Weather School: Heat waves

Weather School: Heat waves

AUGUSTA, GA (WFXG) - It's something that seems to happen at least once a summer: a sweltering heat wave gripped the country. But what is exactly is it? The National Weather Service officially defines a heat wave as three or more days in a row with temperatures above 90 degrees.

In Augusta, we see an average of 76 days with temperatures at or above 90 degrees. A heat wave here is measured by the heat index. The National Weather Service in Columbia issues heat advisories when the heat index jumps to 110. They'll issue excessive heat watches or warnings when the heat index hits 115 or higher.

Heat waves usually happen when air is trapped. Summertime high pressure systems can sometimes stay parked over an area for several days. Instead of moving, the air mass with these systems stays in place and warms up, like an oven. This can cause temperatures to spike well above normal. Also, the air sinks under high pressure, preventing clouds and rain, so there’s not much to cool things off until the system breaks down or finally moves on.