The formation of storms

2020 was a record breaking year for storms in the Atlantic, with dozens of named storms and cyclones spawning and making their way across the ocean. But how do they get going in the first place, and why are some more destructive than others? Chris Smith spoke to Conni Klein from the UK Centre for Ecology and Hydrology...

Conni - So tropical cyclones over the Atlantic, they generally develop from tropical disturbances, so Africa. And those disturbances are thunderstorms that are just packed together to a cluster and they travel with an area of low pressure over the continent. And those disturbances are actually the seedlings for tropical cyclones of which hurricanes are just a very extreme version. So those disturbances, they can strengthen over the Atlantic once they leave West Africa and they might develop into a hurricane when some ingredients are present in the atmosphere, but probably the most important ingredients, the warm, moist air over the ocean as a fuel for the storm. And such conditions we actually only find over tropical oceans with an ocean surface temperature of about 26 degrees and above. And so when the tropical disturbance moves over the ocean, it's sucking in air towards this low pressure center that already exists. You know, areas moving from high to low pressure. This air is sucked in, it's warm and moist, and this basically just comes from the evaporation of the very warm ocean. And once this air is lifted, in the case of a hurricane development, actually this air is highly unstable. So it continues to rise into this disturbance. And unstable really means that the rising air parcel remains warmer, which means it has lower density than the surrounding air masses. And it rises more rapidly into this storm. the greater the density difference to the surroundings is.

Chris - So the key ingredients then are you need something like a seed crystal, you get your initial storm over Africa, some disturbance. And when it meets this killer combination of some nice, warm seawater, which is warming the air above the water and making it saturated with moisture, it's that combination that then really accelerates things and you end up with a hurricane brewing off the back of it?

Conni - Yeah, exactly. So what is actually then happening is once this air gets moving into the storm, because the air is so moist, it releases additional energy from the water vapour in the air. Because water vapour is basically stored energy that was needed to evaporate the water from the ocean. And it's then released once the air rises and cools, it's released again in liquid water droplets

Chris - It's like the reverse of sweating, isn't it? So when you sweat you rob energy away from the skin to turn your sweat into water vapour, but when it turns back into a liquid in the atmosphere, it gives you extra energy and that's going to give the hurricane an extra thump.

Conni - Yeah!

Chris - Yeah. So it's reverse sweating for your hurricane, but what are the implications then for climate change? Because obviously if the killer ingredients are high levels of sea temperature, sustained high sea temperatures, does that mean then that if we warm the world up, we're probably on a course for more hurricanes with more intensity more often?

Conni - Yes. So the biggest factor that is also pretty likely to change rainfall intensities and hurricane intensities in general is just the higher water vapour content that we find in warmer atmosphere in the future. So by every degree that we warm the atmosphere, we actually increase the water vapor content that the atmosphere can hold by 7%. And that also means the atmosphere can rain out more water. And also there is more energy to actually be released in hurricanes. So that would drive stronger circulations and that's very likely to happen.

Chris - So it's a double-whammy then in some respects, isn't it, you're going to get more intense storms more often, but they're also potentially capable of dumping more rainfall when they do make landfall?

Conni - Yes, absolutely. And also stronger winds. Yeah. So those two factors are very likely to happen. What is not very clear yet is whether we will see a higher frequency in hurricane numbers overall. But it's quite likely that of the hurricanes we get a higher proportion is quite extreme.

Chris - Have we gotten any evidence that this is the case? Because obviously the earth has gone through patterns of changing climate over many, many thousands to millions of years. Is there evidence that what we're predicting with climate change has happened in the past with high temperatures meaning higher storm intensities and frequencies and therefore more destruction?

Conni - Yeah. So unfortunately the past sort of observations of hurricanes are not that dense or reliable that we can sufficiently or robustly relate it to anthropogenic forcing. But since the seventies, we actually have observed an increase in very extreme hurricanes above the category of three. So the ones that are really destructive. And otherwise we see this sort of behavior in models and projections yeah.

Chris - Can you tell us, Conni, the intriguing thing about how these storms get their names?

Conni - Tropical cyclones generally get a name when they develop winds that are stronger than 60 kilometres per hour, and then they might develop into hurricanes. And that's when they reach 120 kilometres per hour. So the world meteorological organization actually has lists for that. And it's six lists, they are being rotated over six years and it's very boring actually. It's an alphabetical sort of order of names.