The Driest Place in Pennsylvania

Tioga county is the driest county in Pennsylvania.

No, I'm not talking about those "dry counties" where it is challenging to get your drink on--I'm talking about precipitation. Pennsylvania actually does not permit counties or other localities to prohibit alcohol sales.

People often are curious about extremes and records--highest and lowest points in a state, or the wettest and driest places on earth or on a continent. What about the driest place in Pennsylvania? I don't know about others, but I find this question interesting, in part because I grew up in PA...and also because, for the most part, the climate doesn't vary much across the state, so where it does vary, it can be quite interesting.

View full (un-annotated) map.

An analysis by Oregon State University that combined data from NOAA, SnoTel, and other local data, between 1961 and 1990, the precipitation in PA varied between just under 34 inches annually to just over 50 inches for most of the state. These differences are subtle compared to the precipitation differences in regions like the American west: many people might not even notice the difference between 34 and 50 inches of rain annually. I have annotated this map to give a sense of what's where. If interested, you can view the project's original precipitation map for Pennsylvania.

One note: the wettest places on the map can be mostly explained by orographic lift, the process by which clouds drop their precipitation when they are forced up to higher altitudes.

Tioga County is the Driest County in PA, but Why?

The county circled in the map, Tioga County, is clearly significantly drier than any other county in the state; it's the only county where most of the area averages less than 34 inches of precipitation annually. To give a little background on this county, the local public transit system in Tioga and surrounding counties is called Endless Mountains Transportation Authority. The county seat, Wellsboro, isn't exactly a thriving metropolis; it had a population of 3,328 in 2000. Tioga county is also home to the "Grand Canyon of Pennsylvania".

To get a sense of why this county is so dry, I checked out some altitude data, including a topographical (elevation) map of PA. I also looked at some temperature data.

Elevation: Rain Shadow? Tioga county does appear to be surrounded by areas of slightly higher elevation--could this be a rain shadow effect? Rain shadow is so pronounced on the west coast, especially the pacific northwest, where rainforests grow on the windward side of mountains, and deserts can be found on the leeward side. In PA, the effect is subtle at best--the whole state is forested and receives ample rainfall year round.

Temperature? Look at Weather.com's data on weather averages on Wellsboro, and you see something interesting--and very different from the rest of the state: Wellsboro has a strongly seasonal pattern of precipitation, receiving an average of 4.56 inches of rain in the wettest month, June, and only 1.72 inches (rain equivalent) of precipitation in the driest month, February. This pattern looks superficially similar to the precipitation pattern in very cold regions, such as the upper midwest. Could the low precipitation be due to colder winters? Colder winters can result in lower precipitation, as cold air holds less water. Connected to this is the fact that snowfall tends to have a much lower volume (and thus a much smaller rainfall equivalent) than rain--areas that consistently stay below freezing in the winter tend to have less winter precipitation.

Taking a look at the averages for Smethport, a town two counties west (in the wetter zone) we see a similar temperature profile in the winter, although the daytime highs are a couple degrees higher on average--perhaps there's a higher likelihood of rain here. For reference, both of these towns have similar elevation. Smethport has a flatter precipitation pattern except for a dry February (like most of Pennsylvania), although it does share the unusually wet June with Wellsboro (unlike other parts of PA where June has similar precipitation to other months).

My current guess is that the low precipitation in this county is some combination of rain shadow and temperature. However, I'm no expert on meteorology or climatology, so I'd welcome a more thorough and rigorous explanation if anyone would care to offer one!

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