The operating principle of the barometer is simple. The pressure of the atmosphere is essential for predicting the weather. When the barometers pressure is greater than the pressure of the air, the pencil will go up from the normal position and the weather is bound to change. Since each area has a different interpretation of high and low temperature, the barometer takes some time as we have to note down weather at each gradient with the help of descriptions. Once the instrument has been calibrated, it will give you accurate predictions of the weather around you.
So now you have a reliable weather warning system present right on your table. Take a look at it before going outside and you will be saved from the extreme conditions a lot of times!