From the ABC 7 Weather team

Happy vernal equinox (or first day of spring)!

March 20, 2011 - 08:01 AM
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We've made it through winter and now spring is knocking on our doorstep later today. Did you know, however, there are two different starts to the spring season?

After all the snow, wind and cold, spring is in the air... or at least it will be in the coming weeks! March 20 or 21 is the traditional start of the spring season, but in reality we're already 20 days into spring. Are you confused? Let me explain!

Meteorological Spring is defined as the transition between the three coldest months of the year and the three warmest months of the year. So, December, January and February (Meteorological Winter) are the coldest three months of the year in the northern hemisphere and June, July and August, Meteorological Summer, are the three warmest months. In between, you have Meteorological Spring: March, April and May. So, in that regards, yes, spring has been in the air for the past 20 days!!

Your calendar says "First day of spring" because today marks the traditional vernal equinox. It's the day in late March when the sun is directly overhead at the equator, having moved north steadily over the last three months from the Tropic of Capricorn at 23.5 S (south latitude) on Dec. 21, the first day of winter, known as the Winter Solstice. Yes, this justifies that the seasons revolve around how solar radiation is distributed in both hemispheres, which stems from the sun angle and length of day.


Courtesy of UC Santa Barbara Department of Geography

The prefix "equi" means equal. Applying this to the word equinox, such as the vernal equinox, which arrives today at 7:21 p.m., and the autumnal equinox that occurs on September 20 or 21, there will be an equal amount of daylight and darkness on the globe; 12 hours of each. During the equinox, the sun at high Noon shines directly over the equator (0 degrees latitude). The sun will continue its journey north and by June 21, the Summer Solstice, it will reach its northern most point at 23.5 N, the Tropic of Cancer. While the northern hemisphere will be into the summer season at this point with a higher sun angle and longer days, the southern hemisphere will arrive into winter... shorter days and less sunlight.

Here's a fun activity for you and your friends. Want to know just how far from directly overhead (known as zenith) and the horizon the sun is at high noon today, or the noon solar zenith angle? It's really simple to figure out. Find out your latitude on the globe... (here in the District, we are approximately 39 degrees North). OK, now use this formula to determine the sun angle with respect to your location. Solar noon zenith angle is your latitude (39 N in the District)  -  March 21 noon zenith angle (0 degrees). Simply put, the sun at Noon today will be about 39 degrees from directly overhead. So, how far from the horizon does this make the sun at high Noon? Remember from Geometry class there are 90 degrees in a right angle, so subtract the number you just got (39 degrees) from 90 to get 51 degrees from the horizon.

The days will continue to get longer and by June 21st, we'll have 14 hours, 53 minutes and 48 seconds of daylight.

Don't forget with the spring season comes a variety of activities, including the highly popular Cherry Blossom Festival in the District from March 26 to April 10.

Have a great spring season and be safe!

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