We are keenly aware of the devastating destruction that hurricanes can and have caused. We also know that they are graded on the Saffir-Simpson Hurricane Wind Scale, a scale that is designed to estimate the storm surge and minimum expected wind speeds. We fear hurricanes, and justifiably so, but how many people actually know how hurricanes really form? This is something we just don't consider, and perhaps by understanding how they do form, we can learn more about how these powerful storms are easily one the greatest and most intense causes of natural disasters.
*Where tropical cyclones form -
Hurricanes are called typhoons and cyclones, but the proper scientific term for a hurricane is a tropical cyclone. Tropical cyclones that form over the Atlantic Ocean or the eastern part of the Pacific Ocean are called hurricanes. They are fueled by warm moist air and for that reason, they only form in the warm ocean waters near the equator.
*The formation of a tropical cyclone -
Warm moist air rises up over the ocean from near the water's surface. Because the warm air is rising, a low pressure area is left below. Air that has air pressure from other areas pushes its way into the low pressure area. We know that many storms form by a collision between high pressure and low pressure and cool air coming into contact with warmer moist air. This creates incredible instability in the air's atmosphere.
A new system that consists of the mixture of low and high pressure becomes warm and moist as it continues to rise. While the warm moist air continues to rise, other air that surrounds this warm air mass swirls in to replace it. Meanwhile, that warm and moist air is continuing to rise, and as it does, it cools of the moisture in the air, and in the process, creates clouds. The result of the process that created the clouds mixes with the wind and continues to grow, all the while, increasing in size. The heat and water that evaporates from the ocean's surface continues to feed the storm, fueling both strength and size.
*How the storms rotate -
Because of the way the earth rotates on its axis, storms that develop north of the equator will spin in a counterclockwise motion. Storms that form to the south of the equator will spin in a clockwise motion. In addition to the rotation of the storm itself, the eye begins to form as the storm rotates, causing the rotation to get progressively faster. In the area nearest to the eye, there is very low pressure and the air and atmosphere are calm and clear. That won't last for long, however, because the high pressure from above is sucked down into the eye, fueling the rotation even more.
*The stages of a tropical cyclone -
Until the wind speeds of the rotating storm reach 39 mph, the storm is considered a tropical depression. This is still a potentially severe storm, but the damage inflicted by a tropical depression will be much less severe than it would with more well developed storms. Once the storm's wind speeds reach 39 mph, the storm becomes a tropical storm. From 39 mph until the storm reaches 73 mph, it will remain a tropical storm. When the storm's wind speeds finally reach 74 mph, the storm is officially declared a "tropical cyclone " or a full blown hurricane.
*From landfall and beyond -
By the time a tropical cyclone or hurricane makes landfall, it is already starting to weaken because it no longer has the warmth of the ocean waters to fuel it. This doesn't mean that as a storm moves further inland that it can't dump many inches of rain fall and cause severe flooding. It also doesn't mean that the storm isn't capable of causing considerable wind damage from high wind speeds and even tornadoes in some cases. People shouldn't get lax about heeding weather warnings after a hurricane has made landfall. There is still the potential to cause major damage and kill people, livestock and destroy vegetation in the area.
Hurricanes are arguably one of the most dangerous types of storms. Although there have been great advances in hurricane forecasting and in the prediction of the path of hurricanes, there is still much that needs to be done to ensure the safety of people in the path of these storms. It is always a good idea for people who might be affected by any aspect of a hurricane (whether it be the storm itself, the feeder bands, or the residual effects,) prepare themselves by planning an evacuation route well in advance, making arrangements to go someplace for safety, and keeping a fully stocked hurricane readiness and survival kit on hand. The more prepared you are, the safer you and your possessions will be and the easier it will be for you to get through the storm and aftermath.
NASA's The Space Place: How Hurricanes Form