Thursday, March 20th marks the spring equinox – the first day of spring according to most calendars. But does spring really start on March 20th? I would argue the answer is “yes, no, and maybe.” March and early April is possibly the most confusing, yet most important time of year for Colorado climatologically.

[Double rainbow over eastern Colorado in May. Photo credit: Allie Mazurek]

Three Ways to Define Spring

The start of spring may be defined in several ways:

Astronomical Spring uses the sun’s position. The astronomical calendar divides the year using solstices and equinoxes. Our summer solstice occurs when Earth’s northern hemisphere’s tilt towards the sun is maximized. Our winter solstice occurs when Earth’s northern hemisphere’s tilt away from the sun is maximized. The spring equinox (late March) and fall equinox (late September) occur when Earth’s tilt is tangential to the sun, resulting in 12 hours of daylight everywhere.

[Schematic of Earth’s orbit and astronomical seasons. Source: wikipedia, public domain]

Climatological Spring is simply the three-month period of March, April, and May. This definition doesn’t follow Earth’s orbit but aligns better with our temperature experience. Astronomical seasons can be counterintuitive: December 19th is fall, but March 19th is winter? Yet March 19th is typically warmer than December 19th in mid-latitude locations.

Phenological Spring focuses on natural changes – new grass, blooming flowers, and leafing trees. The National Phenology Network tracks leaf-out dates using satellite data. The 30-year average for the Front Range (Fort Collins to Colorado Springs) is between April 1-15. In eastern Colorado and southwestern valleys, it’s March 15-31. In the high country, it might be as late as May or early June. The phenological processes of spring vary yearly based on weather conditions.

[30-year average leaf-out date. Source: National Phenology Network]

Beyond These Definitions

These three definitions do not cover everything. Farmers might define spring by planting time – commonly late April or early May in Colorado. Others may focus on when snow stops falling. On average, the Front Range sees its last snow around the third week of April – earlier for the eastern plains and southwestern valleys, but much later for the mountains. Some areas, like the aptly named “Never Summer Range,” may see snow year-round. As recently as 2019, even the lowest elevations of eastern Colorado had snow as late as May’s fourth week, disrupting graduations, weddings, and “summer break” plans.

[May 21st, 2019 west of Fort Collins, CO. Photo credit: Allie Mazurek]

Alternative Approaches

I might define spring on the northern Front Range as starting when the average minimum temperature rises above freezing and ending when the average daily maximum temperature hits 80°F. PRISM data shows this works reasonably at lower elevations but less so for higher ones.

[First day of calendar year with average minimum daily temperatures above freezing. Created by Colorado Climate Center. Gridded data source: PRISM. Station data source: SCACIS]

Climatologist Brian Brettschneider suggests defining winter as days in the bottom 25% of the temperature distribution, summer as the top 25%, and spring/fall as everything between. By this measure, Denver’s spring begins in March’s third week and ends in June’s second week. Check out his blog for more details.

The Fifth Season?

If we define spring phenologically using leaf-out dates (late March/April or later in mountains), what do we call mid-March? With sunset at 7:00 PM and warming temperatures, it doesn’t feel like winter, yet spring’s phenological processes have not begun. Perhaps it’s a fifth season – “wind season” (Colorado’s windiest time) or “water season” (crucial for our state’s water supply).

Colorado’s Most Important Season

This transitional period is extremely important for Colorado. Most of our usable water comes from mountain snowfall that melts in spring and flows into reservoirs. April is the wettest month for much of our high country, with March close behind.

[Month of year with highest average precipitation. Created by Colorado Climate Center. Data source: PRISM]

When March and April are dry, mountain snowpack suffers and snow melts early. When they’re wet, snowpack is likelier to peak at or above average, and snowmelt comes later. This shortens our high-elevation fire season and leaves more water in our reservoirs through summer and fall.

So while we debate whether it’s technically spring yet, remember that March and early April – whatever we call this season – plays a crucial role in Colorado’s water security and ecological health for the entire year.