- from Hurricanes
Why don’t hurricanes ever form in Kansas, Oregon, or Vermont? Why do they always start in the tropics?
It’s because of a part of the tropics that lies between five degrees north and five degrees south latitude of the equator. There, ocean waters are at least 80°F over a large area. That is the first ingredient a hurricane needs to develop. The second ingredient is wind blowing westward off the continent of Africa.
Hurricanes feed on warm moist air rising from the Atlantic Ocean. Warm water evaporates (turns into water vapor) and rises from the surface of the ocean. As it rises, it cools. This causes the water vapor to condense (to become more dense or compact). Then, cumulonimbus clouds form. A cumulonimbus is a very large thunderstorm cloud that rises to a great height. Once these clouds form, the first stage of hurricane development has begun.
When water condenses to form clouds, it releases heat into the air. As air warms, it rises and is pulled into the cloud columns. The cloud columns grow larger and higher. Evaporation and condensation keep going in a cycle. This creates a pattern of wind that circulates around a center (like water going down a drain). As the moving column of air finds more clouds, it becomes a cluster of thunderstorm clouds. That cluster is called a tropical disturbance. ▶
When winds inside the cloud mass reach 25 to 38 mph, weather forecasters call the storm a tropical depression. How do the winds get that fast? Air molecules are always in motion, pushing and pulling when they hit objects. This push-or-pull force is called air pressure. There aren’t as many air molecules at higher elevations, so air pressure decreases. Air flows from high- to low-pressure regions. This flow of air is called wind. The greater the difference between the high- and low-pressure areas, the stronger the wind. As the thunderstorm grows bigger, the air at the top of the cloud column is cooling and creating an area of low pressure. This area draws more and more warm air up toward it. Winds in the storm cloud column spin faster and faster, whipping around in a giant circular motion. ▼
▲ Tropical Storm
When wind speeds reach 39 mph, the tropical depression becomes a tropical storm. The winds move faster and begin twisting and turning around the eye, or calm center of the storm. Wind direction is counterclockwise (west to east) in the northern hemisphere. But it is clockwise (east to west) in the southern hemisphere. This is known as the Coriolis effect.
After wind speeds top 74 mph, the tropical storm is officially called a hurricane. Now it’s not just high, it’s also wide. It’s over 50,000 feet tall and about 125 miles in diameter! Air at the top of the turning column merges with high air currents that do not change much in direction and speed. When that happens, the air goes in the opposite direction. However, some of the now-dry air is pushed back down the center of the spiral. The center is the eye, a low-pressure area where things are calmer. It’s anywhere from 5 to 30 miles wide. Trade winds from the western coast of Africa move the hurricane’s track from east to west.