One drawback of living at 7,100 feet is that spring drags its feet. I see the blue sky outside and assume warm sunshine to go with it. Yet, I step one foot out the door and my teeth start to chatter—as much because of the icy winds as the frigid temperatures. We may have two or more months of snow yet to endure, but I’m ready for spring. There’s only one solution.
I can’t afford a plane ticket to Cancún, or even a road trip to San Diego, but I can drive to a (relatively) lower elevation. While the eastern states’ climate zones are determined by latitude, ours are determined by altitude. It’s amazing how much impact a couple thousand feet can have on the arrival of spring.
How does this work?
It’s all about the “adiabatic lapse rate.” Lapse rate is defined as “the rate at which Earth’s atmospheric temperature decreases with an increase in altitude.” (Note that this only applies to the lowest layer of the atmosphere, the troposphere. Higher up, things are different.)
In general, air temperature falls about 5.4°F for every thousand feet you ascend. This is why snow lingers on mountaintops long after it has melted in the valleys.
The temperature would fall faster, except air happens to be a poor conductor of heat. Air masses tend to stay the same temperature as they move around. If you’re inclined to physics, know that an adiabatic process is “one that occurs without transfer of heat or matter between a thermodynamic system and its surroundings.”
In other words, if a cold air mass arrives in my front yard from Canada, it’s still going to be cold when it arrives. And if a warm air mass blows in from Oklahoma, it will retain its heat. This effect will even overcome that of the sun (or its absence at night). I’ve watched the temperature on our patio thermometer actually rise as the night wears on, then fall once the sun comes up. What a weird place we live!
Various additional factors influence this temperature differential. For one, hot air rises. While air is generally warmer lower down, it’s tendency to expand causes it to carry that heat upward. At the same time, the colder air higher up sinks. Without this convection, the average ground temperature would be significantly hotter—approximately 140°F. This convection also has a lot to do with the formation of hail and thunderstorms.
Another influencing factor is humidity. Air that carries a lot of water is more stable, temperature-wise, than dry air. (That’s because it contains what is called the heat of vaporization, another physics-y concept.) As a result, a humid air mass will only drop approximately 3° per thousand feet.
What does that means to us gardeners? For one thing, I should avoid plants that dislike sudden temperature changes. Where my daughter lives in rainy western Washington, winter’s daily highs and lows vary by a mere 4°, while ours here in arid Colorado can vary by as much as 50°!
What all this boils down to is that, on a relatively dry day (such as is typical here), if it’s 90° in Pueblo (at 4,692′), then it should be about 77° at home (which, remember, is at 7,100), about 13° cooler, and one reason we live here instead of there. And if we’re still shivering in March here in Colorado Springs, the weather in Denver (at an elevation of 5,280 feet—the famously mile-high city) could be downright spring-like, or at least about 10° warmer.
Thankfully, it only takes about an hour to get to Denver Botanic Gardens (at 5,379 feet), and I can get my fix of crocuses, snowdrops, and other early-bloomers.
Photos, from top: Crocus, snow on the mountaintops in Alaska, convection cells, Siberian Squill (Scilla Siberica), Glory-of-the-Snow (Chionodoxa), Puschkinia, Crabapple buds (Malus), Windflowers (Anemone).