Understanding A Hurricane and Tropical Storms

From How They Form, To How They're Maintained and How They Break Apart

With the summer months and severe weather season coming to an end, a new season is beginning – hurricane season. With the Atlantic hurricane season starting from June and ending in November, the United States has already been hit by a major hurricane. Being the first major hurricane to make US landfall in over a decade, Harvey may have you wondering how these systems are made, how they strengthen and what in the atmosphere is needed to weaken them. In this blog, we’ll cover all of that and more!

Hurricane Irma, Sept. 2nd 2017 via Nasa Rapid Response

In the warm waters of the Atlantic is where we’ll start. Northeasterly trade winds over Africa carry disturbances off the mainland and push them west into the Atlantic; the winds are constant throughout the year however, it’s during the summer and fall months when we these disturbances become more moist and unstable. Generally these disturbances will be marked by a cluster of thunderstorms, often near Cape Verde, every two to three days as the African Easterly Jet undulates (wave like motion) north and south. Sometimes those clusters of storms develops around an area of circulations and continues westward into the Atlantic.

Global Wind Circulations via Wikipedia 

As this organized cluster of thunderstorms continues westward, a handful of conditions are needed to take this group of thunderstorms to the next level – a tropical depression. First, which may be surprising, is warm sea surface temperatures; unlike low pressure systems that move across the continental United States and get their energy from the sun, tropical systems get their energy from the water. Sea surface temperatures have to be at least 80 degrees from the surface to a depth of about 150 feet.

                               Sea Surface Temperatures on September 3rd, via Wunderground 

Secondly, rotation needs to occur and be maintained. As you know, the Earth is constantly rotating but other than changing day to night, the rotation does other things too. Called the Coriolis Effect, the rotation of the Earth helps big systems like tropical storms and hurricanes rotate and effects whether they rotate clockwise or counterclockwise depending on the hemisphere they’re in. The perfect placement for these disturbances to feel the positive effects of the Coriolis Effect is about 300 miles from the Equator.
Third, there needs to minimal vertical wind shear. Strong vertical wind shear and tropical storms are like oil and water, they do not mix. Tropical systems like hurricanes have thunderstorms and need minimal vertical wind shear to maintain their updrafts. Too much vertical wind shear will essentially rip the tropical storm/hurricane/typhoon apart.

       Vertical Wind Shear, Green Being Favorable, Red Being Unfavorable via CIMSS Tropical Cyclones

Say these conditions were met, now our cluster of organized thunderstorms has become a tropical depression. It may look the same from a satellite and radar view but, by definition from NOAA, a tropical depression is a tropical cyclone that has maximum surface winds of 38 mph or less for at least a minute. As the tropical depression continues to move westward, while still staying in warm waters, it may become more organized with more defined rotation and stronger winds. If that’s the case, and those winds top out 39 mph and 73 mph, the tropical depression will be upgraded to a tropical storm. It’s at this point that storm surge becomes a concern.


New Disturbance Off of African Coast, via National Hurricane Center

This tropical storm will continue westward, taking any number of possible paths depending on where the upper level winds are pointed. While moving westward, the tropical storm will move through various areas of stronger/weaker vertical wind shear, warmer and cooler sea surface temperatures as well as air that’s drier or more moist. Each of these will influence its life but if the tropical storms path takes it through low vertical wind shear, warm sea surface temperatures and a moist environment, the tropical storm will most likely turn into a hurricane.

All Paths of Hurricanes and Tropical Storms from 1851 to 2015 via National Hurricane Center

Getting a name, meteorologists around the clock will be watching satellite loops, flying planes into the hurricane for atmospheric readings trying to stay ahead of the storm with accurate forecasts. Starting as a Category 1, with sustained wind speeds between 74 mph and 95 mph, this newly formed hurricane could (something that has happened with past hurricanes) quickly intensify if conditions are right.

Saffir-Simpson Hurricane Scale via National Hurricane Center 

Unlike tornadoes, hurricanes are categorized by their wind speed NOT the destruction they cause. From Category 1 to Category 5, with winds ranging from 74 mph to 156 mph and storm surges getting as high or higher than 18 feet, hurricanes generally go through multiple categories before their life comes to an end.

It’s here that we’ll briefly touch on what makes hurricanes dangerous – storm surge as well as flooding. With winds at the surface potentially reaching speeds as high as 156 mph, the westward moving hurricane is also taking with it water at the surface. With no obstacles in the waters way, like the shoreline, the waves will continue to gain and maintain their speed and height as they move with the hurricane. They maintain their speed and height, upwards of 18 feet or more with Category 5 hurricanes, up until the point they crash into the shore, since that’s the first obstacle they’ll face and continue to impact the shore for as long as the winds blow onshore.
While storm surge is occurring, so are rains. Heavy at times, even well after the hurricane has downgraded back into a tropical depression, these rains fall quickly causing dangerous flooding. And depending on how fast the hurricane/post hurricane is moving, these heavy rains can fall over the same area for several hours or even days.

Storm Surge Diagram via Wikipedia

With the Atlantic hurricane season beginning and the United States potentially seeing a second major hurricane make landfall, we wanted to explain what goes into making a hurricane. We also would like to stress that it’s important to make sure you know where the latest forecast is coming from. Trusted sources for forecast, especially with hurricanes, are the National Weather Service as well as the National Hurricane Center. There are a lot of fake forecast already circulating about the possible path of Irma, like the one below. It is not accurate and incites fear, if you see see it, do not share it.

Blaise Keller
KDLT Morning Meteorologist
Twitter – @blaisekellerr

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