Sunrise ... Sunset - differences

Red Sky at Night - New York Times: "Red Sky at Night
Pretty good explanation of atmospherics
I'd go mostly for the humidity factors, at least here at the lake

By C. CLAIBORNE RAY
Published: August 9, 2005

Q. Why does a sunrise look so different from a sunset?

A. It depends on a number of factors that vary and interact, said Geoff Cornish, a meteorologist at Penn State.

'Two factors are air quality, which is typically worse at sunset, changing the color of sunlight as the setting sun reaches the horizon, and the fact that the sky is frequently more complex in terms of cloud formations late in the day,' he said.

First, more industrial activity and driving take place in the daytime, he said, adding particles to the atmosphere that distribute and reflect the light of sunset differently. This difference may not be so apparent on a ship far at sea, as the impact of pollution would be less.

Second, the sun is more likely to disappear into clouds at night than to emerge from clouds in the morning, Mr. Cornish said.

'Typically, fair-weather cumulus clouds dissipate a few hours after sunset because their lifeblood, the contrast of the warm ground and the cooler air aloft, has been cut off,' he said. The atmosphere is least stable in the afternoon, with the freest exchange of air from low levels to high.

Also, the ground is cooler in the morning, leading to condensation and low-level patchy fog.

'A localized haze over the lowest few score feet above the ground is typically a morning phenomenon,' Mr. Cornish said, producing the 'misty moisty morning' of the nursery rhyme.

He suggested still another possible factor, on the chemical level. When the atmosphere is warmer, the heat differences may catalyze different chemical reactions, so that plants and trees emit different chemicals into the atmosphere over the course of the day.

Readers are invited to submit questions by mail to Question, Science Times, The New York Times, 229 West 43rd Street, New York, N.Y. 10036-3959, or by e-mail to question@nytimes.com."