During the summer months, most areas in the United States approach their highest temperatures for the year.
To give you a better idea of the warmest time of year for your area, NCEI has created “Warmest Day of the Year” maps with the help of Climate.gov for the contiguous United States, Alaska, Hawaii, and Puerto Rico. The maps are derived from the 1991–2020 U.S. Climate Normals, NCEI's 30-year averages of climatological variables including the average high temperature for every day. From these values, scientists can identify which day of the year, on average, has the highest maximum temperature, referred to here as the “warmest day.”
Although the amount of solar radiation reaching Earth peaks at the summer solstice on June 21 in the Northern Hemisphere, temperatures for most of the United States tend to keep increasing into July. The temperature increase after the solstice occurs because the rate of heat input from the sun during the day continues to be greater than the cooling at night for several weeks, until temperatures start to descend in late July and early August.
But, this isn't the case everywhere! The “Warmest Day of the Year” maps show just how variable the climate of the United States can be. For instance, the June values in New Mexico and Arizona reflect the North American Monsoon, a period of increased rainfall affecting the U.S. Southwest. Because these areas tend to be cloudier and wetter from July through September, the temperature is highest on average in June.
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