MTPR

Field Notes: Winter Clouds

Feb 16, 2016

At no other time is the parting of clouds felt more powerfully than outdoors, at the height of winter.

On this particular day, the clouds break intermittently and when they do, motion ensues. The peeps and chatters of birds start, and you can see them dart and cling through a white and shifting world. The snow itself starts to awaken and come alive - melty, unstable layers slide down the steeples of trees.

We are on the banks of the Lochsa river and my feet have been numb since I left my sleeping bag. Today’s clouds are by field guide standards fair weather cumulus. Due to our late start of the morning, the ground has had time to warm slightly. When the cold surface air meets the warmer ground, warmed pockets of air break away and rise. As the air rises, it cools and condenses into these fairly stable clouds which resemble lumpy cotton.

Today the clouds are scant enough to give us the opportunity to watch birds without getting frostbite on our binocular-clutching fingers, and even to find a dry spot under a cedar to write. Yet they are numerous enough to remind us of our very different experience the day before.

The crest of Lolo Pass was thick with snow the day we arrived. There was so much fresh powder in fact that we had to dig ourselves a place to park. The storm wasn’t over and we eagerly strapped on our snowshoes to walk out into it. The clouds moved in on us and suddenly there was scant difference between the white on the ground and the white of the sky.

Precipitation-generating clouds are termed nimbus. The most dynamic, or storm producing, are cumulonimbus. These are the ones that look like giant cauliflower from a distance and can consume whole vistas and mountains. Inside young cumulonimbus, glaciation can occur, when water particles are converted to ice.

When the cumulonimbus upon us that day on the Montana/Idaho border began to let down its confetti, it had reached full maturity. But today on the banks of the river, a large puff of fair weather cumulus envelops the ridge-tops and another patch threatens to cover the sun. Birds and small mammals that live above ground and overwinter in these harsh conditions depend on breaks in storm clouds to forage for food. But a late morning sky of fair weather cumulus is no guarantee of good hunting conditions. With changeability being the primary nature of clouds, it’s not uncommon for them to build through their stages quickly, morph into cumulonimbus, and leave creatures to hunker through another storm while snow lovers get another day of good skiing.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Fieldnotes," 1/10/16 & 1/15/16. Listen on air or online Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or via podcast.)