Although your calendar at home says summer begins on June 21st, meteorologists consider June 1st to be the start of summer! Part of this has to do with simplifying climatological data, since meteorologists and climatologists collect data on a month-by-month basis. Another reason is that the June 1st through August 31st time period is climatologically warmer in most areas of the northern hemisphere than the June 21st through September 22nd time period. Rockford, for example, has its climatologically warmest day around July 18th when our average high is 83 and our average low is 63. And the 18th of July is at the heart of the June 1st - August 31st time frame.
In addition, I found this information from a Wikipedia article that gives more insight into the start of meteorological summer:
"In 1780 the Societas Meteorologica Palatina, an early international organization for meteorology, defined seasons as groupings of three whole months. Ever since, professional meteorologists all over the world have used this definition.[5] So, in meteorology for the Northern hemisphere: spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December."
The June 21st date is called the astronomical first day of summer. This occurs when the suns rays are most direct in the northern hemisphere, or when they are directly above the Tropic of Cancer. From June 21st through the first astronomical day of Autumn (Sept. 23rd this year), the sun's most direct light gradually receeds from the Tropic of Cancer to the equator. This system has worked for astronomers for centuries. But now meteorologists and climatologists have come along, and they find that the June 1st date works better for them. --TS
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