Nighttime heat is the star of summer in NYC

As June comes to a close and summer gets into full swing, we're entering the warmest month of the year in New York City. Both our average high and low temperature are at their warmest in July; the high temperature averages 85°, and the 30-year average for low temperature is 70°. At summer's peak, we don't get a lot of relief from the heat during the overnight hours. But it hasn't always been that way. 

The chart below shows the average low temperatures in July at Central Park's official observation site, which has been at Belvedere Castle since 1920.

  

Yes, that incredible building is located inside of the park!
Photo of Belvedere Castle from Central Park Conservancy

Low temperature data from NWS New York City

Other than a brief dip in low temperatures in the early 2000s, the upward trend in overnight lows has been pretty consistent. Follow the trend line, and you'll see an increase of about 5 degrees since 1920. 

The Urban Heat Island effect is one of the reasons for this increase. Central Park may be green this time of year, but it is surrounded by paved roads (there's even a few paved roads within the park) and high-rise buildings. Dark colored pavement absorbs heat during the day and slowly releases it after sunset; asphalt is one of the most efficient substances on Earth for absorbing heat from the sun. When the Belvedere Castle observation site was established, uptown NYC looked like this: 

Images of Broadway at 90th & 96th St in 1920 from the Museum of the City of New York

Most side streets are still unpaved, and even the main thoroughfares are not as busy as they are today. In the 21st century, there are more cars and high rises in Manhattan than ever before. Every automobile and building adds an incremental amount of heat, and in the city that never sleeps, all that infrastructure keeps pumping in the heat well after sunset.

Another warming factor is humidity, which is higher in the Tri-State area when evaporation increases off the waters of the Atlantic Ocean and Long Island Sound. It's a vicious cycle that leads to steamy summer nights: warmer air holds more water vapor, and water vapor molecules hold onto almost double the amount of heat energy of dry air (the fancy term for this in physics is "specific heat capacity"). As temperatures have risen in New York in recent decades, so has the humidity, as this dew point graphic from Climate Central shows:

Data from Oak Ridge National Laboratory, graphic by Climate Central

Higher dew points = higher moisture, which means more heat is being stored in the air, both day and night. 

We've managed to make it through June without a heat wave (a stretch of at least 3 consecutive days with high temperatures in the 90s), but July is the most likely month to produce these hot stretches. If you don't have air conditioning at home, make sure you know the location of your nearest cooling center, and follow the National Weather Service's tips for beating the heat.