Tuesday, March 20 is the first day of spring with the vernal (spring) equinox occurring at 10:15 a.m. M.D.T. At that precise moment, the Earth’s equator intersects the plane of its orbit around the sun – meaning the sun is positioned directly over the equator. After that, it begins slowly moving north toward the Tropic of Cancer, arriving at its highest position in the sky on the summer solstice, which will be on June 21.
On the first day of spring, all over the world (except at the poles), day and night are each twelve hours in length. At the North Pole, the sun rises above the horizon and does not set for six months, while the South Pole plunges into six months of darkness.
The normal high in Calgary on the first day of spring is 4 C and the overnight low is –7 C. Calgary’s warmest March 20 was in 1928, when it was 20 C; the coldest temperature ever seen on this day was –32 C in 1882.
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The sun is already getting higher in the sky with each passing day, resulting in longer daylight hours and increased solar radiation. This extra solar energy will continue to raise temperatures until we reach the warmest time of the year: mid-July to mid-August.
When we talk about the first day of spring, what we are referring to is the “astronomical” first day of the season. But there is the “meteorological” definition, as well. That is the day when the mean temperature (the 24-hour high, minus the 24-hour low) is 0 C. In Calgary, that day is usually March 30.
It takes the Earth 365 days, five hours, 48 minutes and 46 seconds to complete one orbit of the sun – just in case you care.
Because the Earth’s orbit is not a perfect circle, but an ellipse, the Earth travels more slowly when it is further away from the sun (during the summer months) than when it is closer to the sun (during the winter months). So it takes an extra seven days to go from the spring to the fall equinox than it does to go from the fall to the spring equinox. In other words, spring and summer last a week longer than fall and winter.
Calgary’s frost-free period usually doesn’t begin until May 23 and on average, runs 115 days, with the first frost occurring on Sept. 15. So to be safe, we recommend waiting until the May long weekend before planting.
The fact that it is finally spring is, in itself, worth celebrating. Let’s hope spring is kinder to us than this winter was.