拉斯维加斯111度对纽约93度：数字更低的纽约，反而更危险

On the same day: Las Vegas a hundred and eleven degrees, New York ninety-three.

By the numbers that is an eighteen-degree gap. The winner looks obvious.

But ask an emergency physician which of those two cities was more likely to put someone in hospital that day, and the answer is probably New York.

The conclusion is counterintuitive, so it is worth taking apart properly.

First, units: these are Fahrenheit.

A hundred and eleven Fahrenheit is about forty-four Celsius; ninety-three is about thirty-four.

Now the human side.

Humans are warm-blooded. Core temperature has to hold around thirty-seven degrees, with very little room to drift.

Once the outside air is hotter than your body, you can no longer cool down by shedding heat into the air, because heat flows from hot to cold.

That leaves one route: sweating.

But what actually cools you is the instant the sweat evaporates. The sweat coming out does nothing by itself.

Turning a liquid into a gas takes energy, and that energy is drawn straight off the skin, which lowers skin temperature.

Put another way: sweat sitting on you does not cool you. Sweat evaporating does.

And here is where it breaks: the rate of evaporation is not up to you. It is up to the air.

There is a ceiling on how much water air can hold, and how close you are to that ceiling is what relative humidity means.

Dry air is nowhere near full, so sweat is carried off the moment it appears. Sweating is extremely efficient.

Humid air is already near saturation and cannot take much more, so the sweat just runs down your skin.

The result is a body continuously losing water and salt while buying almost no cooling with it.

That is the difference between Las Vegas and New York.

Las Vegas is in a desert, and the air is extremely dry.

A hundred and eleven sounds appalling, but with water and shade, the body's cooling channel is still open.

New York is coastal, and summer humidity is high.

Ninety-three plus high humidity means that cooling channel is essentially blocked.

The 'feels like' figure in a forecast is calculating exactly this: temperature and humidity combined into the load a body actually carries.

These days New York feels like ninety-three, Washington reached a hundred, Philadelphia is close behind.

So what happens when the heat cannot leave?

The body's first response is to divert large volumes of blood to the skin, trying to release heat at the surface.

The cost is less blood to the organs and the brain — hence dizziness, nausea and weakness, the early warnings of heatstroke.

If the core keeps climbing past forty degrees Celsius, proteins begin to denature and organ function is damaged. At that point it stops being uncomfortable and becomes fatal.

It is worth stressing that this does not require extreme heat.

At high humidity, the low thirties Celsius is enough.

Last Saturday, at an outdoor event in Colorado, seventy-two people were treated for heat and seven were taken to hospital.

The temperature there that day was nowhere near a hundred and eleven.

Heat warnings currently run from California through the Midwest, and the east coast is broadly four to eight degrees above normal.

The forecast has New York falling back into the eighties by Wednesday, but there are a few days to get through first.

Which lands on one practical rule: to decide whether to go outside, do not read the temperature alone. Read the felt temperature.

Dry heat and humid heat are two different things, and the second one is the one people underestimate.

Think about it: where you live, is the summer dry heat or humid heat?

Which do you find harder to take?