Nigel might be shocked but I am with him on this one - even the red zone (which we have been in and out of over the last 2 days) is so unscientific as to be laughable.
There is no measuring going on to validate the zones - it is all the prediction of the oh so fabulous Met Office computer - you know, the one that said we would have a BBQ Summer last year. As with all computer models of the atmosphere it is 10% fact and 90% guess-work - they still can't forecast the weather with any accuracy more than 12 hours ahead!
Did we notice a change in the atmosphere as the red zone came and then cleared? Er No... it still looked 8/8 blue on Tuesday and it is clear as a bell today whilst the met man 'accurately' states we will be under it until 1800Z.