When we think of “summer,” we normally think of “hot,” and for good reason. The vast majority of the U.S. experiences its warmest conditions during the months of June, July, and August, which comprise the meteorological summer months in the Northern Hemisphere. But did you know that the climatological “warmest day of the year” varies a great deal across the United States? It can occur any time between early June in some areas to as late as September and October in others!
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 smoothed values, scientists can identify which day of the year has the highest maximum temperature normal, 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 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, Arizona, and Texas reflect the impact of 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. For example, the warmest day of the year in Terlingua, TX is June 11th. Conversely, the persistence of the marine layer along the Pacific Coast leads to cool temperatures in early summer, with the warmest days on average occurring as late as October. In fact, daily high temperature normals peak on October 2 in San Francisco.
Temperature Normals are important indicators that are used in forecasting and monitoring by many U.S. economic sectors. The latest 30-year averages were released in May 2021 to encompass the 1991–2020 period of record. Knowing the warmest day of the year helps energy companies prepare for peak electricity demand (e.g., peak air conditioning load) and helps farmers monitor conditions for heat-sensitive crops. They are also useful planning tools for the healthcare, construction, and tourism industries. Be sure to check the Normals before planning your next event or vacation.
While the map shows warmest days of the year on average throughout the United States, this year's actual conditions may vary widely based on weather and climate patterns. For a prediction of your actual local daily temperature, and to see how it matches up with the Climate Normals, check out your local forecast at Weather.gov.