The combination of increased sprawl and an enlarged heat island in concert with the general trends of a warmer climate globally, plus the warmer ebbs of the Atlantic ocean, have combined to produce more warm nights in the city. Philadelphia's streak of 70 degree days having ended, I thought it would be a good idea to look at the increased trend of warm nights in Philadelphia.
For this article, 75 degrees is the definition of a warm night...it's an easier data spit than 70.
Historically, Philadelphia has seen an average that's just shy of five nights a year where the temperature fails to drop below 75 degrees (4.68 is the exact number). Over the years, we've gone through summers where we fail to have a warm night -- in some cases, as many as three consecutive summers as 1962 to 1964 each failed to produce a warm night. However, as Delaware County, Gloucester County, and Chester County have developed further through the 70's and 80's, the concrete and pavement from that development has provided a little extra warming at night. The warming trend in our global climate has also generated an extra bit of influence in temperatures.
These factors have gone a long way to increasing the number of nights that fail to drop below 75 in Philadelphia. We've failed to go through a year without a warm night since 1996 -- yeah, it's been 17 years! And, over the years, the number of nights at or above 75 have increased.
Prior to 1940, Philadelphia's temperature was measured in various parts of the city but generally near or around Center City in some location. Since 1940, it's been at the Airport...the pre 1940 data is used to show the influence of concrete and city on the warmth at night on low temperatures (the 1930's were also a warm decade in general so there's some influence on the temperature from that standpoint). After three decades of relatively lower totals of warm nights, with the lowest ebb in the 1960's, we have an increase in the number of warm nights each year...and also in the number of nights where we failed to drop below 80 degrees.
We've notched one third of our 75 degree (or warmer) nights since 1990, along with 24 of our 48 nights historically where we have failed to drop below 80.
Just in the last four years, Philadelphia has failed to drop below 80 on eight occasions, almost as many as in the entirety of the 1990's! We've also seen two 83 degree nights so far in the last four years (one in July 2010 and the other in July 2011), the first times that has occurred in Philly's recorded temperature history.
The most 75+ degree lows in a year in Philly was in 2010 -- 22. This year, we're at 15, more than 2012's total of 11.
I mentioned back at the beginning of the article the warm pattern in the Atlantic -- the warm phase of the Atlantic Multidecadal Oscillation -- as a driver in the warmth at night. Given that ocean temperatures in the Atlantic overall, not solely along the Shore, are running warmer than average...there is more atmospheric moisture and more warmth available for transport into Philadelphia during the winter. The warm phase of the AMO kicked in around 1995 or so and these phases historically can run as long as 30 years. The AMO is a driver to enhance tropical activity in the Atlantic (more warmth = more potential for tropical systems to sustain themselves) but also helps provide additional warmth along the East Coast through the general circulation pattern of the Bermuda High in the summer pumping humidity northward.
The AMO is not the sole driver -- overall warming of the globe compared to the 60's has helped as well as more suburban sprawl around the Airport. These pieces have helped provide for a warmer overall climate at night in Philly -- one that sometimes struggles to cool off at a more frequent clip.