Clouds in Washington State in Winter
Judy Garland was quoted to have said, “Behind every cloud is another cloud”. The residents of Washington state can appreciate that statement during winter. A map of the mean number of cloudy days in January for the US is shown in Figure 1, and indeed the Pacific Northwest is the cloudiest part of the country, with only a few regions around the Great Lakes reaching the values observed across Washington, Oregon and much of Idaho. But there are local variations in cloudiness in WA state, and some of these variations might be surprising.
Before diving into the data, it bears mentioning that there exists potential health issues associated with our gray skies in winter. While it is easy to receive too much exposure to sunlight around here in the summer, there is the opposite problem in winter. Deficiencies in Vitamin D have been related to an increased risk of cardiovascular disease, cognitive impairment in older adults, and notably, higher incidence of various autoimmune diseases. The farther one lives from the equator the higher the chance of contracting multiple sclerosis (MS), considering populations as a whole. There is a large amount of information on this subject with an example cited below (Aranow 2011). You are forewarned: this paper and others of its ilk have even more jargon and acronyms than those in the fields of climatology and meteorology, which is saying something. But if dropping everything and heading to Hawaii is not really an option, are there local variations in sky conditions that can be put to advantage?
The short answer is yes, but only to a limited extent. Even though average precipitation totals for the month range greatly across the state, mean values of cloud cover are much more uniform. A map of WA state with the 26 stations for which long-term averages of cloud cover fraction in January are available is shown in Figure 2. Note that these averages are in oktas or one-eighths, which is the traditional unit for fractional sky cover. The averages are based on data between 1973-2000, and while they are not exactly up-to-date they should still be representative of present conditions. Of the 26 stations included in Figure 2, the least cloudy location is Omak in the northern part of the state east of the Cascade Mountain crest, and the cloudiest location is Walla Walla near the southeast corner of the state. The latter result was not anticipated. Perhaps it can be attributed to the presence of the Blue Mountains just to the east, and specifically their upstream effects in the presence of mean westerly flow, but those effects are not that profound in terms of the precipitation, which averages only slightly above 2” in Walla Walla during the month of January.
We were struck, in general, by how cloudy it is in eastern Washington during January. There are probably different reasons for different locations. In the eastern foothills of the Cascades, the mean pressure gradients causing low-level easterly winds would serve to help keep places like Yakima and Wenatchee cloudy. In the lower elevation Columbia Basin, a “cold pool” forms in otherwise clear and calm conditions and once formed, is often accompanied by persistent decks of low stratus and fog. The Spokane area is essentially on the western approach of the Rocky Mountains and may be subject to the same kind of orographic effects on the flow aloft that are presumed to cause it to be so cloudy in Walla Walla.
The cloudiness across the west side of the state has its own quirks. Shelton checks in with the lowest average cloud cover and Olympia the greatest with Toledo right behind. The high values at Olympia and Toledo appear to be due in large part to those stations being situated in interior basins prone to the development of fog. It is not clear why Shelton enjoys fewer clouds (pun intended) because it is no slouch in terms of rain, with an average of almost 12” during the month of January. Shelton tends to clear out during northwesterly flow due to downsloping off the Olympic Mountains, and perhaps that happens often enough to bring down the mean value. Another interesting result is that the coastal locations of Quillayute and Astoria, Oregon (data were not found for the Aberdeen/Hoquiam area) have cloud fractions in the middle of the pack. This part of the state also receives abundant precipitation, of course, but also apparently some chinks in the cloud armor.
This segment on the dismal subject of our gray skies in winter should not be misconstrued; the OWSC is not in the pocket of the suntan parlor industry. Readers of this newsletter are encouraged to get outside during those infrequent, but by no means rare, intervals when the skies are bright this time of year. Short exposures to sunlight can provide a meaningful dose of Vitamin D, with benefits for one’s mood and health.
Reference:
Aranow, C. (2011): Vitamin D and the Immune System. J. Investig. Med., 59(6): 881-886.doi:10.231/JIM.0b013e31821b8755.