Questions of Light
Up in the Northwest folks are coming down with Seasonal Depression. I can tell you this because I grew up there. Months of dripping rain grows glorious Douglas fir trees hundreds of feet tall and carpets the forest floor with a lush understory, but it can be a downer for the human population. Even gloomier than rain are the damp and low hanging clouds. A terrific remedy for seasonal depression is time spent in a sunny clime, so we have enjoyed visitors from the Northwest in the past weeks. It is a gladdening sight to see our dear friends and family straighten their shoulders and turn their faces to the warm sunlight. Everyone exclaims about the blue, blue sky. I looked this up and the sky really is bluer here. To put it very simply, the air everywhere is full of invisible floating particles and droplets. In more humid areas water vapor clings to these aerosols and creates layers of haze. The air above the Sonoran desert also has tiny particulates made up of pollen, pollution and dust, but drier conditions make the air appear very clear and the sky bright blue. A wonderful aspect of winters in the desert that visitors may not appreciate is the soft quality of the light. Winter days can be warm but not hot, and we don’t yet have that glaring light that causes squinting and life lines (wrinkles). Arizonans know that the Vernal Equinox is just a month away and that the sun is stepping steadily north through the latitudes. In a couple of months it will be hot. Heat in the air affects light and gives rise to classic desert attributes such as mirages. Those shimmering puddles that beckon in the distance are caused by layers of superheated air lying near the pavement or the surface of the desert floor. When sunlight strikes these patches of heat it is refracted, or bent, creating a mirror image. Our brains interpret this reflection as water. The heat generated on the ground expands and rises in bubbles of hot air. Light passing through this air is refracted in random directions and creates a shimmering effect – the characteristic high noon visual of a sweltering day. These pockets of roasting air rise and expand as the day advances and become thermals that hawks and turkey vultures turn to their advantage. They circle effortlessly aloft, broad wings trapping the hot air that carries them higher and higher. When I see mirages and the air shimmering before my squinted eyes, it’ll be time to pay a visit to the folks up north.
If this information leaves you with more questions than answers check out David W. Lazaroff’s article Desert Air and Light in A Natural History of the Sonoran Desert.
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