# Relationship between atmospheric pressure and density

### Atmospheric Pressure and Other State Variables

How the pressure and density of the atmosphere varies. Looks at the relationships between altitude, air pressure, density, and their causes. Gas Pressure; Atmospheric Pressure – Pressure Profile; Hydrostatic Between 6 and 12 km the overload (and the pressure) is halved again: only ¼ of the . relationships linking the three variables temperature, pressure and density that. The relationship between pressure,density, and temperature. P=dRT They are inversely proportional, so air pressure decreases with altitude. (tends to be.

The upper boundary of the air column that gives rise to atmospheric pressure is the vacuum of space.

**Atmospheric Pressure and Boiling**

Being rather light, the mass of a column of air with a 1 cm2 cross section is almost exactly 1 kg. If a much heavier liquid substance is used to balance this air column, only a relatively small length would be needed.

### CHAPTER 2. ATMOSPHERIC PRESSURE

In addition, because the density of liquids does not change with height most liquids are incompressiblesuch an equivalent liquid column has a well defined upper boundary below a vacuumOne of the heaviest liquids at room temperature is mercury Hg and the height of the Hg-column that is equivalent to normal pressure mb is only mm long For this reason, columns of mercury, "hanging" in an inverted vacuum tube, can be used as practical instruments to measure atmospheric pressure see FigureLutgens and Tarbuck, If water were used instead of mercury, the height of the column equivalent to normal pressure would be The Gas Laws The example of the gas-filled balloon can also be used to explore the basic gas laws see also Appendix D, p.

In the following, lets assume that the balloon is tight, so that the amount or mass of air in it stays the same: With density being the ratio of mass per volume, the gas density of the balloon thus varies only with its volume when mass is held constant.

If we squeeze the balloon, we compress the air and two things will happen: The density of humid air can be calculated as the sum of the densities of the two gases, dry air and water vapour in proportion with their partial pressures. Calculating Water Vapour Pressure The amount of water vapour that a parcel of air can hold varies with temperature. The warmer the air the more moisture that it can hold.

Air is said to be saturated when the temperature and dew point of the parcel are equal. There are a number of algorithms available for calculating vapour pressure but we shall use a polynomial developed by Herman Wobus.

This tool will calculate the saturated water vapour pressure given the dewpoint. Click To Tweet The following tools allow you to calculate the density of moist air by knowing either the temperature, pressure and relative humidity OR temperature, dew point and pressure.

The density of moist air is calculated as the sum of the density of the dry air component of the mixture plus the density of the saturated component of the mixture. In the first calculator, the vapour pressure of water vapour in saturated air at the nominated temperature is calculated and multiplied by the relative humidity to give the actual water vapour pressure.

The water vapour pressure is then subtracted from the total pressure to give the pressure of the dry component of the parcel. Densities of the two components are then calculated and summed to give the final answer.

Moist Air Density 1 Calculator This tool will calculate density of moist air given temperature, air pressure and relative humidity.