Start of the astronomical summer
Today, 20th of June 2016, marks the official start of the summer.
In everyone’s calendar, the start of the summer corresponds do the “astronomical summer” and comes every year between June 20 and June 22.
However, in a meteorologist’s calendar, the seasons are listed quite differently.
But what is that difference exactly?
Astronomical seasons depend on the position of the Earth relative to the sun, while meteorological seasons are based on the annual temperature cycle around the globe.
For astronomical seasons, this means that as the Earth revolves around the sun, seasons change gradually along the two solstices and two equinoxes: these are based on the Earth’s tilt and the sun’s alignment over the equator. As we can see from the graphics above, the solstices are the times when the Earth’s equator is tilted farthest, north or south, away from the sun, while the equinoxes happen when the equator is aligned directly with the sun.
Meteorological seasons on the other hand are divided into groups of 3 months each, based on the annual temperature cycle around the globe.
Meteorological summer goes from June 1 to Aug. 31, meteorological fall from Sept. 1 to Nov. 30, meteorological winter from Dec. 1 to Feb. 28 and meteorological spring from March 1 to May 31.
These four meteorological seasons were derived for weather-observing and forecasting purposes.
The meteorological seasons are always 90 to 92 days, depending on whether or not it’s a leap year.
Top image credit NOAA/NWS