While I don’t know a whole lot about the demographics of my readership, I’m going to assume that most of you have at least a passing familiarity with the water cycle. Maybe it’s just me, but when I learned about it as a kid, I learned about it as a description of the movement of water around the surface of the planet. The water cycle is about water. Straightforward, yes?

Then, a few years ago, I was writing a climate science lesson, and I had a minor revelation. The water cycle also describes the movement of energy in our atmosphere. When water evaporates, it effectively absorbs the heat required to keep it in a gaseous state. That cools off the place where the evaporation happens, which is why our own ability to regulate our temperatures relies heavily on evaporation. So now you have that water vapor, kept in that state by a combination of temperature and pressure. It rises up, and after a certain point reaches a low enough pressure and temperature to condense, which turns it into water droplets (clouds, rain, mist, etc), and warms up the air around them. That heat was just transported, as water vapor, from one part of the world to another. Of course, that same bit of water might absorb and release heat like that many times over before it falls back to the ground. If you watch clouds for long enough, on a mostly clear day, you can see some of them forming, or even some that fade in and out of existence as they move through pressure gradients shifting from gas to droplets, and back to gas again.

As I said, this may all be obvious to you, but for some reason it never really clicked in my head until I was actually studying the movement of heat energy in our atmosphere. From that perspective, when enough water fell on Pakistan to submerge one third of the entire country, a huge amount of heat was released into the air above. It’s been interesting to think about, but I couldn’t begin to tell you what predictive value that has for our day to day lives. It could mean that we should expect heat to follow flooding, but if the air warmed is pretty high up to begin with, would that follow? Further, all that water starts evaporating again, sucking up more heat. Fortunately, the world need not wait for people like me to puzzle this stuff out. A team out of Arizona has  found that when a tropical cyclone hits a city, it causes a spike in temperature in the days that follow. 

Three days after Hurricane Fiona struck Puerto Rico in mid-September, the National Weather Service issued an extreme heat advisory, warning that the heat index – which incorporates humidity to calculate perceived temperature – could reach up to 109 degrees.

Above-average temperatures almost always follow tropical cyclones – which by definition include tropical storms and hurricanes – and may soar to nearly 10 degrees Fahrenheit higher than average, according to a new University of Arizona-led study published in the Journal of Geophysical Research. The study’s authors stressed that their results are likely conservative estimates of just how high temperatures can climb following a cyclone.

Tropical cyclones often cause damage from strong winds, storm surges, intense rain and flooding, but extreme heat is an additional hazard, the researchers found. Above-average temperatures can occur days later and even in nearby areas that were not directly impacted by the storm.

“Multiple extreme events happening within a very short window of time can complicate disaster recovery,” said lead study author Zackry Guido, an assistant research professor in the university’s School of Natural Resources and Environment and the Arizona Institutes for Resilience: Solutions for Environment and Societies, or AIRES. “To medical providers, heat is a concern. Our results suggest that tropical cyclone preparedness should also include public information about heat risk.”

The research team analyzed 53 tropical cyclones in the eastern Caribbean between 1991 and 2020 and 205 interactions between the cyclones and 14 Caribbean cities. They found that the cities’ heat index values were always warmer than average after the storm.

“Everyone’s focus is on the destructive power of tropical storms and hurricanes – the storm surge, winds, flooding – and that’s obviously quite substantial, but our focus is on the combined hazard of storm and subsequent heat,” Guido said. “Hurricanes are massive heat pumps, redistributing heat for a large spatial distance around the center of the storm, and they leave massive destruction in their wake that can knock out the energy grid. That combination is often dangerous because it slows recovery and poses risks to human health.”

While the paper doesn’t explore how climate change may be impacting the phenomenon, the authors expect that high heat index values following tropical cyclones will increase in the future.

“It’s very easy to understand the climate change impacts of this,” Guido said. “Our future will likely have hurricanes dropping more intense rain and have more people in harm’s way. Then, if you drape on top of that a hotter environment, you will therefore expect a greater overall impact.”

That makes a lot of sense to me. The proportion of tropical cyclones that become hurricanes or typhoons is increasing in part because weaker ones are being cut off by increased wind shear, and in part because the oceans are warming so rapidly. The strength of the storm generally ties directly to sea surface temperatures, which means that the amount of heat that that storm pumps into an area is also going to go up.

In terms of impacts, a big storm like that means that in addition to the heat dumped, there’s also an increase in humidity (the other factor in the heat index). That means a higher chance of hitting “wet bulb” conditions, in which people can die fast without artificial cooling. Losing power – as so often happens – becomes that much more dangerous. At the same time, floods can contaminate the water supply, which could leave those trying to survive with a choice between lethal dehydration or drinking water that will probably make them sick.

I get why the water cycle wasn’t taught to me as a way that heat moves around in the atmosphere, but it seems that that’s a perspective we’ll need to keep in mind going forward.

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