Water, dirt, smoke color our world

Have you ever looked at the sky and wondered where the different colors come from? Gazing skyward you might see blue skies, fiery red sunsets, bright double rainbows, or even a brilliant circumhorizon arc (and what is a circumhorizon arc anyway?) Clean air makes the first possible. Water, dirt and other particulate matter like smoke from wildfires, are responsible for many of the other interesting colors we see in the sky.

Let’s start with the timeless question, “Why is the sky blue?” We can thank the gases in our atmosphere for that – oxygen and nitrogen. White light from the sun is actually made up of a “rainbow” of colors. Remember the prism experiment? It turns out that our atmospheric gases are better at scattering the shorter wavelengths of visible light which are the violets, blues and greens. Our eyes are more sensitive to the blue light, which makes the sky appear blue to us. If you were standing on the moon, whose atmosphere is a million times less dense than the Earth, there would not be enough particles to scatter the sun’s light. The sun would appear white against a black sky.

The rainbow is another optical effect and one which we haven’t seen much of lately due to the lack of rain. Raindrops in a passing shower, combined with sunlight, produce a rainbow when the light passing through the drops is bent, and the spectrum of colors from red to violet is reflected back to our eyes. This happens when the sun is behind you and the rain is in front of you.

Speaking of rainbows, what about that spectacular rainbow display of color that was seen across the area this past June, in the absence of any rain? There may not have been any rain, but cirrus clouds full of ice crystals acted as the “prism” to produce what is called a circumhorizon arc. The circumhorizon arc is a rare optical phenomenon that can only occur when the sun angle is above 58 degrees (which occurs near midday during the summer months in Coeur d’Alene) and when the ice crystals in the clouds are aligned just the right way.

Finally, across the Inland Northwest, what is usually responsible for the deep red sunsets we sometimes see? Well, what is bad for our lungs makes for dramatic color in the evening sky. Smoke particles in the air from nearby wildfires have the effect of scattering away the shorter wavelengths of light – the yellows, blues and violets, and allowing only the red light to reach our eyes. Dust and ash from volcanic activity can also do the trick, as well as salt particles from the ocean air if you happen to be near the coast.

A look at October

October is a transitional month. We usually leave the warm dry days of summer behind, and welcome the cooler, wetter weather of fall. Average precipitation for October is 1.93 inches. High temperatures average in the mid-60s for the beginning of the month, and fall into the lower 50s by the end of the month. The average date for the first killing freeze is Oct. 9, though snowfall is pretty meager during this month averaging only .2 inches. The Climate Prediction Center’s 30-day outlook for October is pretty wishy-washy for the Inland Northwest, showing equal chances for above- or below-normal temperatures and precipitation. Strengthening El Niño conditions also complicate the long-range forecast, but current computer models do seem to show a trend for drier than normal conditions over the next couple of weeks.