A southerly breeze rustles the boughs of the spruce trees. It is chilly and sustained. Overhead, a great blue circle of sky permits sunlight to pass unfiltered. A few hours later, a white film obscures the blue. Its translucent haze advances steadily across the sky, consistent in texture but for a few undulations, high, extremely high above the peaks. This was the sky yesterday.
Today, the southerly breeze matured to a southerly wind. It whipped steady rain over my right shoulder as I traversed the eastbound ridge. Wet plus wind makes cold. This is the coldest warm front I’ve ever seen, I thought throughout the day.
At the top of the climb up to Roan High Knob – the highest shelter on the AT at 6270 feet – Hayden and I encountered a new obstacle: polished ice sheets across the trail. Liquid water running over the ice would freeze Zamboni-smooth, growing ice layer by layer like a pearl. After first marching confidently across, discovering there was precisely zero friction, and tumbling over, I learned to treat these numerous barriers to camp and dinner with caution. Constantly running through my head was the ice rink campaign scene from Parks and Recreation. All the way to the shelter (warmth, dryness, food), I was hearing “Get on your feet…”
New trail crew
I walked today with Hayden, and we needed the companionship for the climb up Roan Mountain and for the ice crossings. Off and on since Hot Springs but especially the past few days, I’ve fallen in with a great crew: Hayden, Gladiator, One Gear, and Typical. They are easygoing jokesters, fun to be around. There are other peripheral members, but this is the crew I’ve squeezed into shelters with the last few nights.
Meteorological commentary
The forecast indicates this day-long rain is a warm front, heralding the arrival of southern warmth that will melt the ice and generally make life easier after two weeks of cold. It got me thinking about warm fronts and cold fronts.
Last Friday, winds whipped in from the west, sharp and biting. Clouds swooped down to snag on the peaks. Rain fell briefly but hard. A few peals of thunder punctuated the heavy precipitation. Above 3000′, as the rain tapered, it shifted to ice and snow. In the wake of this, clouds hovered, freezing to every surface and forming the thick hoarfrost. Cold air moved in rapidly, resulting in a weekend of frigid lows and near-freezing highs but clear skies. A cold front it was. They say today’s pattern is a warm front.
Warm air is less dense than cold air. When a warm air mass is pushing acold air mass out of the way (a warm front), the cold air resists and clings, wedge-like to the ground. After all, it is denser. Over some point on the ground, then, the warm air mass advances overhead first at altitude. It is forced atop the cold air, so its water vapor condenses, forming the high, thin haze. As the bulk of the warm air mass advances, the cold air near the ground over the point thins, so the clouds move closer to the surface, descending until they comprise a thick band of stratocumulus. The cold air remains along the ground, but the warm air is eventually low enough to form a raining nimbostratus. The rain falls soft, steady, and long. The wind blows cold. Behind this is a promise of warmth.
When cold air, instead, advances, it more easily uplifts the lighter warm air it displaces. Thus, a cold front is more sheer, vertical, and abrupt than a warm front. Warm air is forced upward rapidly, so it rapidly cools. Hence the strong winds and heavy but brief rain and thunderstorm activity. On the backside, a blast of cold air shifts the precipitation to frozen forms.
There are many more complex elements to meteorology – high and low pressure systems, occluded fronts, Rossby waves, instability – but this elementary picture of warm and cold fronts, based only on how the density of air varies with temperature, explains pretty much all of my experience over the past week.
It is spring – officially past the vernal equinox now – a season in which a series of warm and cold fronts of parade across the country. What strikes me is the extremity of air masses forming these fronts. Arctic air lingers from winter, but subtropical heat is well advanced in its formation. At my location, both the highs and lows will differ by about 40° separated by 3 days. You feel these things more acutely when you are outside all day.
