How are hurricanes formed?

A tropical cyclone is the most violent storm on Earth. Also known as hurricane, cyclone, or typhoon, depending on geographical location of origin, a tropical cyclone 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 up to 2000 kilometers in diameter.

The eye of the storm, 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, 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. 

Formed near the equator, hurricanes require a number of factors:

1.    High water temperature: at least 26.5 degrees Celsius dozens of meters down, 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’s surface. The high air-pressure area around the low-pressure area 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 condenses into 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 – making landfall.

When the hurricane is formed, its direction and speed depend on two factors. The first and most pivotal is environmental steering, the movement that occurs due to the flow field in the vicinity of the storm. The second is the beta drift, a north-west-bound motion (in the Northern Hemisphere) due to the Coriolis force and momentum conservation. If two storms come close to one another, another type of movement may occur as a result in their revolving around each other, but this is rare. Ophelia, the storm which recently hit the coasts of Europe, is very rare due to the direction of its movement. The storm formed at a relatively large distance in the east Atlantic Ocean, and was mainly moving north, probably due to changes in the winds and currents. These changes are still not fully understood by scientists, and will undoubtedly 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 dissipates if one of its essential driving factors disappears. The most common cause of dissipation is making landfall, which deprives the storm of the warm water feeding it. If the storm returns to the water, it might "recharge" itself.  A hurricane will also dissipate 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, both of which sustain it.

Over the years, numerous ideas have been proposed for artificial hurricane dissipation. These included attempts to seed hurricanes 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 approaches – from towing icebergs under the base of the hurricane to blasting it with nuclear weapons – present 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 not fully understood, with extensive studies underway. Global warming leads to changes in the composition of the atmosphere, the temperature of the water and the air, and the state of ocean flows, among other things. Recent studies using satellite images show that the frequency of cyclones has actually decreased in recent years, but their magnitudes has increased. 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