To add to that...
Flying towards an area either of low pressure or low temperature, the altimeter will overread. This is not very nice in mountainous areas, 'cos you will be lower than you think.
Reason is, if you were to keep a constant ACTUAL altitude, as the pressure drops the altimeter shows a climb. So you descend to keep the same reading.
The reason for the original phenomenon lies in the universal gas law, which states that P1*V1/T1 = P2*V2/T2 = R (the universal gas constant).
Since we're dealing with (for simplicity's sake) constant volumes, P*T = 1/R.
Hence, as temperature drops, pressure increases to keep the same ratio.