What defines a heat wave? The answer could decide where disaster dollars go.

Another brutal summer is shattering temperature records, broiling over a third of Americans under extended heat advisories. As smoke from wildfires begins to choke skies and death counts tick upwards, affected states say they need more help from the federal government. 

During most climate-driven emergencies, such as hurricanes or floods, local governments can rely on the Federal Emergency Management Agency to fund relief efforts. But heat waves and wildfire smoke aren’t explicitly listed...