Nashville just neared its dew point record. Why that’s dangerous for heat, flooding

Nashville reached tropical moisture levels during the latest summer heatwave this week.

To understand the general feeling of mugginess, weather forecasters look to the “dew point,” which measures when air can hold no more moisture at a given temperature. 

The higher the dew point, the higher the heat index, and the harder it becomes for the human body to cool off through the evaporation of sweat. Higher dew points also raise the potential for high amounts of rainfall during a storm.

The National Weather Service says humidity may start to feel oppressive with dew points at or above 65 degrees. The dew point reached 77 degrees at Nashville’s airport station on Monday, not far off from the city’s record.

For reference, an actual temperature of at least 88 degrees Fahrenheit paired with that dew point translates to a heat index of 100 degrees. In fact, the city reached 93 degrees earlier that day. On Tuesday, the city’s heat index peaked at about 105 degrees, with a dew point peak of 75. Nearly 1.5 inches of rain fell by day’s end. 

Near-record territory

Nashville recorded its highest dew point of 82 degrees on August 15, 1995. The city has only reached or exceeded a dew point of 80 degrees during two heatwave events, on July 14, 2015, and August 13-15, 1995, according to city data stored with the Iowa Environmental Mesonet. 

The city also neared its record for “precipitable water” value (PWAT), which measures the amount of moisture in the lower level of the atmosphere. Nashville’s average PWAT this time of year is a level of about 1.4 inches, and the city measured “at least” 2.2 inches on Monday, according to NWS. 

This week, dew points climbed even higher elsewhere in Middle Tennessee. Clarksville’s dew point reached at least 79 degrees, Spring Hill hit 80, Dickson County and Maury County reached 82, and Sparta reached 82.4 degrees, according to NWS data. 

Some counties in western Kentucky broke records for dew points in recent days. Hopkins County set a new record Tuesday with a dew point of 85 degrees, according to the Kentucky Mesonet, which has station data dating back to 2008. At least seven other stations recorded dew points of at least 83 degrees this week, bringing heat indices over 110 degrees.

Globally, the highest dew point ever recorded was 95 degrees Fahrenheit at Dhahran, Saudi Arabia in 2003. Combined with the actual air temperature of 108 degrees, the city’s heat index reached 178 degrees — the highest ever documented on Earth.