Meteorological vs. astronomical spring
By Jared Shelton News-Press NOW meteorologist
For much of the Midwest and Great Plains, it may have felt like this year’s spring season began in February.
Mild temperatures almost became the new normal last month here in Northwest Missouri, as daily highs averaged out to approximately 59 degrees. Not surprisingly, snowfall was also in short supply, falling on only two occasions and totaling between one and two inches for the entire month here in the St. Joseph area.
Regardless of temperature and precipitation trends, March brings the “official” start of spring in the northern hemisphere. Yet the day on which it arrives depends on who you ask. For most, spring begins with the vernal equinox, which takes place between March 19 and 21, depending on the year. For meteorologists and weather enthusiasts alike, the first day of spring falls on March 1. To understand this conundrum, we must first get a grip on the concept of astronomical seasons as a whole, defined by the position of Earth relative to the sun throughout the year.
Earth’s axis is tilted by about 23.5 degrees, orienting the northern hemisphere toward the sun for half of the year and away for the other. The summer solstice takes place when the northern hemisphere is pointed most directly toward the sun, receiving the most solar energy and the longest duration of daylight. Opposite is the winter solstice, when the northern hemisphere is pointed most away from the sun, receiving the least amount of solar energy and shortest duration of daylight.
Halfway between the solstices are equinoxes, when Earth receives roughly the same amount of daylight from pole to pole, marking the start of astronomical spring and fall.
The date of equinoxes and solstices change within a range of two to three days from year to year. Hence the recent equinox was said to be the earliest start to spring since 1896, taking place on March 19 at 10:06 p.m. The elliptical shape of Earth’s orbit also adds variability between the length of astronomical seasons, ranging from 89 to 94 days.
Although small, the inconsistencies in astronomical seasons are problematic when it comes to calculating weather-related averages and figures, a primary reason for the advent of meteorological seasons, established in the early 1900s.
Defined more simply, meteorological seasons are divided into three-month increments: spring starting March 1, summer on June 1, fall on Sept. 1, and winter beginning Dec. 1. In addition to cleaner climate stats, meteorological seasons are also in better alignment with annual changes in temperature.
Some perceive the concept of meteorological seasons to be a needless layer of complexity. Even so, there’s no denying that cyclic shifts in weather are hallmarks of seasons themselves. Especially here in the mid-latitudes, where they are undoubtedly distinct.
