It sure feels hot

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LINDSAY MAST, HOST: Up next, with record temperatures the last few weeks across portions of the US, we’re not just hearing about the temperature, but what it supposedly feels like to the human body.

BUTLER: Joining us now to help us understand what exactly the heat index is… is David Legates, retired Professor of Geography and Climatology at the University of Delaware. He’s also the Director of Research and Education at the Cornwall Alliance and specializes in long-term weather and climate data. Good morning, David.

DAVID LEGATES: Good morning.

BUTLER: Well, let's start with just a little history, shall we? When exactly did climate scientists begin tracking what we now call the heat index?

LEGATES: Well, it goes back a while because people sort of recognize if it's 95 degrees in New Orleans and it's 95 degrees in Tucson, there's quite a bit of difference between the two climates. So you can sort of, you can put up with 95 degrees in Tucson. I've lived in Baton Rouge, 95 degrees in New Orleans is nearly unbearable. It's like walking outside into underwater conditions. It's just massive humidity.

And the idea is the human body cools based upon perspiration. So you perspire, water evaporates, it cools the surface, works very well in the desert, not so much in a swamp where the air is already saturated or near saturated, and there's just no way to get that moisture to evaporate well.

So back, I guess, in the 1950s or 1960s, they came up with the idea of the temperature humidity index. The idea was that we can figure temperature, but temperature doesn't have to tell the whole story. We want to add in humidity as well. And then back in the late 70s, Robert Stedman came along. He tried to look at some things associated with human heat stress. The idea is that looking at the amount of cooling that would take place on a human being based upon the humidity characteristics. It's really the temperature combined with the humidity, and that eventually morphed into the heat index that National Weather Service and everybody else reports today.

BUTLER: I have to admit that I’m a little skeptical about why so many news reports include the heat index…making the temperature data more sensational…but is it a helpful measure?

LEGATES: It can be if you understand just what it is and what it isn't. I mean, we often say “it feels like temperature.” So it's 95, but it feels like it's 110. Well, what does that really mean? I mean, you have to go back to Stedman's original work, which virtually nobody does. He says it's the equivalent heat loss that a person would experience if they were naked, standing prone outside and associated with the current conditions. If you were doing so in Tucson at 115, then if you were in New Orleans at 115. Temperature in New Orleans is 115, but because there's so much more humidity, your body can't get rid of energy or heat by evaporating water. And since that's the case, it feels like it would be if you were standing in an oven at 115.

So it's sort of like what it would be in the optimal sense of somebody standing in Tucson at the same temperature, how much energy would they release? Of course, the condition is it's never the same because at 115 and no humidity, it's still different than 95 and very high humidity.

BUTLER: So is it a helpful measure or is there something more helpful?

LEGATES: To the general public, it probably isn't because I don't think they ever understand what it means. So in some senses, it's a sensational figure because it always seems to go up and the windchill always seems to go down. So the idea is it always sounds colder. Or always sounds warmer than it really is.

There are studies that use heat index, but most of them tend to break the humidity out separately from the temperature and treat them as several variables rather than combining them. So I think the heat index is more of a colloquial way of saying feels like. It makes the television meteorologists and the radio meteorologists sort of sound like they know what they're talking about, but the general public doesn't know what that really means.

BUTLER: I suppose we could also make a safety argument for reporting the heat index…as meteorologists aren’t just reporting on the weather conditions but what we should do in response…

LEGATES: Yeah, because we report that the temperature is 105 Fahrenheit, then somebody's likely to say, wow, that's hot. Maybe I should make sure I've got extra water. Maybe I should take breaks. And hopefully that would be the kind of thing that would take the precautions they're supposed to take. So maybe it is scaring them into doing what's right in this case.

BUTLER: David Legates is a retired professor of climatology and co-editor of the helpful book: Climate and Energy: The Case for Realism. David, thanks so much for joining us today.

LEGATES: Thank you.

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