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The Making of a Tropical Storm

written by: Dr Mike C•edited by: Donna Cosmato•updated: 7/17/2010

Tropical storms can be among the most destructive weather events on the planet. This article explores just how do tropical storms start and reveals the staggering energy that they can contain.

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    Birthplace Of The Storm

    As the name implies, a tropical storm originates in the tropics; the region of the earth either side of the equator between the latitudes of 23.5° north (tropic of Cancer) and south (tropic of Capricorn) of the equator. During the spring and winter solstices, the sun is directly overhead at the tropic of Cancer and Capricorn, respectively. Since the sun is always overhead at the equator, the tropics do not experience any significant seasonal variation (i.e. no summer or winter) and the region is always warm. In this brief article, we shall explore the answer to the question of just how do tropical storms form.

    A tropical storm is potentially one of the most powerful and destructive weather events on earth, should it make landfall. Typically, somewhere between 80 and 100 topical storms develop each year. So, how do tropical storms start?

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    Coriolis Force

    A tropical storm forms over the warm waters of the tropics. The power of a tropical storm is generated from the thermal energy held within the ocean. The sea temperature needs to be at least 27 °C to be able to trigger a tropical storm. The storms are generated over waters that have a depth of at least 50m and are at least 500km distant from the equator. At these distances, the Coriolis force can permit the low pressure zones required for the formation of a tropical storm to persist in one location. The Coriolis force is caused by the earth’s rotation. Air movement in the atmosphere is caused by pressure gradients and the Coriolis force causes these air streams to be deflected in a westerly direction in both hemispheres, in the opposite direction to the earth’s rotation. At the equator, there is no Coriolis effect.

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    What Is A Tropical Storm?

    A tropical storm is essentially a very intense thunderstorm which involves extremely low pressure and cyclonic wind rotation. Sometimes, tropical storms are referred to as tropical cyclones, but this term also embraces other weather systems such as tropical depressions; typhoons and hurricanes. A tropical storm requires a sustained wind speed of 39 miles per hour – if this increases to 74 miles per hour it is classified as a hurricane. So much for definitions, but just how do tropical storms start? Well, as you'll see, a number of conditions all have to be met before nature can unleash its destructive power.

    The warm waters over which the tropical storm forms generate a lot of warm moist air (through the process of evaporation of the surface waters). The heat in the system causes the moisture laden air to rise. As it does so, a low pressure zone (depression) is created, which causes surrounding air to flow in to balance the pressure difference, generating a spinning weather system. As the warm air rises into the atmosphere, it cools and the moisture condenses out, forming clouds.

    For a tropical storm to form, there need to be surface level winds blowing from various directions which converge and assist the process of warm, moist air rising. An additional condition is that there should be little wind shear at higher altitudes as this allows the storm clouds to rise vertically to high levels. A tropical storm system may be six miles in height and up to 400 miles across. The storm will cause very large waves if the storm remains at sea, but its full destructive force is unleashed when the very strong winds, waves and torrential rain associated with the storm encounter land and human habitations.

    In a tropical cyclone, as little as 3% of the heat energy, contained in the rising moist air, may be converted into mechanical energy of the circulating winds. This relatively small amount of mechanical energy equates to a power supply of 1.5x1012 Watts - equivalent to about half the world-wide electrical generating capacity. No wonder tropical storms and hurricanes can be so destructive!

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    1. Geoscience Australia:
    2. Atlantic Oceanographic and Meteorology Laboratory:
    3. Coriolis Force, University of Illinois:
    4. The Met Office:

Weather Systems

The weather has a dramatic effect on life on planet earth. This series of article looks at various weather systems and offers an explanation of them.
  1. The Monsoon - A Critical Event For Millions
  2. The Nature of Thunderstorms
  3. The Making of a Tropical Storm
  4. The Impact of a Tsunami