When Does Spring Begin?

The “official” first day of spring is widely known as the vernal equinox. This year, the vernal equinox will commence on Tuesday, March 19th, at 11:06 p.m. EDT (8:06 p.m. PDT). 
 
Why does this occasion mark the beginning of spring? Astronomically speaking, the Sun crosses the equator going from south to north. The Earth maintains its 23.5-degree tilt, but it is neither pointed away or toward the Sun.

As such, the Earth is oriented that the Sun is directly facing the Equator allowing for the Northern Hemisphere to enjoy springtime weather while the Southern Hemisphere enters fall. The day the Sun is directly overhead at noon over the Tropic of Cancer and Capricorn (23.5 degrees N and S latitude) are the summer and winter solstices, respectively.
 
Many refer to Astronomical Spring on March 19, as the first "official" day of spring. Most meteorologists will argue that spring begins a few weeks before, on March 1st. Meteorological spring began March 1, summer begins June 1, fall begins on September 1 and winter begins on December 1.

There are a couple of very important reasons why this is the case. The most important is for climate record-keeping. Climatologists require set time periods to calculate averages and do seasonal comparisons over the years. Since astronomical dates change to different days depending on the year, it would be difficult and confusing for seasonal climate records to be based on those dates.
 
A second reason is that weather-wise, it makes more sense around the globe. In spring, mild surges of air from the south are becoming a regular occurrence and severe weather threats begin to kick in by March 1. The heat of summer has been experienced in most areas across the country by June 1 and the heat of summer is waning by September 1.

Story image: WeatherBug user Pat S. captures a plant blooming in Columbia, Md., on February 24, 2017.