Skip to main content

On A Clear Day, You Can't Necessarily See Forever

In a Hank Ketcham cartoon several years ago, Dennis the Menace and a friend are standing outside looking at the stars and moon. Dennis is telling his friend, "They can't be more'n ten or fifteen miles away. My dad says that's as far as you can see on a clear day."

Dennis had clearly drawn an incorrect conclusion from a casual remark made by his father. But during the day, there is an upper limit to the distance at which objects can be discerned on the earth's surface. Craig Bohren, writing in Weatherwise magazine, states that this is about 200 miles under ideal conditions.

The limiting factor is the loss of contrast between the object (presumably a mountain in this case) and the background of the horizon. Most of us have had the unsettling experience of driving during a winter "whiteout." Whiteouts do not necessarily involve snowstorms. They may occur any time that the shade of an overcast sky so perfectly matches that of the terrain that it is impossible to distinguish between them. A sense of disorientation is the inevitable result. Dennis' observation is just another manifestation of this. The stars shine just as brightly during the day as they do at night, but the difference in contrast with the background is enormous.

But why (neglecting curvature of the earth) shouldn't you be able to see a perfectly black mountain against a perfectly white sky through a perfectly clear atmosphere more than 200 miles away?

The answer, as Bohren explains, is that when you look at any object, the light that you perceive is scattered by all the molecules in the air along your line of sight. This causes a "fuzziness" which increases with distance, and reduces contrast between even a perfectly black object and the horizon sky. Eventually, the object just fades into the background.

The point that emerges is that the reason for whiteouts, which can occur at distances of a few feet to many miles, is much the same as why one could not see Mt. McKinley from the Canadian border (even if the topography permitted). That reason is the loss of contrast between object and background, even though the underlying causes for that loss may be different.