September 2 - Hurricane Eye

Hurricane Eye - Hurricanes start over the oceans as a collection of storms in the tropics. The deepening low-pressure center takes in moist air and thermal energy from the ocean surface, convection lifts the air, and high pressure higher in the atmosphere pushes it outward. Rotation of the wind currents tends to spin the clouds into a tight curl; as the winds reach gale force, the depression becomes a tropical storm. The mature hurricane is nearly circularly symmetrical, and its influence often extends over an area 500 miles in diameter. As a result of the extremely low central pressure surface air spirals inward cyclonically (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), converging on a circle of about 20 miles diameter that surrounds the hurricane's “eye.” The circumference of this circle defines the eye wall, where the inward-spiraling, moisture-laden air is forced aloft; after reaching altitudes of tens of thousands of feet above the surface, this air is finally expelled toward the storm's periphery and eventually creates the spiral bands of clouds easily identifiable in satellite photographs. Because the outward increase in pressure is greatest there, the eye wall is also the region of maximum wind speed. By contrast, the hurricane eye is almost calm, experiences little or no precipitation, and is often exposed to blue sky.

This satellite image shows the progression of the hurricane as it moves from the Atlantic ocean across the Gulf of Mexico. The warm waters of the Gulf increase the power and size of the hurricane. Notice the size and clarity of the hurricane's eye increase from the category 4 hurricane in the Atlantic to the category 5 in the Gulf.