CLEAR AIR RAINBOW

CLEAR AIR RAINBOW: Two days ago, photographer Martin McKenna was driving down a country road near Maghera, Northern Ireland, when he saw a curious thing. "There was a faint rainbow arcing through the crystal-clear blue sky," he says. Rainbows usually require rain, so a clear air rainbow is a curious thing indeed. Where did it come from? The answer may be found below McKenna's picture of the phenomenon
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"There was some rain," explains McKenna. "Strong winds had blown some precipitation over from the northwest where showers where gathering in the distance, and this is what caused the 'bow. It looked most unusual."

Atmospheric optics expert Les Cowley says that there is another way to make clear air rainbows. The method requires conditions of high humidity: "A rainbow can be formed by droplets condensing in a layer of saturated and otherwise apparently clear air. Droplets formed in this way tend to be small, and the rainbows they make are relatively broad."

"Clear air rainbows are always a surprise," he adds, "and where the raindrops come from can be puzzling." Sky watchers should look for them any time the sky is blue.


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