Question

when a cold front passes your loction,the temperature falls as a result air density increases,decreases or stays the same

Answer 1

When a gas is cooled, it contracts. In a sealed environment, the pressure would therefore reduce, as the molecules try to get closer together, that is, push outwards less. But in the atmosphere, the closer-packed molecules (or atoms, of course) make the air more dense, therefore heavier.

If you were at a higher elevation, the cooling is caused by the reduced pressure causing the molecules to move further apart. The temperature reduces about 1 degree Centigrade for every 100metres gain in elevation.

Any good?

Answer 2

the answer is that it decreases