The birth of a hurricane, having only a slight circulation with no closed isobars around an area of low pressure . Tropical disturbances commonly exist in the tropical trade winds at any one time and are often accompanied by clouds and precipitation. Surface wind speeds vary between 20 and 34 knots and a tropical depression has at least one closed isobar that accompanies a drop in pressure in the center of the storm. If sustained wind speeds increase to at least 35 knots, a tropical depression is upgraded to a tropical storm. Surface wind speeds vary between 35 and 64 knots and the storm becomes more organized.
Tropical storms resemble the appearance of hurricanes due to the intensified circulation.
As surface pressures continue to drop, a tropical storm becomes a hurricane when sustained wind speeds exceed 64 knots. A pronounced rotation develops around the central core as spiral rain bands rotate around the eye of the storm. The heaviest precipitation and strongest winds are associated with the eye wall . This information is used to provide advanced warnings to those populations at risk. Unfortunately, despite the advanced warning systems, hurricanes still claim the lives of hundreds, even thousands of people each year.