Thursday, March 12, 2015

When does Spring really start?

We're often asked the question "when does Spring really start?"  The easy answer is to look at your calendar.  It says March 20th.  This is "Astronomical Spring", which is the vernal equinox.  It's defined as the day when the sun is directly over the equator, and the length of a day is 12 hours no matter what your latitude (this isn't quite correct, but we won't go into that here).  But it doesn't take much thought to realize that this definition is only remotely linked to the weather.  And having one date for the entire Northern Hemisphere seems in some ways, ridiculous.  Spring starts in Miami and Fairbanks on the same day?

The "Meteorological Spring" is another definition.  It's defined as the months of March, April, May.  But again, this suffers from a "one size fits all" problem.  Surely there's a better way of defining Spring that varies from one location to another.  The Capital Weather Gang at the Washington Post addressed this same topic, and had some other ideas.  You can read their blog here.  

Some of their suggestions have this natural variability built in, such as the last day of measurable snow, last freeze, soil temperature, and first green leaves.  But many of these would be difficult to apply to all locations.  What about locations that don't receive snow, never freeze, or don't lose all of their leaves?  And if a location receives snow in May, does that mean that it's still Winter?

For central Washington, the Tree Fruit Research Center in Wenatchee tracks the average first bloom of the apple trees.  You can see those dates at this link.  The nice thing about a measure that tracks leaves, flowers, or soil temperatures, is that it will vary year-to-year (i.e. spring doesn't start on the same day every year).

The intriguing measure in the Capital Weather Gang blog was defining seasons based on average temperature.  They took the average daily temperatures at a location, and divided them into the coldest 1/4th (winter) and warmest 1/4th (summer), with spring and fall as the seasons in between.  If we do this for the Spokane Airport, we get the following:

Winter: Nov 11th - Mar 6th (116 days)
Spring: Mar 7th - Jun 8th (93 days)
Summer: Jun 9th - Sep 15th (99 days)
Fall: Sep 16th - Nov 10th (57 days)

While this is somewhat interesting, we're not quite sure if we agree with its accuracy.  Fall is the shortest season, no doubt about that.  But in reality, spring is the longest of all season in the Inland Northwest.  It typically starts in late February or early March, but it often lasts all the way through June.  We have a saying that summer in the Inland Northwest doesn't start until after the 4th of July, in Seattle, they claim that their summer starts on the 12th of July.  This means that in reality, spring lasts about 4 months while summer is just a shade over 2 months long.

For us, it's often easiest to classify the seasons by holidays:

Winter: Thanksgiving to President's Day
Spring: President's Day to the 4th of July
Summer: 4th of July to Labor Day
Fall: Labor Day to Thanksgiving

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