When does spring begin?

The 2018 vernal equinox occurs Tuesday March 20th at 11:15 a.m.

Civilizations throughout the long span of human history in our hemisphere have celebrated this day as it marks the beginning of a continuous six-month period during which the daytime is longer than the nighttime.

The consequences of this dramatic change are many. Of course, the weather begins to noticeably warm around this time of year, reaching an annual maximum approximately four months from now before slipping, at first slowly, back toward the coolness of fall.

Since the weather-producing jet stream is located on the warm edge of the cold air that caps the high latitudes of our hemisphere, this warming drives the jet stream poleward, resulting in a reduction in the frequency of large-scale cyclones across North America as we head into spring and summer.

As the sun continues to climb higher into the sky after the equinox, melting begins in the ice-choked Arctic and increases its pace throughout the summer before finally experiencing the first freezes of the next cold season in mid-September. So, though the occasional cold morning or dreary, raw day may yet remind us of the wisdom of the old saying, “The first day of spring is one thing, but the first spring day is quite another,” take heart in the fact that, as of March 20th, we have turned the page to an entirely different, more benevolent meteorological reality.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.
Category: Seasons

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