Some of the most important factors that affects the height of Mercury in the Barometer are:
The density of air and the atmosphere above becomes less with height. Since pressure depends on weight, the atmospheric pressure also decreases with height. Therefore, the reading in the barometer tube falls as it is taken up in the atmosphere. It falls by about 1 cm for every 110 meters of ascent.
2. Tilting the barometric tube:
At any place, the atmospheric pressure supports a vertical column of mercury. If the tube is titled as the mercury will occupy a longer length in the tube, but the vertical height (h) remains the same, since the atmospheric pressure has not changed. The vacuum in the tube, therefore, decreases. If the tube tilted, so that the vertical height is below h cm, the tube should be filled with mercury. If not, the barometer is faulty, because air must have entered the tube and this act as a cushion to prevent the mercury from filling the tube.
3. Cross-section and length of the tube:
As long as the atmospheric pressure is constant, the vertical height of mercury in the tube must remain the same, irrespective of the cross-sectional area or shape of the tube. If the tube is dipped into the through, the length of the vacuum keeps on decreasing. If the vertical height is below the atmospheric pressure (h cm), a metallic click should be heard and the tube filled with mercury. If not, the barometer is faulty.
4. Dampness of mercury:
If the mercury is damp, a little water will evaporate into the vacuum. The water vapor formed would exert a pressure in the mercury causing its level to fail.
5. Introduction of alcohol into the vacuum:
If a little alcohol is introduced over the mercury, the mercury level falls as the alcohol evaporates, and its vapor exerts a pressure on the mercury. After some time, the vapor is saturated and no further evaporation occurs. The mercury surface then remains steady.
During the hot season, the height of the mercury barometer falls. This is because hot air becomes less dense and rises, and thus the pressure falls.