Lecture Notes: Relative Humidity

Importance of Water Vapor

• Water vapor is crucial in the Earth's atmosphere.
• Enters atmosphere via evaporation, forms clouds, moves over land, exits as precipitation.
• Water vapor usually stays in atmosphere for less than a week.
• Horizontal movement of water vapor is known as advection.

Measurement and Distribution

• Water vapor measured using a hygrometer.
• Global distribution varies by latitude and season.
• Ranges from 4% by volume to nearly none in coldest areas.

Conditions for Evaporation

• Requires water at the surface.
• Positive relationships:
  • More water availability leads to more evaporation.
  • Warmer water facilitates easier escape into atmosphere.
  • Drier atmosphere increases surface evaporation.
  • Faster wind speeds can force water into the air.

Key Concepts in Humidity

• Evaporation: Water changing from liquid to gas.
• Specific Humidity: Mass of water vapor per total mass of air.
• Mixing Ratio: Mass of water vapor per mass of dry air. Measured in grams per kilogram.

Saturation and Relative Humidity

• Saturation: Atmosphere holds all water vapor possible for a given temperature.
• Relative Humidity: Measure of saturation in the atmosphere.
  • Closer to saturation, higher relative humidity.
  • Relationship with air temperature displayed on the Clausius-Clapeyron curve.
  • As temperature increases, potential water holding capacity of atmosphere increases.

Clausius-Clapeyron Curve

• Shows relationship between air temperature and saturation.
• 100% relative humidity occurs at point of saturation (condensation begins).
• Below saturation line = unsaturated (RH < 100%).
• Saturation Mixing Ratio: Potential water holding capacity for a given temperature.

Dew Point Temperature

• Temperature at which condensation or dew begins.
• Indicates saturation; higher dew point = more water vapor.
• Given in Fahrenheit, Celsius, or Kelvin.

Understanding Relative Humidity

• Ratio of actual moisture to potential moisture at saturation.
• Expressed as a percentage.
• Relationship with air temperature:
  • Warmer temperatures lead to lower RH.
  • Colder temperatures lead to higher RH.
• Diurnal changes in RH due to temperature fluctuations.