The heat is on: Boston declares heat emergency starting Tuesday

Take a ride by and let me know what you think. They're back.

Temperature isn't as important as apparent temperature, adjusted for humidity and/or wind.

Why?

Because in order to cool your body off you need your sweat to evaporate. If your sweat can't evaporate efficiently because humidity prevents that, you heat up. This inability to cool your body means that being in 95 degree high humidity weather can be worse than being in 110 degree heat in a dry place.

That is why the professionals who have been studying the health impacts of heat events for decades use temperature corrected for humidity - aka the heat index - as the most robust predictor of potential damage. People who have been studying the body's cooling responses for a century in controlled environments have repeatedly verified that heat index (or similar humidity adjusted measure) is the best predictor of physiologic response to heat. Straight up temperature doesn't work for studying multi-city models and multiple extreme heat events, either.

This is why the NWS uses heat index as a indicator for severity of heat when they issue advisories, watches and warnings. There are decades of research behind this.

I'm guessing that you have never experienced hot weather in multiple climates. I grew up with dry and hot summers and it really is very different. I'll take 105 in a dry place over 95 around here any time.

There is similar research for wind chill as a measure of heat loss. When the wind is blowing your body loses heat much faster. While you can bundle up against the cold to protect yourself, unlike with heat, wind chill factor is still an an appropriate adjustment for heat loss potential at a given temperature.

You might make the argument that those who use these terms don't adequately explain them; on the other hand, there's a whole internet out there that could inform you. They are useful terms because they indicate the degree to which environmental conditions will affect your survivability.

Wind chill is an indication of how quickly an object will cool down to the ambient temperature. If the temperature on the bulb is 20 degrees and there's no wind, a cup of hot coffee will cool down more slowly than it will if there's a 40 mph wind -- but in both cases, it will never cool down below 20 degrees. When people are confused and think that can't possibly be true, I ask them why the Gulf of Mexico doesn't freeze in a category 5 hurricane.

Heat index is different. It's an indication of how slowly (or how poorly) an organism, like a human being, can lose excess heat. Evaporation of sweat from the skin is one of the mechanisms that humans have to lose heat, and as others have pointed out, humidity is a huge factor because it greatly slows the rate of evaporation of sweat from the skin: the more moisture there is in the air, the more slowly sweat (or any other liquid) evaporates, and the slower the heat loss. Wind is another factor, or rather lack of wind: a breeze feels cooling both because it moves away the relatively hotter air around your body and replaces it with cooler air, and because it speeds evaporation. So, in still humid conditions, your body can do much less to get rid of excess heat.

All of this matters because human beings have to regulate their body temperature to within a relatively narrow range. Humans can tolerate an air temperature of 72 degrees just fine, but if that was your body temperature, you'd be a corpse. Likewise with heat: humans can survive in air temperatures up to about 120 degrees if they're not stupid, but a body temperature of 110 will kill you. As you can see, the range that we can tolerate is much narrower on the high side than it is on the low side. Furthermore, apart from our body's thermoregulation, we have more tools for coping with cold than we do with heat. So, yes, heat index matters. It matters a lot, and I'm glad that government agencies are paying attention to it and making people aware of it.