How can I tell whether the sky is clear enough to take sun photometer measurements?
The basic rule is that the sun must not be blocked by clouds during a measurement. It is OK to have clouds near the sun. This can be a difficult decision, because you are never supposed to look directly at the sun. You can look at the sky near the sun by blocking the sun with a book or notebook. An even better idea is to use the comer of a building to block the sun. It is very helpful to wear sunglasses when you make these decisions because they protect your eyes from UV radiation. Orange-tinted sunglasses will help you see faint clouds that might otherwise be invisible. If you have concerns about a measurement, note them in the Comments section of the Aerosols Data Sheet when you report the measurement. Thin cirrus clouds are notoriously difficult to detect, but they can dramatically affect sun photometer measurements. If you see cirrus clouds in the hours before or after a measurement, be sure to include that in your sky description.
