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Specific Humidity Calculator

Calculate the specific humidity of air based on temperature, relative humidity, and atmospheric pressure. Specific humidity represents the mass of water vapor per unit mass of air.

Air temperature in degrees Celsius (°C)
Relative humidity as a percentage (0-100%)
Atmospheric pressure in hectopascals (hPa)

How to Use This Calculator

  1. Enter the air temperature in degrees Celsius
  2. Input the relative humidity as a percentage (0-100%)
  3. Provide the atmospheric pressure in hectopascals (standard is 1013.25 hPa at sea level)
  4. Select your preferred output unit (g/kg or kg/kg)
  5. Click Calculate to see the specific humidity value

Formula Used

q = (0.622 × p_v) / (p_total - p_v)

Where:

  • q = specific humidity (kg/kg or g/kg)
  • p_v = actual vapor pressure (hPa)
  • p_total = total atmospheric pressure (hPa)
  • 0.622 = ratio of specific gas constant of dry air to water vapor

Vapor pressure calculation:

p_v = RH/100 × p_sat

Where:

  • RH = relative humidity (%)
  • p_sat = saturation vapor pressure (hPa)

Saturation vapor pressure (Magnus formula):

p_sat = 6.1078 × 10^((7.5 × T)/(237.3 + T))

Where:

  • T = temperature in Celsius (°C)

Example Calculation

Real-World Scenario:

Let's calculate the specific humidity on a typical summer day in Miami, Florida.

Given:

  • Temperature = 30°C
  • Relative Humidity = 80%
  • Atmospheric Pressure = 1013.25 hPa (sea level)

Calculation:

First, calculate the saturation vapor pressure:
p_sat = 6.1078 × 10^((7.5 × 30)/(237.3 + 30)) = 42.43 hPa

Next, calculate the actual vapor pressure:
p_v = 80/100 × 42.43 = 33.94 hPa

Finally, calculate the specific humidity:
q = (0.622 × 33.94) / (1013.25 - 33.94) = 0.0216 kg/kg = 21.6 g/kg

Result: The specific humidity is 21.6 g/kg, indicating very humid air typical of tropical climates.

Why This Calculation Matters

Practical Applications

  • Weather forecasting and meteorology
  • HVAC system design and optimization
  • Agricultural planning and irrigation
  • Industrial processes requiring precise humidity control

Key Benefits

  • More accurate than relative humidity for certain calculations
  • Essential for calculating air density
  • Remains constant despite temperature changes (unlike relative humidity)
  • Important for heat index and thermal comfort calculations

Common Mistakes & Tips

Relative humidity is the ratio of the current amount of water vapor to the maximum possible at that temperature, while specific humidity is the actual mass of water vapor per unit mass of air. Specific humidity remains constant when air temperature changes, while relative humidity does not.

Always use the actual atmospheric pressure for your location and altitude. Standard sea-level pressure is 1013.25 hPa, but this decreases with altitude. Using an incorrect pressure value will significantly affect your specific humidity calculation.

Frequently Asked Questions

Specific humidity is the mass of water vapor per unit mass of moist air (g/kg), while absolute humidity is the mass of water vapor per unit volume of air (g/m³). Specific humidity is more commonly used in meteorology because it remains constant with temperature changes.

Specific humidity typically ranges from near 0 g/kg in extremely cold, dry conditions to about 30 g/kg in hot, tropical conditions. In temperate climates, values usually range from 5-15 g/kg. The maximum possible specific humidity depends on temperature and pressure.

As altitude increases, atmospheric pressure decreases, which reduces the air's capacity to hold water vapor. This typically results in lower specific humidity values at higher altitudes, even if the relative humidity remains the same. This is why mountain climates often feel drier than low-altitude areas at the same temperature.

References & Disclaimer

Scientific Disclaimer

This calculator provides estimates based on standard meteorological formulas. For precise scientific or engineering applications, consult with a qualified meteorologist or use professional-grade equipment and software.

References

Accuracy Notice

The accuracy of this calculator depends on the precision of input values. The Magnus formula for saturation vapor pressure has an accuracy of approximately ±0.3% for temperatures between -45°C and 60°C. For extreme conditions or professional applications, use more sophisticated calculation methods or direct measurements.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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