# Temperature density and pressure relationship

### temperature - How is pressure related to air density? - Aviation Stack Exchange

Any time you specify a relation between any two of pressure, density or temperature you must hold the third constant or specify its behavior. This ever slower decrease of the overload, of density and pressure, . relationships linking the three variables temperature, pressure and density that describe. For an ideal gas, P = rho R T, where P is the pressure, rho (Greek letter) is density, R is the gas constant for the particular gas, and T is the absolute temperature.

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.

### Pressure/Density/Temperature relationship - ATP Forum

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.

**Pressure, Volume and Temperature Relationships**

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.

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: Since density is mass over volume, and the mass stays constant, the rise in density means that the volume of the balloon decreases: In any given column of atmosphere, if it is warmer than standard a given pressure surface will be higher and when colder than standard the pressure surface will be lower. To illustrate, let's consider you are flying at ft or roughly mb.

## Announcement

Everywhere on this pressure surface will indicate ft on our altimeter for its current setting. If we go somewhere hot, this pressure surface rises, and so we climb though we think we are level with this pressure surface but because the pressure has not changed, we still indicate ft. However, we are higher than ft in reality.

This follows into your next question. Aneroid wafers detect pressure changes and your altimeter displays an altitude not corrected for temperature. This is why your true altitude can vary with temperature for a constant indicated altitude.

### How to Calculate Air Density - Fly Me to the Moon

When you correct the altitude for temperature we call this "density altitude". So back to my example above, your are flying along at mb and indicating ft, and heading into warmer air.

The pressure surface starts to gently rise and as it does you are not yet following that rise and your altimeter will indicate a descent. In true level flight you will begin to fly into higher pressure in this case as the mb surface rises above you and the aneroid wafer in your altimeter will indicate a lower altitude and a descent.

You correct this and climb back up to the mb pressure level so that your altimeter will once again indicate ', all the while actually gently climbing on this pressure surface. You won't really be cognizant of this while flying however, and will just minimize vertical speed and maintain altitude blissfully unaware that you are really flying on a sloping constant pressure surface.