Hurricane season: Different storm types

Huge waves pound the Virginia Beach Oceanfront during Hurricane Sandy. (Facebook/Marc Bianchini)

What is a Tropical Depression?

Just north of the equator during the late summer and early fall, tropical waves move from east to west. These elongated areas of thunderstorms and relatively low air pressure often begin rotating and acquiring steady winds of 25 to 38 mph. Once those wind speeds are reached, the tropical wave becomes a Tropical Depression.

What is a Tropical Storm?

A Tropical Depression becomes a Tropical Storm when thunderstorms move into the center of circulation and steady winds reach to between 39 and 73 mph. At this point, the Tropical Storm is assigned a name. On satellite pictures, Tropical Storms often take on a “comma” shape.

What is a hurricane?

A hurricane is a large rotating storm of tropical origin, centered around an area of very low pressure…the eye. The whole storm system may be five to six miles high and 300 to 600 miles wide with sustained winds reaching 74 mph or greater. It moves like an immense spinning top, with forward speeds of up to 30 mph. As the warm sea heats the air above it, a current of very warm, moist air rises quickly, creating the center of low pressure at the surface. Trade winds rush in toward this low pressure and the inward spiraling winds whirl upward releasing heat and moisture before descending. The rotation of the Earth causes the rising column to twist counterclockwise, gradually taking on the form of a cylinder, whirling around the relatively calm eye.

As a hurricane encounters the friction of land, the winds become more turbulent , creating small, short-lived tornadoes. But the most devastating effect of the hurricane is called storm surge. The 74+ mph winds of a hurricane traverse vast expanses of open water, literally piling up water ahead of it. When this mass of water meets the immovable mass of shoreline, the water climbs up the shore to produce widespread tidal flooding.

Storm surge can continue into the rivers and inlets connected to the ocean and the Chesapeake Bay. Storm surge rises beyond the waterways into our neighborhoods, and often into our homes. The maximum effect of a hurricane is felt within 50 miles of its center…usually just northeast of the eye.

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