Clouds form when warm, moist air rises to a greater altitude, causing it to expand and cool. Eventually, water vapor in the cooled air condenses into the droplets that form clouds.
If you’ve ever been to the Air and Space Museum in Washington DC, perhaps you’ve sampled the kind of freeze-dried ice cream astronauts eat during space missions. A stiff, dry square, the freeze-dried version tastes just like ice cream, except that it isn’t cold, and has no moisture.
Kimberly Sessions, of Atlanta, Georgia wrote to A Moment of Science with the following question: “I know why the ocean is still salty: evaporated water leaves the salt behind. But how did it get salty in the first place?”
Now, at sunset, the lowest mile or so of the atmosphere is filled with things like vehicle exhaust, dust, smoke, and water vapor, and all these pollutants scatter light.