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How unusual was June weather?

June may have seemed unusually "bad" to you this year, with June living up to its reputation for cloudy and cool conditions. However, average temperatures in Coupeville were within one degree of normal, and precipitation was only slightly above normal. My rain gauge near Keystone collected 1.39 inches of rain, which is very close to the Coupeville 100-year average of 1.25 inches and even closer to the 30-year average of 1.31 inches.

One clue to June "badness", however, is that the near-normal amount of precipitation was spread over 13 days. Coupeville would normally have only 9 days with rain. So perhaps June gloom was due to the higher number of cloudy drizzly days. By the way, south Whidbey Island had considerably more rainfall than Coupeville. Two observers measured over 3 inches of rain. Greenbank and Oak Harbor generally got about 2 inches.

Is there a way to confirm that June was more gloomy than usual? It turns out that we have had measurements of sunshine in Coupeville since July 2006, courtesy of WSU Agricultural Weather Network (AgWeatherNet). The data confirm that June was cloudier than normal with a 15% reduction in sunshine (solar radiation). That 15% reduction was relative to the four-year June average from 2007 to 2010.

Another measurement of the AgWeatherNet is evapotranspiration. These data show that soil and plants (grass and alfalfa) thought that it was 22% cooler and wetter than the four-year average. The lack of sunshine and more days of rain combined to produce a significant reduction in water lost through plant leaves (transpiration) plus evaporation from soil and wet leaves.

The WSU AgWeatherNet is a tremendously valuable resource for those who want to keep an eye on Coupeville crop growing conditions. Designed for farmers, it provides hourly measurements of temperature, humidity, wind, and precipitation. In addition to solar radiation and evapotranspiration, measurements are provided of leaf wetness and bare soil temperature (at 8 inch depth). All of this hourly data is available on-line at http://weather.wsu.edu/awn.php. The Whidbey Island station is located just south of Coupeville High School on Engle Road, and gives data that would be representative of conditions on Ebey's Prairie. To access this data you will need to sign up and specify a user name to obtain a password.

Anyone who has lived in western Washington for any length of time is aware of the tendency for June to be cloudy, cool, and somewhat wet. As a young boy growing up in Kirkland near Lake Washington, I noticed that June was often a bad month for water activities. In my university days at the UW studying Atmospheric Sciences, Dr. Phil Church challenged us students to study the "June anomaly," as he called it. He offered a hint: the northward transition of the Pacific high pressure region. While I did not accept the challenge for a detailed study of the June anomaly, I have often thought about it over the years.

Here's what I have concluded about its cause. Satellite pictures clearly show that during spring and early summer the northeastern Pacific Ocean is covered with low-level clouds. The low-level clouds are located in the region of the strengthening high pressure region that gives Hawaii its good weather. High pressure normally is associated with clear skies and warm temperatures, but over the ocean in early summer the high pressure produces a condition where moist air collects near the ocean surface, giving rise to low-level stratus clouds.

In addition, the high pressure offshore means that there is a tendency for the moist, marine air (and associated low clouds) to move inland toward lower pressure in Eastern Washington (due to warm temperatures there). Actually, this occurs all along the Pacific coast from central California northward. Eventually, the Pacific Ocean high pressure region moves north far enough in July and August that the effect is reduced.

The cool moist marine air of the Pacific Ocean is never far away, and can easily move inland if conditions are favorable. On several occasions this summer we will get a strong blast of ocean air down the Strait of Juan de Fuca, and also low clouds and a sudden drop in temperature of 20 degrees or more. This most often will happen after a few days of hot temperatures. The hot temperatures induce low pressure inland, which pulls the marine air toward us. After a few really hot days, most people welcome the transition to cooler air. It's like free air conditioning.

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