The barometer works by measuring the force with which the air pushes something mobile, a very common type of barometer uses a silver liquid, called mercury, as a moving substance. Mercury rises and falls inside a glass tube, inside it there was nothing but mercury, not even air, the tube is closed at the top and open at the bottom, which penetrates into a container full of mercury.
The mercury barometer works like a straw to sip lemonade, by sucking on the straw, you let out all the air that is in it. But the air that surrounds the lemonade presses on it, the pressure of the air forces the lemonade to ascend through the straw to your mouth.
The mercury inside the barometer tube rises through the barometer, depending on the pressure that the air exerts on the mercury in the container. The heavy air presses harder than the light air, and causes the mercury to rise to a greater height in the tube.
Next to the tube there is a graduated scale so that you can know exactly how much the mercury has risen, the dry and heavy air will make the mercury rise in the tube up to about 76 cm. The light and humid air up to about 74 cm, the difference is not much but it is enough for the weather man to know if the day is going to be sunny or rainy.
