It causes the mercury to fall, by evaporating moisture into the air.
708. What effect has cold upon the barometer?
It causes the mercury to rise, by checking evaporation, and increasing the density of the air.
"For so the Lord said unto me, I will take my rest, and I will consider in my dwelling place like a clear heat upon herbs, and like a cloud of dew in the heat of harvest."—Isaiah xviii.
In noting barometrical indications, more attention should be paid to the tendency of the mercury at the time of the observation, than to the actual state of the column, whether it stands high or low. The following rules of barometric reading are given as generally accurate, but liable to exceptions:—
Fair weather indicated by the rise of the mercury.
Foul weather by the fall of the mercury.