GRAND RAPIDS, Mich. (WOOD) — A heat wave is typically known as “three or more days in a row at or above 90 degrees,” but does that hold true for different parts of the nation? That is the basis of this week’s Ask Ellen.
Occasionally, the way weather is defined changes with time. For example, the threshold for a severe thunderstorm was increased from 0.75 inches of hail to 1 inch of hail several years ago.
The definition of heat wave has also experienced some alterations with time to make it more appropriate for each geographical location.
In the American Meteorological Society glossary, a heat wave is a “spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90 (degrees).”
In Grand Rapids, a string of three days in the 90s is fairly rare: Records dating back to the late 1800s show that these were most common in July, with a 40% chance.
In most months, like May, three-day strings of 90s are less frequent: Less than 5% of the years had one.
Compare that to Phoenix, Arizona, where a 90-degree day is not rare. In the desert Southwest, the dry, hot conditions frequently launch highs into the 90s — if not the 100s. According to data that goes back to the 1930s, Phoenix has not gone a summer without a string of three 90s.
In the southeastern United States, increased humidity can make it harder for the thermometer to climb as high as it does in the desert Southwest.
Still, in many spots, 90s are frequent. The increased humidity usually creates difficult days with heat index readings in the 100s. A three-day stretch of 90s in Miami, Florida is historically most frequent in July and August.
Because of the vastness of the United States, the term “heat wave” has often become fairly subjective. Local meteorologists define it as a stretch of weather that is unusually hot or unusually prolonged for an area.
For Grand Rapids and the rest of West Michigan, using a threshold of three or more consecutive 90-degree days is appropriate, since these are relatively rare.
The southwest United States is more likely to brand a span of weather as a heat wave if temperatures are consistently soaring into the 100s for a few days in a row. Places in the nation like the Corn Belt, Deep South and Southeast will likely claim a heat wave if the heat index numbers land in the 100s over a consistent stretch, especially if difficult overnight low temperatures are involved.