Droughts during heat waves can lead to ‘runaway temperatures.’ It’s happening in Tennessee.

Many Tennesseans are experiencing a worsening drought, while some folks are finally seeing some relief.

The latest U.S. Drought Monitor, issued last Thursday, shows a large swatch of the state as abnormally dry or in moderate drought. Several counties, including Chester County, where the recent major oil spill occurred, are in severe drought.

Some parts of the state received sufficient rainfall this past week to improve conditions. But summer storms are spotty, even within cities or counties. Nashville got a few inches in its northern portion — which alleviated but did not end the dryness — while the southern tip remained dry.

At the same time, the state is experiencing a lengthy heat wave, with above-average temperatures, air pollution spikes and a new daily heat record of 101 degrees in Nashville on June 22, the first triple-digit stat for the city since 2012.

Heat waves and drought are very interconnected, and both are worsening because of climate breakdown.

Heat and droughts are part of a feedback loop

During droughts, trees and plants drink up the moisture from the soil, and soil moisture usually soaks up surface heat. So, temperatures rise even faster because of the changes in atmospheric water vapor.

“If we think about all of the energy coming from the sun that gets turned into what we feel as the temperature of the air, if we’ve got a drought going on, all of the energy that would normally go into evaporating water or transpiring water gets just turned into heat energy,” said William Tollefson, Tennessee’s assistant state climatologist.

This dryness can lead to “runaway temperatures,” says state climatologist Andrew Joyner.

On the other hand, the high temperatures also increase the dryness. After a light rain, for example, the water is rapidly absorbed by vegetation and transpired back out as a gas in a process called evapotranspiration. On asphalt that can reach temperatures of 150 degrees, rain can be instantly transformed into steam.

“We don’t really get to use it too much,” Joyner said.

These factors can extend heat waves and droughts in a feedback loop and can also help explain why flash droughts are coming on faster.

The U.S. Drought Monitor will be updated Thursday to reflect how the recent, scattered rains improved conditions in some parts of the state while other areas became drier.

At this time, the streak of 90-plus-degree temperatures looks to be continuing for the next week, according to the National Weather Service.