What is a hurricane, and are they getting stronger?

Hurricane Beryl - the first in this year’s Atlantic season - has wreaked havoc in the Caribbean and the United States. The US National Oceanic and Atmospheric Administration, however, has warned that worse may follow. NOAA, as it’s known, has predicted that the region could have as many as seven major hurricanes this year - which is, on average, three more than it normally would. So, what’s driving this uptick in hurricanes and their earlier arrival? Helen Hooker is a meteorologist at the University of Reading…

Helen - A hurricane is a tropical storm. It entails extremely strong winds. We can categorise hurricanes from one to five. Category one begins around 74 miles an hour.

Chris - When do they tend to happen? And where?

Helen - They're known as hurricanes in the tropical Atlantic and the tropical Pacific. They tend to happen from around August time and then they peak September/October time.

Chris - And why that timing?

Helen - One of the main drivers of forming tropical cyclones or hurricanes is a warm ocean, generally above 26 degrees or so. Normally, it takes the length of the summer for the seas to warm up and this really gives the energy that can then generate the formation of hurricanes.

Chris - I was going to ask you that. So what's actually going on? What is that warm water doing to drive the development of the hurricane?

Helen - To develop a hurricane, you usually need two main drivers: you need energy from beneath and then something else that's going to cause uplift to suck up the air from above. We have these things called waves that are high in the atmosphere and this can cause this uplift. Then, at the same time, if you have a warm ocean, this is going to warm up the air above the ocean and then this air rises. The pressure would then drop at the surface, and this low pressure then draws in more and more air and this air is coming in and lifting and then it becomes a big cycle. So it's kind of self-sustaining

Chris - Presumably also because it's warm, it's going to be very humid. So there's going to be a lot of water going up as water vapour with that rising warm air?

Helen - Yeah, exactly. Which is why we associate extremely heavy rainfall with hurricanes.

Chris - What then determines how powerful that hurricane ultimately becomes?

Helen - It's the combination of that updraft and drivers from above, and the temperatures of the ocean is quite key. We've seen this year that the temperatures have been extremely warm very early in the season in the tropical Pacific. They're about three to four degrees warmer than usual. This really gives ideal conditions for this kind of rapid intensification of hurricanes where they form really, really quickly. Hurricane Beryl went from just a usual tropical storm to a category four hurricane in around 42 hours, which is an incredibly rapid intensification.

Chris - Does this mean then that climate change, rising sea surface temperatures and so on, are likely to make this happen on this sort of scale or greater more often?

Helen - There are regional variations in the expectations of how hurricanes will change in the future. It's likely that hurricanes are becoming more intense, so we're likely to see more of the stronger major hurricanes, but not necessarily a higher frequency overall. I think this is still a very active area of research and I think the general frequency is still very much under debate.