This pressure depends on the density of the air and the speed of movement of the molecules, itself determined by temperature. Since air has weight, each particle is compressed by the ones above it. Density and hence pressure increase with proximity to the Earth's surface. Similarly, when air is cooled, its mass increases, since molecules do not move as quickly. This means that at constant pressure, cold air is denser than warm air.
Atmospheric pressure is exerted in all directions.
The pressure on a surface is at right angles to the exposed plane. Unlike gravitational force, which is always exerted vertically and toward the Earth's centre, pressure can also be horizontal or upward. 4.2 Units Atmospheric pressure is measured with a barometer. The simplest barometer consists of a vertical tube full of mercury, since we know the specific mass of mercury. In addition, we can easily determine its hydrostatic pressure, simply by comparing atmospheric pressure to the pressure of mercury.