Could you use a water u-gauge (manometer), zeroed and with one side of the u-gauge plugged and the other open to air, to show an increase or decrease in barometric pressure after a period of time (e.g. 2 hours)? The idea is that you trap some air at the starting atmospheric pressure on one side of the gauge. Then, if the current atmospheric pressure changes significantly it will cause the water column to rise or fall. At the end of the required period the pressure change can then simply be read off the u-gauge.