Subject: explain how an air mass is formed

I am asking this question because I've been doing a lot of research on 
this and I haven't been able to find any thing. The thing is that I 
have this question on my test that i'm unable to understand and I got it 
wrong and my teacher told me to do it over and that she wasn't going to 
help me with it and she said it don't matter now because i've got what 
i've got on my test but I still want to know what it is.













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Re: explain how an air mass is formed