Cloudy days vs. Sunshine in McKean County Pa.

Cloudy days vs. Sunshine in McKean County Pa.

Many days here in NW Pa., we get up in the morning and see in the forecast mostly cloudy. This unfortunately is a ‘way to often forecast’ for us, and the bodies of water to our West and Northwest are as much to blame as any storm system that may be coming through.


Evaporation, a very important part of the global water cycle, can be defined simply by solar radiation hitting the surface of a body of water and changing that water to a gas.

Even Small Puddles Evaporate Back Into the Air

That gas is then lifted into the atmosphere, which in turn can cause clouds to form as moisture is the main ingredient of all clouds.

Anywhere in the northern hemisphere, there’s a prevailing west to east ‘wind’ just from the rotation of the earth, and coupling it with the rise of the liquid gas off the lakes, we get more than our share of cloudy days.

Many studies have shown that water put back into the the ‘water cycle’ comes from oceans, lakes, rivers and streams. The percent of water back into the cycle from those bodies of water is around 90% of total rainfall around the world and being on the ‘lee side’ of the Great Lakes gives us an even better chance for clouds and precipitation.


The other 10% or so of the water cycle comes from Transpiration. That is the natural occurrence of plants, trees and grasses, taking water in through their roots, moving up through the plant and exiting from arial parts, such as flowers, stems and leaves.

Transpiration example
Transpiration Experiment


A simple experiment to show how transpiration works is in the photo to the left.

Simply cover some stems and leaves with a clear plastic bag and within hours, usually minutes, you will begin to see water droplets forming on the inside of the bag.

Even in the shade, the plant will omit the water vapor to the air, but if you want to increase the water escaping, set the plant in the sunshine for a while and most likely, the water vapor will begin to ‘drip’ back into the bottom of the bag.

The combination of Transpiration and Evaporation gives us Evapotranspiration. The natural evaporation off of the lakes and streams, combined with the transpiration of all of the grasslands, crop fields and trees in the region.


Evaporation + Transpiration

To complete the water cycle, we combine Evaporation with Transpiration by using the term Evapotranspiration.

That is the way all gases off our rivers, streams, lakes and oceans, combine with all gases given off by plants, grasses and trees, and return to the atmosphere to possibly form clouds and give us the much needed rainfall to start the process all over again.

Being in NW Pennsylvania, you can see how we get our we get plenty of forecasts for ‘mostly cloudy’ days, as all of the evaporation and transpiration in our neck of the woods leads to the possibility of mostly cloudy days.

How Records are Kept

Many of the NWS Recording stations, normally at airports, have phased out the use of sun sensors. That began in the 1990’s and now, to get data about how much sun we get, they use a system of clear, partly cloudy and cloudy.

NOAA compiles that data from the airports and submits their Comparative Climate Data summaries. These are put out every year, but usually lag a year or two, as the data is complex and just takes a little time to put together.

For instance, using data through 2015, Erie gets 63 clear days a year, 97 partly cloudy days and 205 mostly cloudy days. Pittsburgh’s average is 59, 103, 203. Buffalo at 52, 102 and 212. Bradford joins the list with 73, 107,  and 185. These are just averages of reported cloud cover over the last 20 years or so, depending on exactly when that station quit using a sun sensor.

At, my station, a Davis Vantage Pro 2, still records actual hours of sunlight using the formula from the World Meteorological Organization W/m2 (watts per square meter). Their recommendation is for 120 watts and the Davis uses 110 watts, to give us actual hours of sunlight through each day.

The sensor, or solar pyranometer, “measures global radiation, the sum at the point of measurement of both the direct and diffuse components of solar irradiance. The sensor’s transducer, which converts incident radiation to electrical current, is a silicon photodiode with wide spectral response.

From the sensor’s output voltage, the console calculates and displays solar irradiance. It also integrates the irradiance values and displays total incident energy over a set period of time.”

In gardener or farmers terms, it’s enough sunshine to cast a shadow of yourself or any object, such as a tree, onto the ground.

At the time of this writing, I have 2,902 days in my database, just short of 8 years and through those days, we average 4.84 hours of sun per day. Now, it’s not everyday we get sun as seen from the data above, but across an entire year and it’s different seasons, that’s the average for my location in McKean County.

So there’s good reasoning, that after a long winter and heading into the spring time, we in this area are clamoring for some sunshine. With the lakes as our neighbors, we get more than our share of clouds through the years.

All of us in McKean County, with towns like Bradford, Smethport, Mt. Jewett and Kane, enjoy the peace and quiet of our ‘mountain’ lifestyle. We could just use a little more sunshine throughout the year.


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