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Air density is the mass of air per unit volume, typically measured in kg/m³. It varies with temperature, pressure, and humidity—lower temperatures and higher pressures increase density, while higher humidity (more water vapor) slightly decreases it due to water's lower molecular weight. Dry air primarily consists of nitrogen and oxygen molecules moving at high speeds. For example, at room temperature, a nitrogen molecule (mass 14 u) moves at about 670 m/s—twice the speed of sound! As temperature rises, these molecules move faster, expanding the gas volume and reducing density (see the ideal gas law). Conversely, increasing pressure compresses the air, increasing the number of molecules per unit volume and thus increasing density.
Altitude significantly affects air density. As you ascend, pressure and temperature drop, reducing the amount of air (and oxygen) per unit volume. At high altitudes, climbers often need oxygen cylinders to breathe. In contrast, airplane cabins are pressurized to maintain air density similar to sea level. The table below shows how dry air density changes with altitude (data from the U.S. Standard Atmosphere, 1976):
| Altitude [ft (m)] | Temperature [°F (°C)] | Pressure [psi (hPa)] | Air Density [lb/cu ft (kg/m³)] |
|---|---|---|---|
| Sea level | 59 (15) | 14.7 (1013.25) | 0.077 (1.23) |
| 2,000 (610) | 51.9 (11.1) | 13.7 (941.7) | 0.072 (1.16) |
| 4,000 (1,219) | 44.7 (7.1) | 12.7 (873.3) | 0.068 (1.09) |
| 6,000 (1,829) | 37.6 (3.1) | 11.7 (808.2) | 0.064 (1.02) |
| 8,000 (2,438) | 30.5 (-0.8) | 10.8 (746.2) | 0.06 (0.95) |
| 10,000 (3,048) | 23.3 (-4.8) | 10 (687.3) | 0.056 (0.9) |
| 12,000 (3,658) | 16.2 (-8.8) | 9.2 (631.6) | 0.052 (0.84) |
| 14,000 (4,267) | 9.1 (-12.8) | 8.4 (579) | 0.048 (0.77) |
| 16,000 (4,877) | 1.9 (-16.7) | 7.7 (530.9) | 0.045 (0.72) |
This table shows that at 16,000 ft (~5 km), the density of dry air is nearly half that at sea level, highlighting the significant impact of altitude.
This calculator supports both dry and moist air, with dew point and water vapor pressure calculated for moist conditions. Select "Dry air" in the "Air Type" field to ignore humidity effects in the computation.
Air Density Calculation: The air density is calculated using the formula:
\(\rho = \left(\frac{p_d}{R_d T}\right) + \left(\frac{p_v}{R_v T}\right)\)
For dry air, the density simplifies to:
\(\rho = \frac{P}{R_d T}\)
where \( R_d = 287.058 \, \text{J/(kg·K)} \). For example, at room temperature (20 °C = 293.15 K) and standard pressure (101325 Pa), the dry air density is:
\(\rho = \frac{101325}{287.058 \cdot 293.15} \approx 1.204 \, \text{kg/m³}\)
Dew Point Calculation: The dew point (DP) for moist air is calculated as:
DP = \(\frac{243.12 \cdot a}{17.62 - a}\)
Where \( a = \ln\left(\frac{\text{RH}}{100}\right) + \frac{17.62 \cdot T}{243.12 + T} \).
Variables:
For moist air, \(p_v = RH \cdot P_{vs}\), where \(P_{vs} = 6.1078 \cdot 10^{\frac{7.5 \cdot T}{T + 237.3}} \cdot 100\) (Pa).
Use the form above to input your values with the desired units, and the calculator will convert them internally to provide the air density, dew point, and water vapor pressure.
The SI unit for density is kilogram per cubic meter (kg/m³). Other convenient units include:
Imperial units for density include:
The calculator displays air density in multiple units for convenience, allowing you to choose the unit that best suits your needs.
Air density varies with temperature and pressure, so standard reference conditions are defined for consistency. Different organizations use different standards:
Choose the appropriate standard conditions based on your application to calculate the standard air density using this calculator.
Air pressure is a physical property of a gas that indicates the force it exerts on its surroundings. Imagine a cubic container with air inside. According to the kinetic theory of gases, air molecules (like nitrogen and oxygen) are in constant motion, colliding with each other and the container walls. Each collision exerts a tiny force, and with ~10²³ molecules (Avogadro's constant order of magnitude), the total force becomes significant and measurable as pressure. Higher temperatures increase molecular speed, raising pressure if volume is constant, while higher pressure compresses air, increasing density.
Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature, expressed as a percentage (0% to 100%). Partial pressure is the pressure of one gas component (e.g., water vapor) if it were alone at the same volume and temperature. The total air pressure is the sum of partial pressures of all gases:
\( p_{\text{total}} = p_{\text{N}_2} + p_{\text{O}_2} + p_{\text{Ar}} + p_{\text{H}_2\text{O}} + \ldots \)
The equilibrium vapor pressure of water increases with temperature, reflecting the tendency of water molecules to evaporate. At RH = 0%, the air is dry; at RH = 100%, the air is fully saturated, and further cooling leads to condensation.
The dew point is the temperature at which water vapor in the air reaches saturation, causing condensation (dew) upon further cooling. It is closely related to humidity. The calculator uses the formula:
DP = \(\frac{243.12 \cdot a}{17.62 - a}\)
Where \( a = \ln\left(\frac{\text{RH}}{100}\right) + \frac{17.62 \cdot T}{243.12 + T} \).
The dew point cannot exceed the air temperature (RH cannot exceed 100%). Low dew points (dry air) can cause skin irritation, while high dew points (humid air) hinder sweat evaporation, affecting comfort. The table below shows dew points at 68°F (20°C) for various relative humidities:
| Dew Point [°F (°C)] | Relative Humidity at 68°F (20°C) [%] |
|---|---|
| Over 60 (16.4) | Over 80 |
| 57.8 (14.3) | 70 |
| 53.6 (12) | 60 |
| 48.7 (9.3) | 50 |
| 42.8 (6) | 40 |
| 35.4 (1.9) | 30 |
| Below 25.4 (-3.7) | Below 20 |
Human comfort is affected by humidity. At 100% RH, sweat doesn't evaporate, causing discomfort. Dry air (low dew point) can lead to skin irritation. Wind can enhance evaporation, providing a cooling effect.
How do you calculate dry air density?
To calculate the density of dry air:
What is the dry air density at room temperature?
The dry air density at room temperature (20 °C = 293.15 K) and standard pressure (101325 Pa) is approximately 1.204 kg/m³. Using the formula:
\(\rho = \frac{101325}{287.058 \cdot 293.15} \approx 1.204 \, \text{kg/m³}\)