What are the Seasons?

The concept of dividing the year into four seasons is reexamined to appraise critically the relative merit of two commonly used definitions of the seasons: 1) the astronomical definition; and 2) the meteorological breakdown into four three-month periods. These are compared with the definition of winter as the coldest season, summer as the warmest season, and spring and autumn as the transition seasons. Observational data on surface temperatures over the entire globe and, in particular, over the United States, are used to determine what the seasons should be. Presented here is an analysis of the amplitude, and phase of and percentage variance explained by the first harmonic of solar radiation at the top of the atmosphere and surface temperatures. Annual changes in surface temperature associated with the seasons are much larger over land than over the oceans. Surface temperatures lag the solar cycle by 27½ days over the United States, compared with 32½ days in mid-latitudes over the Northern Hemisphere as a whole, and 44 days in mid-latitudes of the Southern Hemisphere. The astronomical definition of seasons is appropriate only over the oceanic regions of the Southern Hemisphere. Over the continental regions of the Northern Hemisphere, the "meteorological" seasons in which winter is December, January, and February, etc., agree reasonably well with observed events and are recommended for general usage.