5.1 Water vapor and humidity measurements

Water vapor is crucial in atmospheric science. It affects weather patterns, climate, and energy transfer. This section explores different humidity measurements, instruments used to detect moisture, and how humidity varies across space and time.

Understanding humidity calculations is key for meteorologists and climatologists. We'll look at important equations like Clausius-Clapeyron and how to use psychrometric charts. These tools help predict weather and analyze climate trends.

Water Vapor and Humidity

Types of humidity measurements

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• Absolute humidity measures actual amount of water vapor in a given volume of air, usually expressed in $g/m^3$
• Specific humidity calculates ratio of water vapor mass to total mass of moist air, expressed in $g/kg$
  • Remains constant unless water vapor is added or removed from the air parcel
• Relative humidity represents ratio of actual water vapor pressure to saturation vapor pressure at a given temperature, expressed as a percentage
  • Indicates how close the air is to being fully saturated with water vapor (100% relative humidity)
  • Changes with temperature fluctuations even if absolute amount of water vapor remains constant
• Dew point temperature defines the temperature at which air becomes saturated (reaches 100% relative humidity) when cooled at constant pressure
  • Directly measures absolute moisture content of the air
  • When air temperature cools to the dew point, water vapor condenses into liquid water droplets (dew, fog, clouds)

Instruments for humidity detection

• Hygrometers directly measure humidity levels
  • Capacitive hygrometers utilize a thin polymer film that absorbs or releases water vapor as relative humidity changes, altering the film's capacitance which is measured and converted to relative humidity value
  • Resistive hygrometers employ a hygroscopic material whose electrical resistance varies with humidity levels, allowing resistance measurement to be converted to relative humidity
• Psychrometers indirectly measure humidity by comparing the cooling effect of evaporation between two thermometers
  • Consist of a dry-bulb thermometer measuring air temperature and a wet-bulb thermometer with a moistened wick around its bulb
  • As water evaporates from the wet-bulb wick, it cools the thermometer
  • The difference in temperature readings between the dry-bulb and wet-bulb (wet-bulb depression) is used to calculate relative humidity or dew point
• Dew point sensors directly measure the dew point temperature
  • Chilled mirror hygrometers use a cooled mirror surface and optical sensor to detect condensation
  • The mirror is progressively cooled until water droplets form on its surface, with the temperature at which condensation occurs representing the dew point

Patterns of atmospheric humidity

• Spatial variations in humidity levels
  • Generally decreases with increasing latitude as colder temperatures reduce evaporation rates
  • Higher humidity over oceans and large water bodies compared to land surfaces due to increased evaporation
  • Coastal regions typically experience higher humidity than inland areas
  • Tends to decrease with increasing elevation as air pressure and temperature drop
• Temporal variations in humidity
  • Diurnal changes see humidity increase at night and decrease during the day in response to temperature fluctuations
  • Seasonal changes result in higher humidity in summer and lower in winter, especially in mid-latitude regions
  • Weather patterns influence humidity levels
    • Low-pressure systems, fronts, and convective activity often associated with high humidity
    • High-pressure systems and clear, stable conditions typically bring lower humidity
• Atmospheric processes impacting humidity
  • Evaporation from water surfaces and transpiration from plants add moisture to the atmosphere, increasing humidity
  • Condensation and precipitation remove moisture from the air, decreasing humidity
  • Advection transports humid or dry air masses between regions, altering local humidity levels

Calculations with humidity data

• Thermodynamic equations quantify relationships between humidity, temperature, and pressure
  • Clausius-Clapeyron equation relates saturation vapor pressure ($e_s$) to temperature ($T$)
    • $e_s = e_0 \exp\left(\frac{L_v}{R_v}\left(\frac{1}{T_0}-\frac{1}{T}\right)\right)$
      • $e_0$: reference vapor pressure at temperature $T_0$
      • $L_v$: latent heat of vaporization
      • $R_v$: gas constant for water vapor
  • Relative humidity ($RH$) calculated from actual vapor pressure ($e$) and saturation vapor pressure ($e_s$)
    • $RH = \frac{e}{e_s} \times 100\%$
  • Specific humidity ($q$) determined by vapor pressure ($e$) and total atmospheric pressure ($p$)
    • $q = \frac{0.622e}{p - 0.378e}$
• Psychrometric charts graphically represent relationships between temperature, humidity, and thermodynamic properties of moist air
  • Used to determine humidity parameters (relative humidity, specific humidity, dew point) by plotting dry-bulb and wet-bulb temperatures on the chart and following the lines to the desired value