PITTSBURGH — Why is the Steel City so grey?
According to the National Center for Environmental Information, Pittsburgh averages 59 clear days per year and 306 cloudy days. However, the 306 cloudy days includes partly cloudy days, too, in which clouds cover 40 to 70 percent of the sky.
If only overcast days are taken into consideration, the city averages 203 per year.
So why do we see so many cloudy days? One of the reasons has to do with wind flow and another has to do with what’s going on just above the surface of the atmosphere.
Wind flow from the northwest, which we often see in the winter, pulls moisture from Lake Erie. Air is forced to rise, and clouds are created.
Another reason for the stubborn clouds in our area has to do with something we call an inversion. An inversion is when there is a warmer layer of air above the surface. Inversions are known to lead to cloudy days.
Look at this image of data collected through the atmosphere the morning of Jan. 12. The green line is the dew point temperature as you ascend through the atmosphere. The red line is the temperature line. The numbers at the bottom of the image are the temperatures, in Celsius.
Notice how the temperature starts to decrease with height and then increases as you get higher in the atmosphere. This is called an inversion.
The inversion can be broken by warming surface temperatures or by some sort of lift in the atmosphere, whether it be a cold front or weather disturbance in the jet stream pattern.