Tropical Depression. The first sign that a hurricane may be in the making is the appearance of an organized cluster of thunderclouds over tropical seas. This region of convective activity is labeled a tropical disturbance if a center of low pressure is detectable at the surface. A Tropical depression has a highest wind speed of 38 miles per hour (33 knots), with some rotary circulation and one or more closed isobars. When viewed from a satellite, tropical depressions appear to have little organization. However, the slightest amount of rotation can usually be perceived when looking at a series of satellite images. Instead of a round appearance similar to hurricanes, tropical depressions look like individual thunderstorms that are grouped together. Chances are that the tropical depression was triggered by the ITCZ, by a trough in the westerlies intruding into the tropics from midlatitudes, or by a wave (or ripple) in the easterly trade winds, called an easterly wave.
Tropical Storm. Once a tropical depression has intensified to the point where its maximum sustained winds are between 35-64 knots (39-73 mph), it becomes a tropical storm. It is at this time that it is assigned a name. During this time, the storm itself becomes more organized and begins to become more circular in shape -- resembling a hurricane. The tropical storm stage may persist for up to 45 days, whereas the hurricane stage usually lasts for several days, up to more than two weeks, and longer in the Eastern Pacific. The main energy source is latent heat derived from condensed water vapor, and for this reason hurricanes are generated and continue to gather strength only within the confines of warm oceans.
A low pressure system which develops into a hurricane may have less than hurricane intensity for considerable proportion of its life. Even out in the sea before striking the coast and dissipating, the system fluctuates in its intensity , sometimes going up and sometimes going down on the Beaufort wind scale (see below). Sudden intensification into a hurricane may take place at considerable distance from the place where it deepened into a depression.
Definition of Various Stages by Beaufort Wind Scale. Out in the sea, ships report winds mostly by the appearance of the state of the sea, using the Beaufort scale. In terms of wind speeds, these Beaufort scale numbers are:
|B.F. No.||Speed (Knots)||Stage|
|12||>64||Cyclone, Hurricane, Typhoon, etc.|
By and large, a tropical circulation with B.F. 6-7 is called "Depression" and with B.F. 8-11 is called "Storm". A system with B.F.¹ 12 is called "Hurricane". Some meteorological Services further subdivide depression and storm stages as shown below :
|Severe Cyclonic Storm|
Out in the sea, classification is by maximum sustained wind speed experienced 10 m above sea level. Over land area, the intensity is judged by winds at about 1 km above the ground. Currently, satellite pictures are used to judge the intensity of a tropical cyclonic vortex.
Hurricanes. Once the sustained winds in a Tropical Storm have reached at least 64 knots, it is referred to as a hurricane, cyclone or typhoon, depending on its location.
Hurricanes are assigned an intensity rating between 1 and 5 using the Saffir-Simpson Hurricane Scale. This is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region.