I realise that Met is not an exact science but a defined answer would be nice.
I agree with you that it would be nice... but unfortunately there isn't one.
In meteorology there are very few, if any, "magic numbers" or thresholds at which things happen. There are just TOO MANY variables, more than ANY other science.
Learning or memorizing such useless numbers is not only pointless, but can also be dangerous if one trusts them when making operational decisions. Save the brainpower for some UNDERSTANDING of how the very basic principles or meteorology create most of the weather around us. Look beyond the literature written for pilots. The majority of pilots I came across had a very poor understanding of the most basic meteorology principles. All of them THOUGHT thew knew more than they actually did.
palgia