South Carolina’s long, hot summers are typically defined by temperatures routinely climbing to above 90 degrees, but reaching the threshold can occur well before the start of the warm season.
According to climatological data, the thermometer does not typically reach 90 degrees across much of the state until May, with most observation sites reporting their earliest occurrences during March.
In Columbia and Charleston, the earliest reported 90-degree readings occurred on March 10, 1974, while farther north, Greenville's earliest 90-degree day traces back to March 23, 1907.
The variation in temperature extremes across the Palmetto state is due in part to the unique terrain which stretches from mountains to the Atlantic Ocean.
Inland locations with lower elevations, such as Columbia, often heat up more quickly because of the lack of influence from the Atlantic.
Coastal communities like Myrtle Beach, Charleston and Hilton Head are more frequently influenced by sea breeze activity, which can help to limit daytime heating and keep temperatures cooler.
Once the first 90-degree day of the year occurs, the frequency of warm days typically increases and often persists through the early fall.
During the warm period, Columbia averages roughly 75 days every year at or above 90 degrees, making it one of the warmer locations in the state.
Many coastal communities usually experience about half that number due to the impacts associated with the Atlantic Ocean.
90-degree temperatures tell only part of the story - back as the state routinely experiences extreme heat events that pushes temperatures well beyond the threshold.
In 2012, Columbia reached a scorching 113 degrees, which remains the state’s all-time record high temperature.
Historically, some of the state’s most intense heat waves have occurred during the 1950s, 1980s and 2010s.
Many of the events have occurred when strong ridges of high pressure develop over Canada, sending warm air southward.
As the air descends over the slopes of the Appalachian Mountains, it undergoes additional heating, leading to record temperatures for communities east and south of the mountains.
According to National Weather Service meteorologists, this pattern can suppress cloud formation and persist for many days, resulting in an extended heat wave.
The NWS identifies a heat wave as a period of abnormally warm weather that lasts more than two days.
Depending on air temperatures and the heat index, forecasters may issue Extreme Heat Warnings, Extreme Heat Watches or Heat Advisories, depending on the severity.
All of these alerts are meant to give residents a heads up to make plans to avoid outdoor activities and to check on those who may not have access to air conditioning.
