Remnants of Hurricane Hilary
October 25, 2011
As Hurricane Hilary (2011) drifted northward in the Pacific off Baja, Mexico, it weakened and became a tropical storm. Little squalls from the dying system gave southern California monsoon rains and spectacular sunsets. Here are two pictures showing crepuscular rays and glowing clouds on the western horizon.
The crepuscular rays in the top photo are shadows of clouds cast upon lower, thin clouds. They're all parallel but perspective makes them appear to diverge. Tracing them downward, they all seem to intersect at the Sun below the local horizon. The rays are blue because without direct sunlight only the blue skylight can reach the areas in shadow.
The redness of the clouds in the bottom photo is due to several factors. The low Sun is fiery red because absorption by atmospheric water vapor and scattering by air molecules and aerosol particles remove much of the blue and green sunlight. Earlier in the day, this photograph was taken rain washed much of the smog and dust particles from the air leaving it unusually clear. This allowed more sunlight to penetrate the atmosphere, so the Sun appeared brighter than normal. The clouds themselves also played a role: they were thick enough to intercept and scatter the crimson glow, yet thin enough that they did not absorb much sunlight. As a result, they lit up brilliantly, their flamboyant reds making lovely contrast with the blue sky beyond. The texture and color variations in the cloud are the result of self-shadowing and different thicknesses of the cloud.
And here’s a tip on displaying your own pictures. Make them big! Small pictures are nice enough, but when enlarged, they become spectacular. Try it yourself by filling your screen with the red cloud picture.