Weather usually, such as wind or visibility.
Westward-bound is ~95% time
Lindbergh Field weather observers – humans – in conjunction with a modern computer weather monitoring system – determine when the airport is “flipped” – that is, when the take-offs and landings are reversed. There are factors that they observe and consider, such as:
visibility drops below two miles or
the ceiling drops below 700 feet
if fog ceiling is less than 700 feet and visibility is less than 1 mile, then it’s a double-flip – planes land from and depart to the west
(if tail winds exceed 10 mph)
If visibility gets down to a mile or less and the ceiling is 300 or 400 feet, then the airport shuts down.
googled
http://obrag.org/?p=73256