Waves Depressions And Storms
All hurricanes begin as tropical waves. These are first observed as irregularities, or "bumps," in the tropical isobars (Fig. 6-2). Showers and thunderstorms develop on the eastern, or trailing, side of the disturbance. Tropical waves, also called easterly waves, are common during the summer months, and they usually move from east to west without intensifying. But when conditions are favorable for intensification, an easterly wave can develop a cyclonic circulation. When some of the isobars become closed curves, indicating that a cyclonic circulation has developed, the wave becomes a tropical depression.
A tropical depression is an area of low pressure, similar to a temperate low but without cold fronts or warm fronts. Such a disturbance can continue to strengthen. When this occurs, the central pressure keeps falling, and the wind speed increases. Warm air near the center tends to rise, and the surface winds
- Fig. 6-2. A tropical wave appears as a "bump" in the isobars on the equatorial side of an oceanic high-pressure system.
spiral inward. When the maximum sustained wind reaches 34 kt (39 mi/h), the disturbance becomes a tropical storm, and meteorologists give it a name. If intensification continues until the sustained wind reaches 64 kt (74 mi/h), the storm officially becomes a hurricane.
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