For the continental U.S. as a whole, the hottest part of summer arrives in mid to late July. On the regional level, however, there are significant differences in the timing of the hottest part of the season.
Based on historical averages, NOAA’s National Centers for Environmental Information (NCEI) produced a map showing how the warmest days of summer vary across the country. The desert southwest, for example, is an early bloomer in terms of heat. It’s warmest days typically arrive in June. By early July, the North American Monsoon – a seasonal shift in wind direction – kicks in and helps keep the region relatively cool and rainy through September.
In parts of the south central states, a persistent area of high pressure typically builds over the region in August. This limits cloud formation and dries out the soil, which helps to send temperatures skyrocketing.
On the other far end of the spectrum, temperatures along the west coast do not peak until September. This lag is the result of hot air rising over interior deserts and cool Pacific air flowing in-land. By September, this upper-level wind pattern usually weakens and allows warm air to flow toward the coast.
While the NCEI map is derived from long-term climate averages, it is important to note that short-term variability can cause the date of peak temperatures to vary in any given year.