Absolute air refers to completely dry air with no water vapor content. It is an important concept in meteorology, aviation, engineering and other fields that deal with the properties of air. Some key facts about absolute air:
- Absolute air contains only permanent gases like nitrogen, oxygen, argon, carbon dioxide etc. and no water vapor.
- It has a specific gas constant R of 287 J/kg.K.
- Its density at standard sea level pressure and 15°C temperature is 1.225 kg/m3.
- Absolute air serves as a reference for calculating the properties of moist air in the atmosphere.
- The amount of water vapor in a parcel of air is measured relative to absolute air to determine the humidity.
Now let’s explore absolute air and its relevance in more detail.
Composition of Absolute Air
Absolute air is composed of the following permanent or dry gases:
- Nitrogen (N2) – 78.09%
- Oxygen (O2) – 20.95%
- Argon (Ar) – 0.93%
- Carbon dioxide (CO2) – 0.03%
Along with trace amounts of neon, helium, methane, krypton and other gases. The main distinction is the complete lack of water vapor (H2O), which can range from 0-5% in atmospheric air.
The dry gas composition gives absolute air a specific gas constant of 287 J/kg.K. This value is used in thermodynamic equations involving an ideal gas. It is lower than the gas constant of individual constituents because it represents the weighted average for the mixture.
Variable Composition of Moist Air
In contrast to the fixed composition of dry gases, the water vapor content of moist air varies greatly. This depends on factors like:
- Temperature – Ability to hold moisture increases with heat
- Humidity – Amount of moisture present relative to saturation
- Altitude – Decreases rapidly with height
- Weather phenomena – Thunderstorms, fronts produce varied concentrations
The variability of H2O makes moist air calculations more complex. Absolute air provides a stable reference point to determine the actual water vapor mass.
Density of Absolute Air
The density of a gas is its mass per unit volume. For absolute air at standard sea level pressure (101.325 kPa) and standard temperature (15°C or 288.15 K), the density is:
- Mass of 1 mole = 28.97 g (molar mass from composition)
- Volume of 1 mole at STP = 22.4 L
- ∴ Density = Mass/Volume = 28.97 g / 22.4 L = 1.225 kg/m3
This standard density value allows conversions between mass and volume of absolute air at STP conditions. However, the actual density varies with temperature, pressure and altitude. Lower density reduces aerodynamic forces on aircraft.
Density Variation with Altitude
| Altitude (m) | Pressure (kPa) | Temperature (°C) | Density (kg/m3) |
|---|---|---|---|
| 0 | 101.325 | 15 | 1.225 |
| 1000 | 89.88 | 12.5 | 1.112 |
| 2000 | 79.54 | 10 | 1.007 |
| 3000 | 70.2 | 7.5 | 0.9093 |
The table shows how absolute air density reduces with height, as pressure decreases and temperature drops. This impacts engine and wing performance.
Uses of Absolute Air
The defined properties of absolute air make it useful for:
- Meteorology – Calculating humidity, water vapor content of air
- Aeronautics – Airspeed indicators, engine air intakes
- HVAC and Building Services – Designing ventilation systems
- Environmental Engineering – Standard conditions for emissions
- Physics and Chemistry – Gas law relationships, calculations
Let us look at some applications in further detail:
Meteorology
In meteorology, absolute air provides a fixed reference for determining the humidity or moisture content of air. The mass ratio of water vapor to dry air gives the specific humidity. Comparing the vapor pressure to saturation vapor pressure over water gives the relative humidity.
These humidity values are essential for forecasting dew, clouds, precipitation and storms. They also drive evaporation and plant transpiration.
Aeronautics
In aviation, absolute air properties like density are needed to calibrate airspeed indicators. At higher altitudes, the lower density causes the plane to travel faster relative to the ground for the same indicated airspeed.
Aircraft engines also need to compensate for the lower air density by optimizing the fuel-air mixture intake. Absolute air is used as the reference instead of moist ambient air.
HVAC and Building Services
HVAC systems circulating conditioned air in buildings rely on absolute air parameters. The required ventilation rates and air distribution velocities are calculated using STP density.
Ducts and openings must be properly sized. Fans, blowers and other equipment are also selected based on moving standard absolute air. This makes the systems adaptable to different climates and operating conditions.
Absolute Humidity vs Relative Humidity
Humidity describes the amount of water vapor in air. There are two main measures of humidity:
- Absolute humidity – The mass of water vapor per unit volume of moist air. Grams per cubic meter (g/m3) are the typical units.
- Relative humidity – The ratio of partial pressure of water vapor to the equilibrium vapor pressure at the same temperature. Expressed as a percentage (%).
Absolute humidity represents the actual moisture content. Relative humidity measures how close the air is to being saturated.
Compare and Contrast
| Absolute Humidity | Relative Humidity | |
|---|---|---|
| Units | g/m3 | % |
| Measures | Moisture content | Degree of saturation |
| Depends on | Temperature only | Temperature and pressure |
| Range | 0 to 30 g/m3 | 0% to 100% |
Although interrelated, these parameters have different interpretations and applications. Absolute humidity is used more in engineering calculations while relative humidity is often reported in weather forecasts.
Conclusions
To summarize key points about absolute air:
- It only contains permanent dry gases like nitrogen, oxygen etc.
- The fixed composition gives absolute air a specific gas constant of 287 J/kg.K.
- Standard density at sea level is 1.225 kg/m3.
- It provides a stable reference for calculating moist air properties.
- Absolute air is applied in meteorology, aeronautics, HVAC and engineering.
- Comparing moisture content to absolute air determines the humidity.
Understanding the behavior and parameters of absolute air is essential for various scientific and technical fields. This idealized dry air construct allows meaningful analysis of real moist atmospheric air and its thermodynamic processes.