The Flagstaff area’s high elevation and dry continental air intensify the effect of the sun and wind on large open areas and south- and west-facing slopes. Soils on the sunny (south) side of slopes will be drier and have the lowest organic content than soils in protected areas. Soils on the windy (usually southwest) side of slopes tend to be shallower than on the sheltered side of slopes, where the wind slows and drops airborne particles.
SUN: Flagstaff enjoys mostly bright sunny days throughout the year. Plants require sunlight, but—especially during the dry and windy months of May and June—intense sun on a south-facing slope can shrivel vulnerable plants in a day. In winter, the dry air and low angle of the sun can sublimate (vaporize) snow, exposing and stimulating dormant plants and making them susceptible to deep overnight freezes and frost-heaving. Successful gardening requires taking into account how our dry air and high elevation above sea level amplify the sun’s impact on plants.
WIND: From April through June, prevailing winds come from the southwest. These southwesterly winds combine with strong solar radiation to create extremely challenging conditions for plants on south- and west-facing slopes.
On the other hand, north- and east-facing slopes are shady and remain fairly cool and moist as do draws, ravines, and canyons. Cold air sinks at night, pooling in basins and around the north-facing base of barriers such as walls and hedges.
To measure the steepness of your slope, see the worksheet Measure Your Slope.
Just for fun… use the illustration below to imagine the intensity of solar radiation in your garden, which depends upon the steepness and exposure of your slope. Imagine each circle as a hill that is level at the very bottom and becomes increasingly steep until it reaches 80° (almost vertical) at the top. Flagstaff is at approximately 35° north, so the second row of circles in the left-hand group most closely resembles our area’s seasonal shifts in solar radiation. Click on the diagram to see it enlarged on a separate page.
— Illustration courtesy of Timothy Liebermann, Senior Geographer, South Florida Water Management District.