Subject: If warm air molecules are spread far apart, then why high in the sky are co

If warm air molecules are spread far apart, then why high in the sky are cooler ones spread far apart 
too? I have recently read a book that had said, molecules in warm air are spread farther apart than cool 
air molecules. And that the higher you get, the cooler it gets. So up higher in the air where it is coolest, 
you would assume that the air molecules would be closer together. Yet in this book it also said the 
higher you get the more spread out the air molecules get. And if it is cold air how can this be? 

               Thanks, 
                   
                   Ian Rider
  

Re: If warm air molecules are spread far apart, then why high in the sky are co