Charles' Law states the relationship between temperature and volume of a gas at a constant pressure.
T/V = k
Where T is the temperature of the gas,
V is the volume
and k is a constant.
In a nutshell, Charles' law says that cooling a gas decreases its volume. If the volume remains fixed then the pressure must decrease. A suitable example of this phenomenon is a scuba tank bursting because it is left in the trunk of a hot car in the tropical sun (temperature rises, volume is constant and pressure increases).
See our choices for scuba training