Doesn't really make sense. You can say "The air is fresh" if you want. A better sentence would be: The weather is good in Britain. Better than Spain. The weather is nice in Britain. Nicer than in Spain.

'Fresh' is a false friend for speakers of Latin languages. 'Fresh' in English means clean or new (as opposed to old, tired, dirty, used or stale). For example, 'fresh fruit and vegetables' have been recently picked, and 'fresh bread' has been recently baked. 'Fresh towels' in a hotel means clean ones that have just been laundered, as opposed to the old ones which are dirty/used. Having a shower makes you feel fresh. A fresh start means starting again. A fresh sheet of paper is a new, blank sheet that hasn't been written on. A fresh approach is an innovative one. Fresh air is clean air that isn't polluted, and so on. 'Fresh' can't refer to temperature. The temperatures are lower in Britain and the weather is cooler. Today it is 20 degrees in London and 36 degrees in Cadiz, so it is much cooler in Britain than in Spain. Sometimes weather forecasters will say that it will feel 'fresher' tomorrow, but that's not the same as cooler. If you use the word 'fresh' in English with regard to weather, you're talking about air quality or wind speed, or a lack of 'heaviness' in the air.

It would be: The weather is cool in Britain. Cooler than in Spain. The British informal usage of "fresh" to describe the weather is usually an example of British understatement, and means the weather is cold. It is only ever used to describe the conditions at an instant in time. It is not used to compare general weather patterns, such as to say that overall the weather in Britain is cooler than in Spain.

You might would like to add "chilly" to your atmospheric terminology -- http://www.freethesaurus.com/chilly I hope I remember it well -- "chilly" is right the term used to describe a peculiar cool air, in Great Britain. Glad to receive comments