The weather report comes out with “fair, and temperature of 65 degrees Fahrenheit”. When you step outside and see the clouds, you might wonder: Why isn’t it considered cloudy, or partly cloudy? The answer is that the cloud information in weather reports deals with clouds that have a base below 20,000 feet – the point at which they become a concern to aviation. And while high level clouds aren’t included in the observations, obviously a report of clear skies wouldn’t be accurate either. The result: a condition known as ‘Fair’. Which means there aren’t enough clouds to be a concern to aviation, but the skies aren’t perfectly clear either.
Mary Knapp, Weather Data Library