How do Meteorological and Astronomical Seasons Differ?

The Position of the Earth during the year    Source: NASA

People sometimes hear different definitions of the seasons when talking or reading about the weather.  Some scientists refer to “meteorological” summer, while others strictly refer to “astronomical” summer.  They are two different concepts.

Meteorologists and climatologists commonly look at seasons in three month blocks for ease of calculating average temperatures and precipitation amounts for a given location.  For instance, “meteorological” summer runs from June 1 through August 31 – so today is the unofficial halfway point of the summer.  A column I wrote earlier in the week discussed how much wetter than average the first half of the summer has been in the Nation’s Capital.  I was going by the definition of “meteorological” summer.  Using the same standard, September 1 is when “meteorological” fall begins; December 1 is when “meteorological” winter commences; and “meteorological” spring arrives on March 1.

There are also “astronomical” seasons.  Those are what many people are more familiar with.  The “astronomical” seasons are dependent on the position of the Earth in relation to the sun.  The summer and winter solstices occur when the sun’s path is farthest north or south of the Earth’s equator.  The summer solstice – or first day of “astronomical” summer – occurs on or about June 21 in the Northern Hemisphere.  That’s the first day of winter in the Southern Hemisphere.  The equinoxes occur when the sun passes directly over the Earth’s equator.  In the Northern Hemisphere the spring (vernal) equinox occurs around March 21 (which is the first day of “astronomical” spring) while the fall (autumnal) equinox occurs on or near September 22. 

Since it takes 365.24 days for the Earth to travel around the sun, an extra day is added to the calendar every four years in what are called “Leap Years.”  That’s why the astronomical seasons begin on approximate dates.  According to NOAA (which keeps climate records for the United States), it’s easier to measure seasonal temperatures over an exact three month period.

Summer is Half Over: Taking Stock

The Mid-Atlantic Region is quite green amid ample rainfall (July 2015)

The last six weeks have been significantly wetter and warmer than average in the Nation’s Capital.  With the exception of a nearly two-week stretch in late June and early July when temperatures remained below 90 degrees, the weather has been quite warm.  Although no record highs have been set at National Airport, there has been a prodigious amount of rainfall that’s approaching record territory in the Mid-Atlantic Region.

June 2015 was the second wettest on record in Washington, D.C., and the wettest on record in Baltimore, MD when 13.09” of rain fell.  Measureable rainfall fell at National Airport on 18 days last month and the 7.44” that occurred at Dulles Airport in June 2015 made it their wettest June since 2006.  Meteorological summer runs from June 1 through August 31.

Not only has there been above average rainfall during the last six weeks, but much of that rainfall has occurred in thunderstorms.  There have been several days since June 1 that severe weather occurred in the Mid-Atlantic Region.  In fact, two weak tornadoes touched down in southern Maryland on June 27 and caused minor damage.  This dramatic shift in weather from being warmer and drier than average in April and May to being warmer and significantly wetter than average since then, has had both positive and negative repercussions.  The good news is that area gardens are thriving and local reservoirs are in good shape.  The bad news is that localized flooding has occurred and areas that had severe weather saw property damage.

The National Weather Service reports that 15.4” of rain has fallen in the Nation’s Capital since June 1.  Their records also indicate that Los Angeles, California, has seen only 14.53” of rain since January 1, 2013.  So, Washingtonians have had more rain during the last six weeks than Los Angeles has had in the last two and a half years.

Although Washington, D.C. averages 3.73” of rain during the month of July, 3.46” has already occurred during the first 11 days of the month.  With more rain expected at times this week, it will be interesting to see just how wet this month ends up being.  The wettest July on record in the Washington, D.C., was in 1945 when 11.06” of rain fell.

Just How Hot Can It Be in July?

Temperatures don't often reach 100 degrees in the Mid-Atlantic Region

July typically features some of the hottest weather of the year across much of the country, including in the Mid-Atlantic Region.  Although it hasn’t been 100 degrees in the Nation’s Capital since 2012, July has seen some extreme and record setting heat during the last five years.

To read more, click on this link to see the story I wrote for the WUSA9 website.