AUSTIN, Texas - As hurricane season approaches, the busiest time of year the weather will be full of different terms for bunches of clouds in the tropics. Each bunch of clouds is a distinct step on the path to a hurricane and it is good to know what characteristics each ‘step’ will have.
We’ll focus on the tropical Atlantic since those are the storms that affect Texas, but the principles are the same in each ocean.
1. An ‘invest area’
An invest area is named as an area that the National Hurricane Center is investigating. The investigation usually involves flying an airplane through the region of interest but can also involve satellite reconnaissance and data from ships and buoys.
Invest areas are typically unorganized areas in the tropics. They always start as a tropical wave.
2. A tropical wave
A tropical wave is a low-pressure area in that moves across the Atlantic ocean. It’s basically a large cluster of thunderstorms that aren’t particularly well organized. They create a lot of rain with gusty winds, lightning and flooding possible if they make it to land.
If the tropical wave starts to organize, it makes it to the next step.
3. The tropical disturbance
If a tropical wave is a low-pressure system that isn’t well organized a tropical disturbance is slightly more organized. The thunderstorms are a little longer lived and slightly more intense and the whole disturbance may have some faint rotation at the upper levels, but the surface winds aren’t strong, and the thunderstorms don’t have much power to them.
If the tropical disturbance starts to rotate, it climbs to the next level.
4. A tropical depression
A tropical depression is the first level that can be called a "tropical cyclone". At this stage there is an easily defined rotation in the clouds and winds at the surface are swirling around a central point. Those winds must be weaker than 38 mph. There may be some parts of the depression that has rain and thunderstorms, but the overall structure is disorganized.
Tropical depressions get lots of attention and are assigned a number but are not named. Tropical depressions can produce enormous rainfall amounts. Flooding can be very serious in some cases and is, really, the only significant threat if they make landfall.
That’s not the case with the next step.
5. A tropical storm
Here’s where the name comes into play and when things can start to get serious. A tropical storm is a well-organized and strong tropical depression. A tropical storm will have sustained winds between 39 mph and 73 mph and the thunderstorms will be wrapped completely around the storm’s center.
Tropical storms can cause some wind damage but are mostly a flooding threat if they make landfall. Rainfall totals from tropical storms can be extreme. In 1979, Tropical Storm Claudette dropped 45 inches of rain on Alvin, Texas.
The winds in Claudette weren’t that strong, but next the winds really come into play.
6. The hurricane
A hurricane is the strongest and most dangerous single weather system in the world. It is a tropical storm with sustained winds greater than 74 mph, though the winds are frequently stronger than that. The strongest sustained winds ever recorded were in Hurricane Patricia in 2015 which produced 215 mph sustained winds.
The peak gusts are frequently higher than that. The fastest wind gust ever recorded came from Cyclone Oliva in 1996 which produced a 254 mph wind gust as it hit an island off the coast of Australia.
The damage, danger and destruction from hurricanes is well known. In addition to extreme winds hurricanes also bring storm surges as the winds push billions of gallons of water ahead of them. Storm surge flooding often acts in tandem with the flooding rain to clog rivers and streams making the flooding worse.
The strongest hurricanes can cause catastrophic damage with their combination of wind, flood and storm surge damage. Hurricanes and tropical storms have cost the United States nearly $1 trillion in damages.
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