Why is it snowing when it’s way above freezing?

During the winter months, when valley-drenching rain storms march across the flats, the people who love to play in mountain snow will invariably wonder: at what elevation will the rain change to snow? By Kerry McClay, National SnowSchool Director

Photo by Sofia Jaramillo (@sofia_jaramillo5) of athlete Emilé Zynobia (@curlsinthewild)

By Kerry McClay, WWA National SnowSchool Director

A prominent snow line stately draped across a large and imposing mountain range is one of the more dramatic sights observable to those of us who spend time gazing up at peaks from the valley floor. And during the winter months, when valley-drenching rain storms march across the flats, the people who love to play in mountain snow will invariably wonder: at what elevation will the rain change to snow? 

The question does remind me a bit of that old prank mountain-folk would sometimes play on city-slickers telling them something like, “Yep the deer ‘round here turn to elk at about 8,000 feet.” If the prank went on long enough they might even work in a reference to the existence of a high altitude “Doe-Zone Layer.”

But this rain / snow line question is no joke. In fact, it’s such serious business that it’s part of a citizen science project called Mountain Rain or Snow, which was recently funded by NASA and aims to help better understand and predict this complex phenomena. 

During our 3rd Annual National SnowSchool Conference this past November, we featured a presentation focused on this very question provided by Meghan Collins at the Desert Research Institute (DRI), who also leads the engagement portion of the Mountain Rain or Snow project. Each year, we host the National SnowSchool Conference to help equip our educators across the country with the best resources to engage thousands of kids in explorations of their local winter wildlands. The purpose of Collins’ presentation was two-fold: 1) Help SnowSchool educators learn more about winter-specific science; and 2) Equip SnowSchool sites with real citizen science projects to help engage kids and K-12 students.

At first glance, most people probably wonder if this is more a question of temperature than it is elevation. But if you think back, you can likely remember a time when it was snowing around you in the mountains but you knew that the temperature was well above freezing. This experience is at the core of the challenge facing scientists. 

“The phenomena of snow above freezing is not a change in the laws of physics, rather it has to do with evaporative cooling” Collins explained to the SnowSchool audience. In short, it’s about humidity.

Collins says, “We know that water vapor plays a role in transferring heat. You’ve probably experienced this when you sweat- your body cools faster.”

It’s the same principle when a snow crystal is surrounded by more or less water vapor in the air. The lower the humidity in the air the slower heat will transfer to (and melt) falling atmospheric snow crystals. So in more arid mountain regions (or even just amid a less-humid moment in a particular storm) observers might see snow falling to the ground when temps are as high as 39.5F!

Want to explore this humidity-on-snow concept on your own? Try our experiential SnowSchool activity idea for your home or classroom!

Correctly forecasting the rain-transitioning-to-snow elevation line has big implications for hydrology and water resources, transportation safety (road conditions), and avalanche safety. With critical factors like temperature and humidity in the mountains varying by elevation and geographic location, the Mountain Rain or Snow citizen science project is an attempt to help forecasters improve their ability to predict the rain/snow elevation line.

Currently, the temperature thresholds used to predict this are often derived from weather stations sitting in far away locations at much lower elevations. And even those weather stations that sit high in the mountains are still far less reliable than the human eye at telling the difference between a falling raindrop and a snow crystal. 

This discussion of mountain snow and rain was particularly timely this year with a series of high-profile (and dangerous) snowstorms and powerful atmospheric rivers striking various parts of the country throughout the month of December and into January. As human-caused climate change warms the cryosphere, we are experiencing an increase in rain-on-snow type flooding events. This has led to unexpected and destructive impacts to the exact mountain locations where making rain versus snow forecast predictions are most difficult. 

To improve forecast accuracy in these situations, scientists are leaning on citizen- and student-collected observations to inform their models and augment data collected from NASA satellites and remote weather stations.

The beauty of the Mountain Rain or Snow project is that anyone can participate. Contributing data is free and requires no special skills or elaborate excursions. Quite simply, as rain or snow begins to fall in your area, you can look out the window of your house, lodge or school bus to see if it’s raining or snowing and use a smartphone to send your observations to the DRI scientists. The team utilizes GPS to track the location and elevation of incoming observations (over 15,000 received last year) and uses this citizen science data to inform and correct their forecast models.

For over a decade, Winter Wildlands has been teaching K-12 students to be citizen scientists and work with projects like DRI’s Mountain Rain or Snow project (and NASA’s SnowEx program – check it out here!). At 70 sites across the snow belt, our SnowSchool program has directly impacted and empowered the future generation of winter adventurers, snow scientists, and wild winter advocates.

Please consider making a donation directly to our work at SnowSchool headquarters so we can continue sharing our curriculum with kids across the country and opt into programs like Mountain Rain or Snow and NASA’s SnowEx.