This is because molecular weight of water is less than average molecular weight of dry air. Cold, dry air produce higher surface pressure than do warm, humid air. Warm, dry air produces higher surface pressure than equally warm, but more humid air. Divergence at surface results from sinking, downward air motion decreasing air density and air pressure at surface. Convergence at surface results rising, upward air motion increasing air density and air pressure at surface.
Mercury barometer consists of a glass tube and an open container of mercury.
The tube, filled with mercury, is inverted into the container. The height of the mercury column is a measure of the air pressure. The average air pressure at sea level supports the mercury column in the tube to a height of 760 mm. An altimeter typically does not give the true altitude. The altimeter reading is the true altitude only when air temperature and pressure matches the standard atmosphere.