When is the hottest day of the year?

The hottest day of the year varies widely across the country, from as early as late June in portions of the Southwest to as late as September along much of the West Coast.

This "cool" map from the National Centers for Environmental Information (NCEI) shows where your hottest day of the year is, based on historical averages. It includes comments from meteorologist Chad Burke at the Weather Channel.

For folks living in the West, northern Plains, New England and Florida, the hottest day of the year typically occurs around now, in the final few days of July or early August, the map shows.

Surprisingly, people in the the torrid Southwest have likely already endured their hottest day of the year, which tends to occur in late June or early July. The "cooling" effect of the clouds and rain from the midsummer monsoon limits the heat in July and August, Burke said.

Alaska, which has its own map, tends to see its hottest weather in July.

Residents of the Mid-South and southern Plains often see their hottest days in August. This is because the region "browns out" in late summer, usually delaying the worst of the heat until August.

Hawaii also waits until late August or even September for its hottest days.

You'll notice a stripe of purple along the immediate West Coast. This is because temperatures in Pacific coastal cities are strongly modulated by ocean temperatures, which peak in late summer, as late as September, Burks' analysis shows.

"The persistence of the marine layer along the Pacific Coast leads to cool temperatures in early summer, with the warmest days on average later in the season," according to the NCEI.

San Francisco, for instance, usually sees its warmest temperatures in September.

This backs up a famous quote about "summer" in San Francisco, which has been wrongly attributed to Mark Twain (according to Snopes):  "The coldest winter I ever spent was a summer in San Francisco."

The maps are derived from the 1981–2010 U.S. Climate Normals, a 30-year average of climatological variables that includes the average high temperature for every day, the NCEI said.

Thanks to Todd Crawford @tcrawf_nh at WSI for pointing out this map.