The Arrival of Spring

The astronomical spring season officially began in the Rockies with the Vernal Equinox that occurred at 3:37 a.m. MDT on Saturday March 20th. It is that time of year when warm, sunny days can lull you into a bout of spring fever only to be squashed by a return to a stretch of winter-like conditions. Spring has a way of flopping back and forth between seasons in the mountains.

While this post can’t change the weather, it will explain what the Equinox signifies and how this new season changes the look of things in the Rockies and across the globe.

Vernal Equinox

“Vernal” is sometimes used to describe youth and vigor, something fresh. It also means, belonging to spring. When flowers begin to bloom, and everything starts fresh after the colder, snowier and darker winter.

“Equinox” is the Latin term for equal night (aequous nox). Around the date of the Equinoxes, both Vernal and Autumnal, nighttime and daytime are nearly equal everywhere around the globe.  From Ecuador to Alaska, from Nigeria to New Zealand, the sun is up for 12 hours and down for 12 hours. That is because on the Equinox the sun is directly overhead at the Equator, shining down on the disk of planet Earth evenly, from north to south.

Why is that? Because of the tilt of the Earth and its annual orbit around the sun.  At the Equinox, neither hemisphere is tilted away nor towards the sun.

Angle of the Sun

The big difference between how the day looks in Alaska versus the Equator is how high in the sky the sun gets at noontime. That is dependent upon the latitude and the curvature of the Earth.

Here in Jackson, the sun is about 46-degrees above the horizon at noon on the Equinox. Compared to only 23-degrees above the horizon on the Winter Solstice in December and around 70-degrees above the horizon on the Summer Solstice in June.

Up in Fairbanks, Alaska the sun angle on the Equinox is 25-degrees above the horizon at noon. On the Winter Solstice it barely cracks the horizon at 2-degrees. On the Summer Solstice in Fairbanks the sun only gets as high in the sky as it is here in March on the Equinox, around 47-degrees above the horizon. However, their length of day in Alaska increases exponentially between now and the Summer Solstice.

Length of Day

By mid-June, the length of our day in Jackson will increase almost three and a half hours from what we have right now. On the Summer Solstice, the time between sunrise and sunset will be 15 hours and 25 minutes. Add an extra 36 minutes on either end of the day for twilight, and barely leaves enough hours of darkness to get a full 8-hours of sleep here.

Whereas, up in Fairbanks, the length of day on the Summer Solstice is 21 hours and 50 minutes and twilight lasts for the remaining 2 hours and 10 minutes. It’s hard to get any shuteye up there in the summer.

In Ecuador, the days don’t change as dramatically, as a matter of fact, the length of day on the Equinox is just 11 seconds longer than it is on the Summer Solstice down there.

Changing Time

Many people ask why we change from Standard Time for the fall and winter months to Daylight Savings Time (DST) for the spring and summer months? The answer to that question involves some politics, some economics and some science. I’ll allude to the scientific reason here.

Our seasonal time change is mainly related to our biological clocks or more precisely our circadian rhythms. Where that time change is most effective though is between about 35-degrees and 55-degrees North latitude, roughly between northern Arizona and Southern Canada.

In Jackson Hole, Wyoming, where I sit at 43-degrees North latitude, if we remained on Standard Time all summer, we would see the sun rising at around 4:42 a.m. in mid-June, with twilight beginning just after 4:00 a.m. or an hour earlier than we currently have under Daylight Savings Time. That’s probably way too early for most folks to get out of bed.

Sunset here would also be an hour earlier if we stayed on Standard Time, on the longest days in mid-June sunset would be at 8:08 p.m. or similar to the sunset time in Jackson in mid-April, under Daylight Savings Time.

As it is now, sunset will be at 9:08 p.m. in mid-June, extending our playtime well into the evening in the summer. No matter which time we are under, Mountain Daylight Time (MDT) or Mountain Standard Time (MST), the amount of time we get between sunrise and sunset would remain the same. Most of us though would probably sleep through the first couple of hours of the day in summer, if the clocks didn’t change.

Jim is the chief meteorologist at and has forecast the weather in Jackson Hole for the last 30 years.

This article originally appeared in the Jackson Hole News and Guide.