With meteorological fall now underway as of September 1st, many of us are wondering what the difference is between meteorological and astronomical fall. The difference comes down to the weather based on temperature cycles (meteorological) and the natural rotation of the Earth (astronomical). 

• Meteorology is the science pertaining to the processes and phenomena of the atmosphere, especially as a means of weather forecasting.
• Astronomy is the study of objects and matter outside the Earth's atmosphere in relation to their physical and chemical properties.

Both meteorological and astronomical seasons (Winter, Spring, Summer, and Fall) represent Earth’s seasonal temperature cycle but in different ways.

Meteorological Seasons

The meteorological seasons have start dates based on the seasonal cycles of temperatures. This directly relates to what ‘we feel’ through the changes in temperature as the seasons change.

In the Northern Hemisphere, the meteorological seasons are defined by: 

The start dates are on the 1st of the month and the end dates are on the last date of the month. 

Weather observations and forecasting led to the creation of meteorological seasons. Meteorological seasons are closely tied to our civil calendars (not when Halloween stuff is in the grocery store), while astronomical seasons are not.

Since meteorological seasons are in monthly periods, they are used for official climatological and meteorological statistics. 

Astronomical Seasons

The astronomical seasons have start dates that are based on the natural rotation of the Earth. The tilt of the Earth and the alignment of the Sun over the equator determines both the solstices and equinoxes, which are the names given for the astronomical seasons.

The equinox is when the Sun passes directly above the equator. The solstice is when the Sun is directly over the Tropic of Cancer, which is located at 23.5 degrees North.

In the Northern Hemisphere, the astronomical seasons are defined by: 

The time it takes Earth to travel around the Sun is 365.24 days. Since this is not an easy number to divide into months, an extra day is needed every fourth year, creating a Leap Year. This also causes the exact date of the solstices and equinoxes to vary by several days between different years. 

The length of the season will vary by 89-94 days, which is determined by the start and end dates of the astronomical season. 

The differences in astronomical seasons per year cause a headache to compare statistics for climatologists and meteorologists. In order to provide a consistent date and season for all seasons and years, meteorological seasons are used.

Other Facts of Seasons

While for official record keeping, meteorological seasons are used, the weather does not always play by the rules. The seasons apply mostly to mid-latitude regions (30-60 degrees) where the tilt of the Earth allows temperatures to cycle in a natural rhythm. Areas close to the equator (0 - 30 degrees) and poles (60 - 90 degrees) experience less diverse temperature cycles due to the Sun’s angle on these areas.

The tables shown above are related to the seasons in the Northern Hemisphere. In the Southern Hemisphere, the seasons are the same months and dates but change per respective temperature pattern, which is the opposite. 

Additionally, mountainous environments no matter the latitude can have harsh weather due to elevation. This causes the weather to not always ‘feel’ like the associated season. Mountainous environments can cause extreme weather no matter the meteorological or astronomical season.

Zach Butler