Albido is the rate at which heat is reflected back into the atmosphere. A high albido happens with light colours for example grass, whilst a low albido occurs with dark colours such as tarmac. Therefore high albido occurs in the countryside and low albido in the cities.
I won't get into details here as I'll be here all day. The gist of the matter is that every planet has a given mass, and that mass is relatively constant, meaning that the acceleration due to gravity stays that way at a given height above the earth (the height must remain constant for that aforementioned statement to be valid), so at the surface there is actually a give, 'escape velocity' found using Newton's law of Gravitation, that states that if a given mass has this velocity it can escape that planetary body's gravitational pull indefinitely. Now, for earth that is around 11km/s which is very very fast, but think about heat energy inside a system, this time this system will be the earth's atmosphere. When the heat and radiation energy from the sun reaches the earth some of the energy is immediately reflected back into space (the percentage of the energy reflected from the surface is called the 'albido' of a given planet or object in space. Now, as heat energy is seen these days to be an adaptation of kinetic energy but on the atomic scale, if enough energy from the sun hits a certain amount of air molecules in the upper atmosphere, they actually, individually can have this escape velocity, sending some air molecules into space, at a rate that is quite hard to determine exactly but it happens nonetheless. Also, thought this is only accountable for a smaller percentage of the escaped atmosphere, the rotation of the earth coupled with the distance of the molecules in the upper atmosphere being a bit further than that of the surface, this means the gravitational force that acts on these air molecules is that much weaker such that they could potentially just rotate off into space. Think of a merry-go-round and the faster it goes the more you tend to be thrown outwards, same principal, just replace the merry-go-round with the earth, and you with the air molecules. Hope this helps. Cheers.