What is truly behind the alarming high temperatures in the hottest place in South Florida? 

We won’t know for sure until summer is over and we add up all the numbers, but it’s certainly looking like 2023 could be South Florida’s hottest summer on record; both the hottest air temperatures and the hottest water temperatures.

While daily records have been falling at a never-before-seen pace across all of South Florida from Key West to Miami to Fort Lauderdale, there is one spot that stands out: Marathon.

Marathon is different than the rest of South Florida. For one, records only go back to 1950. Secondly, the “Marathon Area” is actually four different locations since 1950: Marathon Shores, Conch Key, Duck Key and the official location since 1998, the east side of Marathon Airport. This current location has proved a challenge for forecasters at the National Weather Service in Key West because the sensor (or thermometer) is located in a vulnerable spot. Because of all the surrounding pavement, the afternoon high temperature in the blazing summer sun can register a few degrees warmer than it actually is. So the plan is to move the sensor from the east side of the airport to the west side of the airport where it may become more accurate. The plan may still be years away, but at least it is being talked about.

The biggest problem with the move is corrupting all of the data since 1998. While it is true that it runs hot by day, at least we know that it has been doing that consistently. If we move the sensor, does it make it harder to compare the numbers that go back to 1998?

As the NWS’s Chip Kasper explains, “The ambient air temperature at that location runs on the ‘hot’ side relative to other official stations in Southern Florida.” He continues, “We have tested and calibrated temperature sensors multiple times over the years, and they are indeed accurate. It is just a little ‘hotter’ at that particular location and exposure.”

The numbers have been simply astounding. From July 4^th to July 21^st, Marathon has recorded 33 warm records, 19 (consecutive) in the afternoons and 14 in the mornings. During this stretch, Marathon had its hottest two days on record. July 13^th was 86° to 99°, averaging out to a shocking 92.5°. On the 18^th, it was 87° to 98°, also averaging out to 92.5°. It should only be 85° for an average July day. And that 87° low was Marathon’s hottest on record as well.

If you add up all of Marathon’s July records from 2016 to 2022, you get 36. This month, we’ve already seen 33.

If you add up all of Marathon’s records at any time of the year from 2016 to 2022, you get 297 over 2,555 days. That’s a record every 8.6 days. In July of 2023, those 33 records happened in 19 days. That’s a record every 0.57 days. That’s essentially two records per day for nearly three straight weeks.

We have done extensive number crunching here at NBC6 and we have concluded that the morning records at 100% accurate because Key West’s records (which go back 150 years) are almost identical and water temps at one point were up near 100°. Key West even had a stretch of 8 days where the temp never dropped below 85°, even at night.

We believe the afternoon records are valid even if the actual number is off by a few degrees. That’s because those records are comparing Marathon to itself. Even if the temps run hot, we can still compare Marathon to its own past. Some 2022 records were broken by several degrees just a year later. In fact, on the 13^th, that 99° high temperature beat the record from a year earlier by 4°! This tells us these records are real.

The conclusion is that even with the skewed high temps, the records are still valid and this summer is legitimately an historically hot one for The Keys. Marathon is South Florida’s “Climate Change Bellwether.” While it may not be accurate to compare Marathon to the rest of South Florida when it comes to afternoon high temps in the summer, by comparing Marathon to itself, we can watch how our climate is changing over time at an alarming rate.