A September Heat Dome Pushes Records Across the South

Much of the south-central United States baked under a heat dome in mid-September, just as summer was nearing its official end. According to NASA’s Earth Observatory, more than 41 million people, about 12 percent of the U.S. population, were under a National Weather Service extreme heat advisory, watch, or warning on September 15. The National Weather Service is the federal agency that issues weather alerts, and these alerts mean dangerous heat is possible or already happening.

The hottest area stretched across large parts of Texas, Oklahoma, Arkansas, Missouri, and Tennessee. NASA’s map showed air temperatures at 4 p.m. Central Time on September 15 across the contiguous United States, meaning the connected 48 states, not Alaska or Hawaii. The darkest red areas were near or above 104ºF. NASA made the map by combining satellite observations with a computer model, which is a set of math equations used to estimate how the atmosphere is behaving.

Several cities set new daily high temperature records that day. Dallas reached 101ºF. Memphis reached 99ºF. Nashville reached 100ºF, breaking a daily record that had stood since 1927. All three cities were at least 12ºF warmer than normal for that date. The day before, Nashville also set a record-high minimum temperature of 75ºF. A minimum temperature is the lowest temperature during a day, usually reached overnight, and a high nighttime low can make it harder for buildings and bodies to cool down.

The cause was a heat dome. A heat dome forms when high pressure high in the atmosphere pushes hot air downward and traps it near the ground. High pressure is an area where air is pressing down more strongly than in nearby places. That sinking air can block convection, the rising movement of warm air that helps form clouds and storms. With fewer clouds and less rain, more sunlight reaches the ground, and temperatures climb even higher.

Humidity made the situation worse in some places. Humidity is water vapor in the air. When the air is humid, sweat does not evaporate as easily, so the body has a harder time cooling itself. NASA said meteorologists warned that high humidity, limited cloud cover, and light winds could make the air feel hotter than the thermometer reading. That can raise the risk of heat-related illness, such as heat exhaustion, when the body overheats.

This event matters because weather is not only something on an app. It changes choices people make about outdoor sports, recess, bus stops, construction work, and electricity use. It also shows how science classes connect to daily life. A heat dome is about air pressure, sunlight, water vapor, and energy moving through Earth’s system.

The event also fits into a larger pattern reported by NOAA, the National Oceanic and Atmospheric Administration, a U.S. science agency that tracks weather and climate. NASA said June through August 2026 was the warmest such three-month period in the contiguous United States in a 132-year record. It was 0.4ºF warmer than the previous records, set in 1936 and 2021. One hot week does not explain climate by itself, but records over many years help scientists see how unusual an event is.