Why is this city in the Keys the hottest spot in Florida? Weather service has answers

In the daily heat records race. it's a marathon. And Marathon is in for the win.

On Wednesday, Miami and the Florida Keys city both set daily heat records, again.

• Miami soared to 97, breaking the July 12 record of 95 set in 1981.
• Marathon baked at 98, the hottest in Florida, breaking the July 12 record of 97 for the Middle Keys city set in 2022.

Marathon stats

Marathon boundaries are from the east end of the Seven Mile Bridge at Mile Marker 47 to the west end of Toms Harbors Bridge, at MM 60, about an hour drive from Key West and Key Largo. Marathon is touted by city officials as "the heart of the Florida Keys—and this heart pumps hot.

According to the National Weather Service in Key West, Marathon has set or tied the daily high temperature record for nine straight days. All of these days this July have been 95 or hotter.

That 98-degree high temperature at Marathon International Airport happened Tuesday and Wednesday. The mark is just one degree from Marathon's all-time hottest high of 99 degrees set in 1987 and 1972.

Thursday's high is forecast to hit the mid-90s again. A heat advisory for the entire Florida Keys chain remains from 10 a.m. to 6 p.m. Thursday, with an index high "feels-like" temperature of 112 possible.

Why is Marathon always hotter?

To some observers, Marathon runs hotter than other parts of the Keys. It's not in your imagination—and there's a reason.

The hot spot has to do with the location of the National Weather Service's sensors, explains National Weather Service in Key West meteorologist Jon Rizzo.

"The temperature of the cities is actually a specific instrument that we maintain at the airport. And in the case of Marathon, the location of that is kind of sandwiched between some pavement including part of the ramp where they park the aircraft and the driveway, which is just to the south," Rizzo said. "So what happens is when the winds are light and over this stretch, in particular, the last couple of weeks, some of the hot air over the payment passes over the sensor, and so it will read higher than perhaps positioned at another part of the airfield that may be over a grassy surface."

"A lot of that has to do with the siting and that's why it's consistently on average two to three degrees warmer than, let's say, our sensor at the Key West airport."

Another factor for falling records: Marathon's period of temperature weather-keeping records is considerably shorter than that of Key West, Rizzo said.

"It's still significant that you're seeing record reports coming out of [Marathon] because even in the history of that location and the station, relatively speaking, it has been hotter this year than others. Every day we're pretty close to tying or breaking a daily high record. Marathon's period of record dates back to 1950, which is kind of medium length. In comparison to Key West, with the records dating back to 1871, it's harder to break records in Key West with its long periods of record. Whereas, Marathon you're dealing with a shorter time period."

Rizzo said the Keys weather service is working with the Federal Aviation Administration to possibly relocate the sensors at Marathon International Airport to a different part of its airfield. But any such move will take some time.

As for Thursday, Rizzo said the forecast high is either 96 or 97. The record high for July 14 in Marathon is 95, set a year ago in 2022.

Expect another record.

Miami forecast

At 6:30 a.m. it was already 86 degrees with a "feels-like" temperature of 97 at Miami International Airport, the National Weather Service in Miami reported.

Thursday will be another stay hydrated day and a day to keep considering indoor activities.

The Miami weather service's heat advisory has been extended through 8 p.m. Friday for Miami-Dade, Broward, Palm Beach and Collier counties. The heat index could hit 110.

The heat index already was in the triple digits at 101 in Kendall, Miami, Fort Lauderdale at 8:30 a.m. Thursday, CBS News Miami meteorologist Lissette Gonzalez reported.

The heat advisory expires at 8 p.m. Thursday for Hendry and Glades counties.

Temperatures will peak in the low- to mid-90s on Thursday and Friday afternoons in South Florida, the service said, with heat index values between 105 and 110.

The highest temperature on record in Miami on a July 13 was 97 degrees in 1933, according to Extreme Weather Watch's tracking site. Franklin Delano Roosevelt was the president of the United States at the time.

At 10 a.m., the weather service posted to Twitter that it was nearly 91 degrees at its office on the main Florida International University campus in Tamiami, with a heat index of 108.

"Take precautions to protect yourself if you will be outdoors today. A hat, sun-glasses, & bottles of water are great items to have on hand," the meteorologists posted atop an image of a toy, stuffed iguana clad in sunglasses. (The creatures won't be falling from trees in Miami these days.)

"Additional heat advisories may be needed through the weekend as well for portions of the area as hot weather is expected to continue," the weather service added.

Hot water

The high temperatures are not just over paved lands. Southwest Florida meteorologist Matt Devitt of WINK News reported a 98-degree water reading at Everglades National Park's Garfied Bight Station on Wednesday.

"One of the hottest readings I've ever seen along our coast," Devitt tweeted.

The message?

The message weather experts have amid all of these daily records to help avoid heat exhaustion and heat stroke?

• "Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors," the weather service in Miami said in its heat advisory report. By fluids, they mean water or electrolyte-replacing beverages. Avoid alcohol and caffeinated drinks, like coffee, because those can dehydrate. It's fine to have your morning coffee, just make sure you're hydrating properly during the day.
• "Young children and pets should never be left unattended in vehicles under any circumstances."
• Take extra precautions if you work or spend time outside and consider rescheduling strenuous activities—like your tennis league matches or pickleball games, for instance—to early morning or evening.
• Wear lightweight, light colored and loose fitting clothing, too.

Everyone in Florida is at risk during this heatwave but the most vulnerable are the elderly, infants and children, people with chronic illness and pregnant women.

2023 Miami Herald.

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