As far as I can tell, weather is a simple humidity number that goes from 0% to 100%. As a Scenario Master in the Ring you even get a slider to set it.
0% to 60% is Fair, 60% to 80% is Rainy, 80% to 100% is Thundery.
The problem, I would guess, is that materials are often programmed to be available only if the weather is, say, between 33% and 37%. So saying those mats are up in Fair weather is like describing a person by saying they're less than 3 meters tall; technically correct but so woefully inaccurate as to be useless information.
0% to 60% is Fair, 60% to 80% is Rainy, 80% to 100% is Thundery.
The problem, I would guess, is that materials are often programmed to be available only if the weather is, say, between 33% and 37%. So saying those mats are up in Fair weather is like describing a person by saying they're less than 3 meters tall; technically correct but so woefully inaccurate as to be useless information.