Specific Humidity

Definition

Specific humidity is a measure of the amount of water vapor present in the air, expressed as the ratio of the mass of water vapor to the total mass of the air-water mixture. It is an important parameter in understanding the behavior of air, particularly in the context of humidity, evaporation, and boiling.

5 Must Know Facts For Your Next Test

1. Specific humidity is measured in grams of water vapor per kilogram of moist air (g/kg) or kilograms of water vapor per kilogram of moist air (kg/kg).
2. Specific humidity is a conserved quantity, meaning it remains constant during many processes, such as cooling or heating of air, as long as no water is added or removed.
3. Specific humidity is an important factor in determining the rate of evaporation, as it represents the driving force for the transfer of water vapor from a surface to the surrounding air.
4. The relationship between specific humidity and boiling point is inverse: as specific humidity increases, the boiling point of water decreases, and vice versa.
5. Specific humidity is a key parameter in the study of psychrometrics, which is the study of the physical and thermodynamic properties of air-water vapor mixtures.

Review Questions

• Explain how specific humidity relates to the process of evaporation.
  • Specific humidity is a crucial factor in determining the rate of evaporation. The difference in specific humidity between the evaporating surface and the surrounding air creates a driving force for the transfer of water vapor from the surface to the air. As the specific humidity of the air increases, the rate of evaporation decreases, as the air becomes more saturated with water vapor. Conversely, as the specific humidity of the air decreases, the rate of evaporation increases, as the air has a greater capacity to absorb water vapor from the surface.
• Describe the relationship between specific humidity and the boiling point of water.
  • The relationship between specific humidity and the boiling point of water is inverse. As the specific humidity of the air increases, the boiling point of water decreases. This is because the presence of water vapor in the air reduces the partial pressure of the air, which in turn lowers the total pressure exerted on the surface of the water. According to Dalton's law of partial pressures, a lower total pressure results in a lower boiling point. Conversely, as the specific humidity decreases, the boiling point of water increases, as the total pressure on the water's surface rises.
• Analyze the role of specific humidity in the study of psychrometrics and its applications in various fields.
  • Specific humidity is a key parameter in the field of psychrometrics, which is the study of the physical and thermodynamic properties of air-water vapor mixtures. Psychrometrics is widely used in fields such as HVAC (Heating, Ventilation, and Air Conditioning) engineering, meteorology, and agriculture to understand and predict the behavior of air-water vapor systems. By understanding the relationship between specific humidity, temperature, and other psychrometric properties, professionals in these fields can design and optimize systems for air conditioning, drying, and moisture control, as well as analyze and forecast weather patterns and agricultural conditions more accurately.

Related terms

Absolute Humidity:

The total mass of water vapor present in a given volume of air, regardless of the total volume of the air-water mixture.

Relative Humidity: The ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature and pressure.

Dew Point:

The temperature at which the air becomes saturated with water vapor, and any further cooling will result in the condensation of water vapor into liquid water.