It was unusually cloudy in December across much of the United States. So much so that some industries and even people were influenced by the persistent cloud cover. The Kansas Mesonet (mesonet.ksu.edu), like everything else, was also subject to these dark times. These weather stations were able to capture clouds via solar radiation measurements.
During the day, if clouds are present, more solar radiation from the sun is reflected back into the atmosphere, not reaching the surface. Some of our Mesonet stations have been around for more than 20 years, observing long-term trends in cloud cover from surface radiation. One of these long-term stations is located at the Agronomy North Farm, Manhattan. In December for the last 28 years, the station has averaged 201.6 MJ/m2 for the month. However, in December 2014, we received 154.3MJ/m2, only 76.5% of our average solar radiation. To put this into perspective – the last (and only other) time it observed less than 160MJ/m2 was 1995. Only 9 times in last 28 years were below 180MJ/m2. December 2013 was very sunny compared to average, receiving a total of 239.7MJ/m2.
Figure 1. 2014 is the 2nd least solar radiation observed in Manhattan mesonet during the month of December.
On a typical day in December the computed maximum possible daily radiation is 10MJ/m2. The average Dec day realized radiation is 6.5MJ/m2 over the last 28 years. In 2014, we only had about 5MJ/m2 each day. With an average of 9.5 hours of daylight each day, that means we lost almost three hours of sunlight lost due to cloud reflection/refraction each day.
Clouds were not only confined to the Manhattan area. Much of eastern Kansas was also subjected to widespread, persistent cloud cover. Parsons, another 28-year station, had the lowest monthly total of solar radiation of all time, 135MJ/m2. That is only 63 percent of the historical average at this station.
Figure 2. Mesonet at Parsons observed the lowest ever radiation in Dec 2014.
Meanwhile, in western Kansas, December was more average, with the Colby site observing 90 percent of its 28-year average, 253.3MJ/m2. Typically, clouds tend to be less persistent across western Kansas in all months, as was the case in December.
Figure 3. Colby mesonet site has a higher average Dec radiation, with no substantial decrease in 2014 due to clouds.
Other than just causing gray days, clouds also have an impact on temperatures. Low temperatures were warmer (2-7 degrees F) than normal. This is because clouds keep longwave radiation concealed to the surface, especially at night. Usually this radiation can escape into the atmosphere, cooling the surface. Other impacts felt across Kansas include: solar panels not operating correctly, light winds, damp conditions, and maybe even mood changes!
Chip Redmond, Weather Data Library