A tropical cyclone is the most violent storm to exist on Earth. The tropical cyclone is also known as a hurricane, cyclone and typhoon, which only differ from one another by geography. The storm is characterized by a low-pressure area at the eye of the storm, surrounded by strong winds. The winds revolve around the low-pressure area due to conservation of angular momentum coming from the Earth's rotation. Hurricanes have a diameter of hundreds of kilometers, with the largest reaching about 2000 kilometers.
At the center of the storm there is an area called "the eye of the storm", which is characterized by a lower pressure than the rest of its environment. Surprisingly, the precise center of the storm is actually a relatively quiet area, with very little wind and rain. The area at the outer edge of the eye is called the "eyewall" – there the winds revolve around the center at very high speeds, of up to 350 kilometers per hour. Hurricanes are rated on a scale of 1-5, according to wind speed. A storm with 100-130 km/h winds is classified as a category 1 hurricane and when wind speed exceeds 220 km/h the hurricane is classified as a category 5.
Hurricanes are formed near the equator and require a number of factors:
1. High water temperature: at least 26.5 degrees centigrade for a depth of dozens of meters, in order to induce instability in the atmosphere.
2. Sharp temperature gradient: atmospheric temperature dropping sharply with elevation, which enables heat to be released as water vapors undergo condensation.
3. High humidity, which is required for the formation a large amount of cumulonimbus clouds.
How does the storm form?
The hot and humid air above the ocean moves upwards, creating a low-pressure area near the water surface. The area surrounding the low-pressure area, which is characterized by high air pressure, pushes air towards the low point, where it heats up and becomes more humid and begins to move upwards. In its place, "new" cooler air is sucked into the low-pressure area. The hot and humid air that moved upwards cools and forms clouds, and then the whole process repeats itself. The storm spins due to momentum conservation and the Coriolis force, and it will dissipate only when one of its essential driving factors ceases to exist: the water cools down, the temperature differences in the air diminish, the humidity decreases and most likely – as the storm hits land.
When the hurricane is formed, its direction and speed depend on two factors. The first and most pivotal is its environmental steering, which is movement that occurs due to the flow field in the vicinity of the storm. The second is the beta drift, which is a north-west-bound motion due to the Coriolis force and momentum conservation. Another type of movement may occur if two storms come close to one another, which will result in them revolving around each other, but that is rare. The storm Ophelia, which has recently hit the coasts of Europe, is very rare due to the direction it is moving in. The storm has formed pretty far in the east Atlantic Ocean, and was mainly moving north, probably due to changes in the winds and currents. These changes are still not completely understood by scientists, and they will surely be the focus of many future climate studies.
Winds at very high speeds. The damages of hurricane Harvey in Texas, August 2017 | Photograph: Shutterstock
When does a hurricane dissipate?
A hurricane can dissipate if one of its essential driving factors disappears. The most common cause of dissipation is the storm reaching land, thus depriving it of the warm water that feeds it. If the storm returns to the water, it might "recharge" itself. A hurricane will dissipate also if it reaches an area of cool water, or an area with strong vertical winds that will destroy its eye-of-the-storm mechanism and the system that raises hot air upwards, which both enable its sustenance.
Over the years, many ideas for artificial dissipation of hurricanes have been proposed. Among others, there was an attempt to seed the hurricane with silver iodide. The idea was that the silver iodide would cause the supercooled water at the edge of the storm to freeze, thus causing the inner eyewall to collapse. These attempts have failed, only causing the storm to slow down or change direction. Other ideas included towing an iceberg under the base of the hurricane and even blasting it with a nuclear weapon. All these solutions pose major technical challenges and have very problematic repercussions. To date, there is no practical solution for the hurricane problem, due to the storm's huge size and short lifespan.
Are hurricanes affected by global warming? First, it is important to understand that hurricanes are still a phenomenon that is not fully understood and still extensively studied. Global warming affects, among other things, the composition of the atmosphere, the temperature of the water and the air, and the state of flows in the ocean. Recent studies using satellite images have shown that the number of storms has actually decreased in recent years, but they have much bigger magnitudes. The direct reason is most likely the elevation of the average temperature at the water surface, due to the greenhouse effect.
Watch the report on BBC about Ophelia hitting Ireland:
Translated by Elee Shimshoni