Someone once told me that the San Francisco Bay Area has the longest time lag between the longest day of the year (the summer solstice, or June 21) and the hottest day of the year. Wikipedia says:
Among major U.S. cities, San Francisco has the coldest daily mean, maximum, and minimum temperatures for June, July and August. During the summer, rising hot air in California’s interior valleys creates a low pressure area that draws winds from the North Pacific High through the Golden Gate, which creates the city’s characteristic cool winds and fog.
Once fall comes around, the interior valleys begin to cool down and the ocean is comparatively warmer, which stops the fog conveyor belt and finally lets temperatures warm up a little. This means that the hottest days occur in late summer and fall.
I was curious whether all of this actually causes a particularly long lag between the longest day and the hottest day. Using publicly available hourly weather data going back about forty years for several cities around the United States, I calculated daily maximum temperatures and then averaged the day of the year for the ten hottest days every year. This tells you when, on average, the ten hottest days of the year occur. I decided to average the ten hottest days each year because there often isn’t a big difference between the hottest day and consecutive hot days, and because particularly hot days often occur in heat waves. Retaining the ten hottest every year emphasizes this.
The results are in a table below. As it turns out, the hottest day in Los Angeles occurs almost two weeks after the hottest day in Oakland (across the Bay from San Francisco)! In other words, the theory that the Bay Area has a particularly long lag between the longest day and the hottest day is busted. This may be because Los Angeles’ weather patterns are influenced by some of the same factors affecting Bay Area weather patterns, but clearly the specific factors governing exactly when the hottest days occur are more pronounced in LA than they are in the Bay Area. Also interesting is that Fresno, the main city in the Central Valley and the other Californian city on the list, has its average high temperature almost exactly a month after the solstice, along with New York and St Louis. This likely means that whatever factors governing when high temperatures occur along the coast are absent in the Central Valley.
City Hottest Date Average High (F) Denver July 10 94.4 Chicago July 17 93.4 Miami July 18 93.0 Fresno July 20 106.1 New York July 20 91.5 St Louis July 20 96.3 Seattle July 26 89.1 Houston July 31 96.5 Oakland August 5 87.0 Los Angeles August 18 88.7