Warmer Summer Nights

Summer nights are warming across the country. We often think about the increasing number of hot days as greenhouse gases build in the atmosphere and the climate warms, but we are also seeing a strong rise in overnight low temperatures. 

This was highlighted last month. According to NOAA, the U.S. June low temperature was the highest on record (back to 1895). The high temperature average for the month were also hotter than usual, but only came in as the 14th-highest on record. So the warm nights propelled the country to its second-warmest recorded June overall.

In our analysis this week, we show that every state in the U.S. is experiencing an upward trend in summer low temperatures.

So far in 2015, the number of monthly warm low-temperature records is outpacing the number of cold low-temperature records by a ratio of 3 to 1.  Looking at individual locations the past 12 months, the number of all-time warm low-temperature records is outpacing the corresponding cold ones by a ratio of 6 to 1.

While the specific amount of warming due to urbanization varies from site to site, on the global scale, the urban heat island effect cannot explain all of the observed warming. Further, temperatures have trended upward in both urban and rural locations. But when added to the warmth from greenhouse gases, the urban heat island effect will make the hot city nights even hotter.