# Relationship between air pressure density temperature

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

How does the density of a substance change as the pressure The density (r) of an object depends on temperature and pressure. that the density of water is approximately 1, kg/m3 and the density of air is kg/m3. Any time you specify a relation between any two of pressure, density or temperature you must hold the third constant or specify its behavior. Air pressure, or atmospheric pressure, is defined as the weight of air in Earth's (or another planet's) atmosphere. Pressure is a force exerted on or against an.

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Included on the page are online calculators. The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure: Remember that to convert oC to oK you need to add Dry Air Density Calculator This tool will calculate density of dry air given temperature and air pressure.

Nitrogen has a molecular weight of 14 so a N2 molecule has a molecular weight of Oxygen has a molecular weight of 16 so an O2 molecule has a molecular weight of Given the mixture of gases found in air the average molecular weight of air is around Water is made up of two hydrogen and one oxygen atoms H2O. Hydrogen is the lightest element and has a molecular weight of one. Water is a very light molecule and much lighter than the average weight of the molecules found in air.

Moist Air In the real world the air always contains some moisture. The addition of water vapour to a mass of air makes it less dense. For example, hot air rises, but why then is it cold on top of a mountain.

The answer is that hot air is less dense than the cold air surrounding it for a constant pressure, and being less dense it rises. With a mountain, the pressure is decreasing, and we likewise find in the atmosphere that temperature decreases with decreasing pressure.

On a hot day what tends to happen is that the surface, which is being warmed by the sun, heats the lowest level of the atmosphere, reducing its density it is at the same pressure as its surroundings and its T rises.

### Density of air - Wikipedia

This will eventually drive convection and mix this warmer air vertically. Given enough time, this will reduce the mass in the column of air and therefore reduce the pressure at the surface. These are called "heat lows" and you can see them forming in the desert areas and they play roles in sea breeze formation and the monsoons.

Weather Variables: Air Pressure, Temperature & Density

To address the expanded question: The point in the FAA written is best understood by forgetting that we fly at constant altitudes -- we don't. In level flight we fly on constant pressure surfaces which we then translate to an altitude. 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. 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.

## Density of air

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.

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.