September 20, 2012 in Washington Voices

Farewell to summer; fall equinox closing in

Randy Mann

It is hard to believe summer is almost over. The autumnal equinox, and the beginning of fall, is at 7:49 a.m. Saturday. For parts of the central U.S., this summer has been one of the worst heat and drought cycles since the 1930s.

With a new season, we should eventually see a change to a cooler and wetter weather pattern, probably around mid- to late October.

Most of us know that we have seasons because the Earth is tilted on its axis by about 23.5 degrees. In our summer, the Northern Hemisphere is tilted toward the sun, allowing us to receive more direct solar radiation. In our winter, the Northern Hemisphere is tilted away from the sun, making for shorter days and a lower sun angle.

Believe it or not, distance from the sun has very little to do with seasonal changes in temperature, as we are actually closer to the sun during our colder season. The Earth is usually closest to the sun in its orbit around Jan. 3 and farthest away on July 4.

On the equinox, a person standing on the equator would see the sun pass directly overhead at 90 degrees. Every part of our planet, including the poles, will experience 12 hours of daylight and 12 hours of darkness around Sept. 22, and again for the vernal equinox around March 21.

Seasonal changes are more drastically felt the farther you go from the equator as colder air and warmer air are in a constant battle for supremacy. This is one reason why we see the severe thunderstorm and tornadic activity during the spring season in the central U.S.

The high pressure ridge that’s currently keeping storm systems from the Gulf of Alaska from entering the Pacific Northwest is very strong and quite large, covering the entire northwestern corner of the U.S. and much of southwestern Canada. Relative humidity levels have been as low as 25 percent in recent weeks. Fire danger levels will remain high until we see some significant precipitation in our area.

A major change to the wetter side of the meteorological scale isn’t expected until the “new moon” phase of Oct. 15-22, a full month into the fall season.

If you have any questions or comments, you can contact Randy Mann at wxmann, or go to www.longrange for additional information.

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