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We’re in the middle of a significant late-summer heat wave across Southern Middle Tennessee, and temperatures are making a run at a number that always gets people’s attention: 100 degrees. We routinely reach the middle and upper 90s during our hottest stretches of summer. Add Tennessee humidity, and it can easily FEEL like 105-110° outside. But getting the actual air temperature to officially reach 100° is surprisingly difficult around here.

It’s certainly not impossible, but compared with places across the Plains and Desert Southwest, triple-digit temperatures are relatively uncommon in our part of the country. A big reason why comes down to our landscape, our moisture, and how the Sun’s energy gets used.

Tennessee Is Really, Really Green

Take a look around Southern Middle Tennessee in the summer. We’ve got forests, pastures, crops, lawns and plenty of vegetation. Unless we’re dealing with a significant drought, there’s also usually moisture available in the soil. That matters because not all of the Sun’s energy goes directly toward heating the air. Some of it is used to evaporate water from the ground and move moisture through plants, a combined process known as evapotranspiration.

You can almost think of the Sun as giving the ground an energy budget every afternoon. If the landscape is relatively moist, part of that budget gets “spent” evaporating water. That means less energy is available to directly heat the ground and the air above it. When the ground gets very dry, that changes. Less moisture means less energy is used for evaporation, allowing more of the Sun’s energy to go toward raising temperatures. That’s one reason drought can make extreme heat easier to achieve.

Humidity Doesn’t Create a Magical Wall at 100°

You’ll sometimes hear people say Tennessee can’t reach 100° because it’s too humid. That’s not quite right… Humidity itself doesn’t prevent the temperature from reaching triple digits. The bigger story is the environment that often comes with our humid summers: moist soils, lush vegetation and plenty of evapotranspiration.

And there’s an important twist here. A humid day that only reaches 97° or 98° can actually be more dangerous than a drier day that reaches 100°. High humidity makes it harder for sweat to evaporate efficiently, which is why our Heat Index can reach 105-110° even when the actual temperature stays below 100°. So if we stop at 99°, that doesn’t mean the heat wave somehow underperformed.

Clouds and Storms Can Ruin a Run at 100°

Another thing working against triple-digit heat around here is our tendency to produce summertime clouds and thunderstorms. Even puffy afternoon cumulus clouds can temporarily reduce the amount of sunshine reaching the ground. And if a thunderstorm develops nearby, rain-cooled air can knock temperatures down very quickly. That’s part of what makes this week’s setup so favorable for extreme heat.

Strong upper-level high pressure is producing sinking air, which suppresses clouds and widespread thunderstorms. Fewer clouds + very little rain + strong sunshine + hot air overhead = a much better chance of flirting with 100°. The Cumberland Plateau has another obstacle: elevation. Locations around Monteagle and across the Plateau sit considerably higher than communities farther west, so afternoon temperatures are typically several degrees cooler there.

And No... Your Car Saying 106° Doesn’t Mean We Reached 106°

This one comes up every single time we get close to 100°. The official high might be 98° or 99°, and suddenly everyone’s dashboard says 104°, 106° or 108°. Your vehicle isn’t necessarily inventing that number, but car thermometers are not official air-temperature observations. Most vehicle sensors are located near the front grille or bumper, and they can be heavily influenced by the environment immediately around the car. Think about where your vehicle is sitting or driving on a hot afternoon: asphalt and concrete.

Those surfaces absorb tremendous amounts of sunlight and can become MUCH hotter than the surrounding air. If your vehicle has been sitting in a sunny parking lot or moving slowly over scorching pavement, the sensor can read several degrees too warm. Once you’ve been driving for a while, the reading may improve, but it still isn’t the same as an official weather observation. Official temperatures are measured under carefully controlled conditions designed to minimize the influence of direct sunlight, pavement, buildings and other artificial heat sources.

The same problem applies to backyard weather stations. Put a temperature sensor next to a brick wall, above a driveway or in direct afternoon sunshine and you may get a very impressive number. It just might not be the actual air temperature. So if the official temperature in Shelbyville is 99° while your truck says 106°, meteorologists aren’t hiding seven degrees from you. 😂

This Week Gives Us a Legitimate Shot

This heat wave contains many of the ingredients needed to make a serious run at the century mark. A strong ridge is firmly established across the eastern United States, widespread rain chances are extremely low, sunshine is plentiful and temperatures throughout the lower atmosphere are very warm.

That’s allowing highs across much of Southern Middle Tennessee to reach the upper 90s, with a few communities potentially flirting with 100° today and Thursday. But whether the official thermometer ultimately stops at 98°, 99° or 100° really doesn’t change the bigger story. Humidity will push Heat Index values into the 105-110° range, and we’re dealing with several consecutive days of significant heat with relatively warm nights providing limited relief.

So don’t let 99° fool you… 99° with Tennessee humidity is PLENTY hot enough. And if we officially hit 100° somewhere? At least now you know Mother Nature had to work a little harder than you might think to get us there. Your truck, meanwhile, will probably say 106°. 😂

The Bottom Line 🧾

🌡️ 100° is absolutely possible here, but it’s less common than temperatures in the 90s.
🌱 Moist soils and vegetation work against extreme temperatures by using some solar energy for evaporation and transpiration.
🏜️ Drier ground can make extreme heat easier to achieve.
💧 Humidity itself doesn’t prevent 100°, but it can make temperatures in the upper 90s feel much worse.
⛈️ Clouds and thunderstorms can interrupt daytime heating, while strong high pressure helps temperatures soar.
⛰️ The Cumberland Plateau’s elevation makes triple-digit temperatures even harder to reach there.
🚗 Your car thermometer doesn’t count as an official temperature. Hot pavement and parking lots can make dashboard readings run several degrees too warm.
🔥 This week: Actual temperatures may flirt with 100°, while Heat Index values could reach 105-110°.
🥵 98°, 99° or 100°... it’s still dangerously hot.

📰 Reminder: The Southern Tennessee Weather Blog, presented by Heritage South Community Credit Union, is updated Monday through Friday with fresh, locally tailored forecasts you can trust.

Why Is 100° Actually So Hard to Reach Around Here?