Tropical Storm

I

Introduction

Tropical Storm, weather system composed of a cluster of thunderstorms and of wind speeds near the surface of between 63 and 119 km/h (39 and 74 mph). Tropical storms develop out of storms called tropical depressions, in which wind speeds are less than 63 km/h (39 mph). If a tropical storm intensifies so that its wind speed reaches 119 km/h (74 mph), the storm becomes a hurricane. In contrast to a hurricane, a tropical storm typically does not have an eye, or calm area, at its center. Tropical storms form over large expanses of warm tropical ocean water. However, they do not form in the regions of the eastern Pacific or the Atlantic oceans near the equator or south of the equator.

Tropical storms cause torrential rainfall and flooding, which pose the gravest threat to populated areas. For example, in 1994, tropical storms Alberto, Beryl, and Gordon caused nearly $1 billion worth of damage in the United States. The flooding caused by Alberto killed 30 people in Alabama and Georgia. In June 1972 tropical storm Agnes killed more than 100 people along the East Coast of the United States and caused catastrophic flooding in the northeastern part of the country.

II

Formation

Most tropical storms form over warm tropical oceans from preexisting regions of low pressure called tropical depressions. In these regions, air is moistened and warmed. The hot air rises, creating an area of low pressure. The rising air forms thunderstorms. Air moving in toward the low-pressure area travels across the warm ocean waters, and is moistened. This air provides more water vapor to fuel a storm.

As the hot, humid air rises, the water vapor in the air condenses to form clouds and rain. When water vapor condenses, it releases energy. This energy is called latent heat, or heat of condensation. It represents the amount of heat that is absorbed or released by a substance when the substance undergoes a change of phase—in this case, a change from water vapor to liquid water (see Heat: Latent Heat). A storm is a type of heat engine. Latent heat powers a storm and intensifies it. A storm will intensify as long as warm, humid air flows toward the center of low pressure that, in turn, continues to attract air. In addition, the change of wind speed and direction with altitude (called wind shear) must be small (change of wind speed of less than 27 km/h, or 17 mph) so that the heat from the thunderstorm remains within the storm’s center.

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The rotation of Earth also affects the movement of air. In the northern hemisphere, Earth’s rotation deflects air from left to right, while in the southern hemisphere, it deflects air from right to left. This deflection is called the Coriolis effect (see Coriolis Force). As air moves toward a low-pressure center, the deflection causes the air to spiral around the center rather than travel straight into the center. The inward spiraling of air causes the formation of circular bands of thunderstorms, which are a distinctive feature of tropical storms and hurricanes, along with spiraling winds. The spiraling winds rotate faster as they approach the center. Centrifugal force flings the rotating air outward, making it increasingly difficult for air to reach the center (see Centripetal Force).

As a storm intensifies and its winds gain strength, the winds reach a speed at which the air can no longer reach the center. The region corresponding to the maximum penetration of the spiraling air is called the eye wall. The eye wall contains the strongest winds. Inside this ring of strong winds, a calm area develops, known as an eye. A major distinction between a tropical storm and a hurricane is the presence of an eye in a hurricane. An eye forms when the winds near the center reach about 119 km/h (74 mph).

Within 5° of latitude north or south of the equator, tropical storms and hurricanes do not occur, despite frequent thunderstorms. The reason for this absence is the weakness of the Coriolis effect within 5° of the equator. In this equatorial region the winds can travel straight into the center of a low-pressure system and eliminate it. This process halts a cluster of thunderstorms before it can intensify into a tropical storm. Tropical storms and hurricanes also do not occur anywhere in the south Atlantic Ocean or in the eastern part of the south Pacific Ocean. This absence is the result of relatively cold ocean temperatures and persistent large wind shear.

III

Death of Tropical Storms

Hurricanes and tropical storms weaken rapidly when they travel over cool water or over land. In these environments they lose their source of hot humid air. They weaken especially fast over land because they lose the evaporation of warm ocean water.