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Today is the first day of spring... maybe

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According to my calendar, today is the first day of spring. And it seems like that is all anyone can talk about today. Most people seem to take for granted that the calendar says today is the first day of spring, so it must be true. However, that is not necessarily true. It all depends on which of many definitions of spring you subscribe to. What? You did not know that there are multiple legitimate definitions of spring? Well then, I think it is time for a little science lesson. Today’s topic: different definitions of spring and their strengths and weaknesses.

1. Astronomical: The seasons are defined based on the location of the sun in the sky. This is the one that you see on your calendar. Using this definition, spring begins on the day of the Vernal Equinox. The Vernal Equinox is that moment when the sun is directly overhead when observed from the equator. This year, that moment occurs at 12:57 EDT on March 20. The term equinox is a bit of a misnomer. The word equinox comes from Latin and means literally “equal night”. It is widely believed that on the day of the equinox that there will be exactly 12 hours of sunlight and 12 hours of dark. However, due to a number of factors, there is actually slightly more than 12 hours of daylight on the day of the Vernal Equinox. Some of the reasons for this are due to atmospheric refraction of sunlight, varying rotational speed of the Earth, relative size of the sun’s radius to the earth’s radius, etc. But I won’t go into the details of that here.

This definition is useful because it is easily recognizable everywhere in the world. However, the date fluctuates slightly from year to year which makes weather data comparisons challenging. Also, the reality of day-to-day weather does not typically follow the calendar that precisely at most locations. (Does today really feel more like spring in CT than it did on March 8 when the temperature was in the 50s?)

2. Meteorological: Each season is defined to be exactly 3 months long. Spring begins on March 1 and lasts until May 31.

This definition is extremely useful for data collection and analyses. Every season is exactly 3 months long, and every season begins on the first day of a month. When compiling data you don’t need to worry about what day of the month the data comes from, just the month.

However, this definition is arbitrary in that it is based solely on the design of the Gregorian calendar. If the calendar had been designed in such a way that March 1 fell on the day that is only 1 week before the vernal equinox rather than the current 3 weeks before the vernal Equinox, then the data we have for meteorological spring would look different. While this definition makes data collection and analyses extremely easy, I personally think that kind of arbitrary definition to be a little lacking from a scientific point of view.

3. Climatological: The seasons are defined by the average daily temperature. Winter is defined as the period of the year when the average daily temperature is consistently 30°F or lower. Therefore, once the temperature is no longer consistently that cold, spring is here.

This definition is useful because it utilizes an actual physical parameter rather than an arbitrary date on the calendar. However, it this definition results in widely varying dates from year to year as well as place to place. For example, based on this definition, spring typically arrives in CT on or around Feb 17. This year spring arrived in central CT on Feb 23. This kind of inconsistency wreaks havoc on conventional understanding of the seasons from a data point of view. But, this is most closely related to how people actually think about the seasons. Have you ever heard someone say “spring came early this year”, or “winter was longer than normal this year”? That kind of statement does not work with the astronomical or meteorological definitions of the seasons, but fits perfectly with the climatological definition.

4. Environmental conditions: This last one is less of a definition, and more of a feeling or point of view. Does it feel like spring? Are the daffodils blooming? Did you see your first robin? (Although – I saw robins out my back window all winter this year, so the accuracy of that particular harbinger of spring might need to be reconsidered.)

Does it really matter what the calendar says, or what any of the technical definitions say? If it feels like spring to you, then I say embrace it no matter what day it is!

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