New Yorkers know that late July and early August are brutal, but the warmest day of the year doesn’t hit every part of the country at the same time. Significant parts of the California coast are hottest in September, while southern Arizona and New Mexico are usually at their most sweltering in June.
The National Climatic Data Center at the National Oceanic and Atmospheric Organisation put together a few maps that use historical data to show when the warmest day of year usually hits each of the 50 states, and the variation is pretty fascinating (click to enlarge).
The summer solstice on June 21 is when solar radiation peaks in the Northern Hemisphere, but for a few weeks, the earth still absorbs more heat during the day than it cools at night, which is why July and August tend to be the hottest in most places.
But other factors come into play too — the North American Monsoon is what makes the Southwest so hot in June, while the cool Pacific waters create a strange effect in early summer where air temperatures increase with height (normally air higher up is cooler), and that warm air traps cool air near the coast, creating the “June gloom” effect.
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