Key West shatters heat index record by 17 degrees

The official Key West weather station reported a heat index of 115°F at 4:53 pm Wednesday. That not only breaks the daily heat index number by an astonishing 17 degrees, it also ties the all-time record for Key West.

What is remarkable about this event is that it happened in mid-May, not June, July or August when the humidity is often higher.

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Another word for the heat index is the feels like temperature. This takes into account both the temperature and also the humidity.

This is actually a better measure than just temperature because it tells us just how oppressive and dangerous the air is that day. That’s because the more humid the air is, the less able our body is cool itself down by sweating. Thus wetter air is more dangerous to our health.

The graphic below from University of Miami scientist Brian McNoldy shows all heat index records since 1948 in Key West. Wednesday’s reading is in a league of its own for late spring.

It’s worth mentioning that when all is said and done, the Key West National Weather Service may record this as 114 degrees rather than 115 degrees, depending on exactly what the heat index number rounds out to be. But either way, the early season heat is hard to fathom.

Miami also had it hottest day on record for the month of May.

So why is it so hot in South Florida? Two reasons: weather and climate.

First, let’s discuss the weather reason. There has been—and is—a large and persistent heat dome across Central America. This produced a high of 124 degrees (actual temperature) late last week in Mexico—its hottest May number on record. The northern edge of heat dome extends all the way to Florida. This heat dome will get stronger over Mexico in the coming week.

Now let’s discuss the climate reason. The water around Key West is 3 to 4 degrees above normal, on the edge of record breaking for this time of year. While this can be due to fluctuations in weekly weather, over the long term (last five decades) the southeastern Gulf of Mexico has warmed a substantial amount. Specifically the Florida Keys have warmed around 3 degrees. This is the human-caused climate change trend.

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When ocean temperatures are warmer, that feeds more moisture into the air, thus creating more humid air and a higher heat index in Key West.

How do we know the record was aided by climate change? We now have real-time tools to gauge if and how much climate change affected the weather on a certain day. It is called the Climate Shift Index, a fancy algorithm developed by Climate Central. You’ll notice below that while northern Florida has no impact from climate change, Central and South Florida did.

In fact, climate change made Key West’s heat at least five times more likely on Wednesday.

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