As the seasons change, you may often hear of the terms “astronomical Spring,” or “meteorological Spring” being used around this time of year. You may then be wondering what the difference between the two are, and which is the correct term to use? The answer is that they’re both correct, but when discussing the different seasons, it’s important to note which definition you’re using and what side of the equator you’re on. Simply, the astronomical seasons are defined by the position of the Earth compared to the sun, and the meteorological seasons are based on the annual cycle of temperatures.
The reason why we have seasons is because the Earth is tilted approximately 23.4 degrees on its axis. As the Earth orbits around the sun, different parts of the Earth receive varying amounts of solar energy at a time. This natural rotation of the Earth around the sun along with its tilt sets the foundation for the astronomical calendar in which we group the seasons into solstices and equinoxes. As you can see in the visual representation of Earth’s seasons below, the tilt of the Earth and alignment of the sun play a key role in defining our seasons.
The seasons are defined by most cultures as Spring, Summer, Fall (Autumn), and Winter. The astronomical definition uses the dates of solstices and equinoxes to define the start and end of the seasons. Spring and Fall (Autumn) begin on the Spring (vernal) and fall equinoxes, and Summer and Winter begin on the Summer and Winter solstices.
The Summer and Winter Solstices mark the longest and shortest days of the year where the Earth is tilted the most towards the Sun during the Summer solstice, and furthest away from the sun during the Winter Solstice. In the Northern hemisphere the summer solstice occurs around June 21st, the winter solstice around December 22nd, the Spring (Vernal) equinox around March 21st, and the Fall (Autumnal) equinox around September 22nd. In the Southern hemisphere the seasons are reversed but begin on the same dates as shown in the image above. However, the timings of the equinoxes and solstices vary each year, which changes the length of the astronomical seasons.
The Earth technically makes a complete orbit around the sun in 365.24 days, so every four years an extra day is needed to balance this out. The Leap Year was then created to accommodate the extra time it takes Earth to fully orbit around the sun. This in turn makes comparing climatological data for each season difficult from one year to the next. Therefore, the meteorological seasons were created.
Meteorological seasons are separated by Meteorologists and Climatologists into four groups of three months based on the annual temperature cycle. It’s based on the observations that summer is the warmest time of the year, winter is the coldest time of the year, and Spring and Fall are the transition seasons. With that being said, the seasons begin on the first day of each month. In the Northern Hemisphere:
Meteorological Spring starts March 1st and ends May 31st
Meteorological Summer starts June 1st and ends August 31st
Meteorological Fall (Autumn) starts September 1st and ends November 30th
Meteorological Winter starts December 1st and ends February 28th (29th during a leap year)
Below is a full chart showing how the seasons are defined by hemisphere, and which definition is being used:
This is why you often hear your local meteorologist inform you about the first day of the meteorological season at the start of each month that contains an astronomical season.
Defining seasons by the meteorological calendar helps long term forecasting and climatic data easier to calculate seasonal statistics in order to better understand and convey average temperatures to the public. This calendar is also useful for other important purposes such as agriculture and commerce.
Different countries around the world define seasons using multiple definitions and factors as well. For example, Australia and New Zealand rely on the meteorological definition to define their seasons, Ireland uses an ancient Celtic calendar, some cultures in South Asia have six seasons, and in Finland and Sweden seasons are based solely on temperatures above and below freezing, so their seasons begin on different days each year depending on their climate.
In summary, the reason for having two (or multiple) ways to express the start and end of the seasons are because of their different start times and durations and based on the climate and culture of an area.
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@ 2019 Weather Forecaster Christine Gregory