to rise into the atmosphere. As it does, more air moves in near the surface to take its place which is the strong wind we feel from these storms. The reason that a hurricane will weaken when its eye moves over land is simply because it doesn’t have the moisture and heat sources that the ocean provides. Simply put: no warm ocean, no hurricane.
Hurricanes are classified based upon the Saffir-Simpson Hurricane Scale. Wind speeds are the determining factor because storm surges are highly dependent upon the slope of the continental shelf in the landfall region.
• Category 1: Sustained winds of 74-95 MPH and storm surge of 4-5 feet
• Category 2: Sustained winds of 96-110 MPH and storm surge of 6-8 feet
• Category 3: Sustained winds of 111-130 MPH and storm surge of 9-12 feet
• Category 4: Sustained winds of 131-155 MPH and storm surge of 13-18 feet
• Category 5: Sustained winds of >155 MPH and storm surge of >18 feet
The biggest hazards associated with hurricanes are storm surges, sustained winds and wind gusts, inland flooding, and tornadoes. The storm surge is basically the water being pushed towards the shore due to the winds circulating around the storm. The storm surge can raise the average sea level by 15 feet with wind driven waves superimposed on top of the surge increasing the height of waves crashing onto the shore even greater. Wind gusts can reach speeds of 250 MPH or more. With wind gusts that great, a 15 lb 2x4 piece of timber would travel through the air at 100 MPH!
In addition to storm surges, hurricanes also cause several other types of flooding including flash flooding, urban area flooding, and river flooding. A slow moving tropical cyclone moving can deposit up to 6 feet of rain in a short amount of time! In fact, nearly 60% of the 600 deaths due to hurricane-induced flooding occurred inland. Tropical cyclones can also produce tornadoes which only add to the powerful destruction. A tornado watch is usually issued before a hurricane moves onshore on the right side of the hurricane’s path.
Neil Opperman
Weather Producer/Meteorologist
