A fluid exerts pressure in all directions, but the pressure is a function of the mass of fluid above a given surface. Pressure is measured in force per unit area. In other words, atmospheric pressure is a measure of the weight of the column of air above a point. Air, unlike water, is a compressible fluid so the rate of pressure decrease as we move up in the atmosphere is not as simple as in water where pressure varies very simply with depth at a rate of one atmosphere of pressure per 33 feet or 10 meters of water. Thus the pressure measured at the depth 50 meters of water would be 5 atmospheres of pressure due to the weight of the overlying water plus one atmosphere's pressure due to the weight of the air in the overlying atmosphere for a total of 6 atmospheres of pressure.
Atmospheric pressure is measured by a barometer.
The ratio of the densities of the two liquids, mercury to water, is about 13.6 to 1. From the early days of meteorology, the relationship between atmospheric pressure and height above sea level has been studied very closely. The pioneering work by the French scientist Pascal is particularly important and that is why he is honored by the naming of the unit of pressure after him. The early meteorologists or aviators ascended mountains or used balloons or aircraft to measure the change in pressure with height. Rocket scientists and space explorers also entered the picture in more recent times as knowledge about the outer fringes of the atmosphere became important.