Saturday, August 10, 2013

A Cloudier Summer Than Past Years

With the way this summer has been in terms of rain in several parts of the region, some of us have been yelling at least figuratively, if not literally.  The rainiest summer on record for Philadelphia (although it may very well not be that for you, it is for the city) has also been cloudier than past years, thanks in large part to being near a persistent frontal boundary that has kept clouds (and showers) in place over the region.

I took a quick look back at the past four summers to see how the years stack up against each other, pretty sure going into this that 2013 was going to be cloudier than other years but wasn't sure to what extent it was.  The information is based on the percentage of daily cloud cover as reported in the National Weather Service climate summaries -- I pulled the info on the last four summers since it was readily available for a quick data pull.

The percentage of cloud cover is defined as the percentage of clouds covering the sky.  10 percent would yield that 90 percent of the sky is clear, 60 percent would yield a mostly cloudy sky with 40 percent clear, and 90 percent would be mainly cloudy with just 10 percent clear.

2013's average through yesterday is 66 percent, which ranks higher than any of the prior four years.  So far this August, we've had clouds covering 77.8 percent of the sky on average and that number will not decrease much despite today and tomorrow's nicer weather.

Last four summer percentage of cloud cover averages. Note August average is 2010-2012 only.

To put these numbers in perspective, the summer of 2012 averaged 62.1 percent cloud cover and the summer of 2011 averaged 55.9 percent. The summer of 2011, which had a very rainy August (Irene and thunderstorms before there) also had a relatively dry June and July that featured quite a bit of sunshine...and the hottest July on record.

While I'm not surprised we're cloudier than the past couple of years given the amount of rain and rainy days that have occurred around the region, the percentage is a bit higher (and overall for each of the years, a bit higher) than I thought it would be.

Some of our historical data is a bit interesting as well.

June 1988 featured just 48 percent cloud cover and clouds covered just 51 percent of the sky on average in the summer of '88.  The summer of '88 was nationally a dry year nationally so I was not surprised to see the percentage of cloud cover as low as it was. It's important to note in 1988 that a large chunk of the summer rainfall occurred in July (eight inches of it, five of which occurred on two stormy days) and that most of the rest of the summer was quite dry.

One of the more intriguing anomalies was the summer of '95, which produced 4.69" of rain and is the third driest on record as well as the prior record holder for warmest summer on record before 2010 bested it. The summer of '95 averaged 57.5 percent cloud cover, more than 2011's average cloud cover.

Long story short is that our summer is cloudier than the past couple of years has been...and answering the why behind it features a mix of factors -- position of jet stream, trough and cut off lows located in the Great Lakes and surface fronts nearby, plus a warm Atlantic providing additional moisture and fuel for precipitation and clouds. These seem to be driving a cloudier bus this if you feel like yelling at the clouds, I don't think many will mind given they've dominated the sky in greater percentage this year than the past few.