Today I received pictures from several of you regarding some wondrous halos in the sky. Others noted an impressive collection of contrails...and there is no accident that both apparitions appeared at the same time: the ultimate cause is the same.
Here is a very nice picture from Brendan Fields showing both appearing at the same time!
A high resolution MODIS image from today shows an extensive collection of high cirrus and cirrostratus clouds and if you look carefully you will see lines: those are contrails!
Halos occur when rising air in the upper tropospherc causes condensation and the formation of ice crystal clouds (cirrus and cirrostratus). When the sun's ray's intersect the randomly oriented ice crystals, the light is bent preferentially by 22 degrees...thus producing a 22 degree halo.
Contrails occur then the air is very near or at saturation, and the addition of water vapor from the combustion in the jet engines causes the formation of a line the of clouds behind the plane.
So both halos and contrails are dependent on the upper troposphere (roughly the layer of the atmosphere from roughly 15,000 ft to 35,000 ft) become saturated or near saturated. We can tell whether this is true of the real atmosphere by taking the observations from balloon-launched weather instrument radiosondes) and plotting the temperature and dew point temperature on a chart (a.k.a. a sounding).
Below is the sounding from Quillayate, on the Washington coast, at 5 PM Saturday. Note the temperature lines (red) and dew point lines (blue) are very close together from 500 hpa (around 18,000 ft) to around 350 hpa (around 25,000 ft). The atmosphere is either at or close to saturation in this layer (saturation is evident when the temperature and dew point are the same) .
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