In photography, the f 16 rule (also known as the Sunny f/16 rule) is a method of estimating correct daylight exposures without a light meter.

The basic rule is, “On a FULL (no high thin clouds) sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight.”[1] For example: for ISO 200 film set camera to f 16 and shutterspeed at 1/250. For 400 ISO film that would be f 16 @ 1/500.

For a day with those high thin cloud covering you loose on stop. So for ISO 200 film it is either f 11 @ 1/250 or f 16 @ 1/125 and so on.
For a more cloudy day, puffy big white clouds in the sky, you loose 2 stops.
An overcast day, 3 stops.
A deep dark over cast day, 4 stops (maybe even more)

Portraits behind a building or large wall you loose 2 stops.

At the beach or on snow you gain a stop. f 22 @ 1/250 or f 16 @ 1/500 for ISO 200 film

If you can find an old film package from kodak or fuji or ilford or agfa, those diagrams printed on the inside of the box surface are right on the money.

Be aware that B&W ISO 400 film should be shot at ISO 200

Geeky stuff
Sunny 16 rule – Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunny_16_rule

Examples
http://www.dgrin.com/showthread.php?t=88199
Simple Exposure Guide Digital Grin Photography Forum