Life on the South Facing Slope: Notes from a Solar Psychonaut

Optical Reality

The equator-facing slope where I pitch my tent receives something like 85% more solar radiation annually than its pole-facing friend, just a football field or two across the canyon. Why? Mostly because of a fundamental optical property of the universe that you likely learned about in high school physics and may occupy a dusty corner of your intuition: the intensity of light incident upon a surface is maximized at right angles to that surface.

Compared to a flat plane at the latitude and elevation of Nugget Hill (~40.1 degrees north, ~7000 feet), sun on the south-facing slope is augmented by about 15% due to the slope’s equatorward inclination. On the far side of the canyon, sun on the north-facing slope is attenuated about 20% by the slope’s poleward inclination. Dense lodgepole pine forest intercepts another 15%. Finally, horizon shading – later sunrises and earlier sunsets created by ridgelines at the crest of the slope – reduces radiation by an additional 5%.

Physical differences create two distinct worlds. The north-facing slope is dark, wet, and mysterious, a moody forest of moss and pine. The south-facing slope is bright, dry, and crisp, a desert landscape of yucca and juniper. On the Colorado Front Range, north-facing slopes are often steeper than their south-facing relatives, perhaps due to dense forest that dams soil and prevents erosion. Water flows differently through the slopes. Snow melts rapidly on the south-facing slope and infiltrates vertically downward through thin, mineral soil, weathering underlying bedrock. On the north-facing slope, snow melts more slowly and seeps laterally downslope through thick, organic soil.

Geologic Genesis

What is unique about the Colorado Front Range? Why don’t all locations on Earth feature steep east-west trending canyons? The story begins around 70 million years ago, when the shallow subduction of the Kula and Farallon plates beneath the North American plate thrust old, hard granitic rock up against young, soft marine sediments from the Cretaceous seaway in a series of mountain building events known as the “Laramide Orogeny”. The orientation of the shallow subduction forced the Front Range mountains upward in a north-south oriented massif.

Pleistocene glaciation near the high peaks gouged wide U-shaped valleys, and melted snow and ice flows eastward below glacier elevations, incising narrow V-shaped canyons. Pre-Columbian people of the Front Range drove elk, moose, and bighorn sheep into these U-shaped valleys during the mild months of August and September and trapped them near the sheer valley walls. Ancient tools, blinds, and campsites have been found throughout the land now known as Rocky Mountain National Park and Indian Peaks Wilderness.

Solar Garden

Fast forward a few years to August 2019, when your friendly neighborhood solar psychonaut pitches his tent on a flat, sunny terrace. The lack of moisture in high Colorado air allows for dramatic daily heat cycles: afternoons are Mexico, nights are Canada. August and September are precious months in the high country – crisp clear mornings rise to spicy shimmering afternoons – a fleeting climax that brings out deep ruby in tomatoes, bronze on skin. This special time comes to an abrupt end one night when frozen air interrupts dreamy expansion and shifts the mountain system into its winter cycle. Growth continues in waves: sun fertilizes, snow irrigates. Rich emerald lodgepoles on the north-facing slope; lime green yuccas on the south-facing slope.

Frost Frame

Among other results, some recent work (Rush et al. in review) explains why soils tend to freeze persistently on north-facing slopes, and only ephemerally on south-facing slopes. Snow insulates the soil from cold air, so we would expect that soils on the south-facing slope should freeze during frequent snow-free periods. However, solar radiation is sufficiently powerful to prevent soil freezing during all but the most intense cold snaps. Can human ingenuity exploit the solar advantage and prevent soil from freezing even during these coldest nights?  

Inspired by wise elders, your friendly neighborhood solar psychonaut is determined to cultivate growth and joy. The frost frame is constructed with a large window ($8), pipe insulation ($2), and 2 x 8’s ($2) from a junkyard in town, as well as lumber lying around the house ($0). The inner walls are lined with extra plastic sheets from the rabbit run and bear-proof compost bin projects ($0) to prevent moisture from seeping into the wood. The structure is set into the hillside and gaps are insulated with ponderosa and piñon pine woodchips left over from fire mitigation activities ($0). Moisture stored in the woodchips will likely rot the lumber of the frame more quickly, but sufficient insulation is important. Compost from horse, rabbit, and goat manure ($0) fills the bottom of the frame and organic topsoil from my advisor’s former lab space ($0) composes the surface. Daikon seeds are planted as a test – mostly to grow what are trendily known as “microgreens”, but maybe a radish root or two will plunge deep into the compost base to collect moisture.

Orion's March 

Nugget Hill nights are of a special kind. Orion rises in the east just above the far wall of the canyon and hikes westward along the ridge. When I go outside to pee, I have an immediate sense of how many hours I have left to sleep. Full moons bring out woodland creatures: owls hoot-hoot into the eerie silence, deer click-clack on the flagstone outside my tent. Cold air outside invites me to burrow deep into my down sleeping bag. Steep topography creates dramatic differences in temperature and pressure that force harsh winds down elevation gradients. I hear the dragon above building and growing strength for minutes before it surges down the canyon in an explosive few seconds. Ear plugs help to keep me asleep during these noisy nights. Sub-freezing tent life is honestly warm and cozy as long as I take a number of pragmatic measures to protect myself. When I go to sleep, I put on fresh socks and warm up my sleeping bag fully dressed. During snowy nights, I take advantage of technology that allows me to avoid going outside to pee. In the morning, I shake snow off the tent from the inside to avoid a snow shower when emerging. When the snow melts, I shovel it away to prevent tent inundation.

Sun sets on Nugget Hill

November, the sun sinks low. In addition to the optical effect I discussed before, low solar angles have other consequences. When the sun is low in the sky, light beams travel a much greater distance through the atmosphere, colliding with particles that reduce the amount of radiation received at the surface. In the steep canyon world, low solar angles allow different ridgelines to cast shadows. Instead of the steady creep of sunset time experienced in flat places, a slight shift in solar angle can lead to a dramatic contraction in the length of the solar day.

Up-Down System

Front Range humans of antiquity likely followed an up-down migration pattern (Benedict, 1992), spending the mild late summer and early fall months hunting game in the high country and moving to lower elevations for the winter. In a strange reversal of this cycle made possible primarily by cheap natural gas, I move to a colder clime for the winter. As the crow flies, the Fraser Valley is just across the Continental Divide from the Front Range urban corridor. Cold air masses from the mountains encircling the valley maintain some of the lowest air temperatures in the lower 48, rivaling International Falls, Minnesota. Each year, the region receives about 30 feet of cold, dry powder that allows for excellent skiing at Winter Park resort, a short 8-minute drive from my home base in Fraser.