39.9526° N, 75.1652° W, the lat/long of Center City. Climatically, Philly is warmer than most places at the same latitude within the continental United States during the cooler months. Geography plays a big role here, our close proximity to the moderating waters of the Atlantic Ocean can explain that, particularly in the wintertime. That makes sense.
However, I've been pondering a question for a couple of years now that deals with Philly's temperatures compared to some other locations, locations with a more fuzzy answer. The question comes from comparing Philly's daytime high temps, not inland areas, but to other East Coast areas; and not during winter, but rather during spring and summer.
Often during warm air advection events on the East Coast during the spring and summer, a cursory look at sfc map temps reveals that the Philly area often matches and sometimes surpasses the temperatures of locations along the East Coast far to the south, and yet areas not nearly as far north of Philly remain significantly cooler. Just north of New York City seems to be the line at which the temperature gradient dramatically decreases during such events. As far as those areas north of NYC, such as Connecticut and up, I think geography here again plays a large role with how given the change in shape of the coastline and raised terrain (and at times backdoor cold fronts can come into play during the spring). But, what about times when Philly has a warmer day than--let's say--Richmond, VA and Raleigh, NC while any difference in cloud cover is negligible? The temp difference can be even more pronounced comparing Philly to rural areas in eastern VA and NC, which lack the urban heat island effect Philly experiences.
The only causes I can think of as to why Philadelphia can achieve such high temperatures during warming events is because of the urban heat island effect and compressional warming. With the large metropolitan area Philly is complete with its low albedo surfaces, combined with its low-lying elevation and its somewhat inland location from the coastline, the urban heat island effect (UHIE) I think has quite an impact on Delaware Valley temps. I think compressional warming also comes into play given how the terrain slopes down from the Appalachians and Piedmont toward the Delaware Valley. But, I don't believe these reasons are sufficient to explain all the data.
There are times when Philly's temps can rival urban centers far to the south, even places such as Atlanta, which are also affected by the UHIE. Mostly rural areas in the Delaware Valley region can at times be on par with their southern counterparts many hours away down the map. All the locations I have discussed so far are set east of the Appalachians, which theoretically should also experience some compressional warming as a result of downloping air movement. So, my point is that I do not think that these two reasons alone can explain everything. What is it about the geography of the Delaware Valley that can make it so warm at times?
I have searched the Interwebs more than once seeing if my specific question regarding Philly's temps during warm periods versus other East Coast areas has been addressed, but I've never found anything. It's not like this is an unknown phenomenon, models accurately project this. So, I'm wondering what the thoughts are of anyone on the board about this matter. Please chime in!