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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measures actual amount of water vapor in a given volume of air, usually expressed in g/m3
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
represents ratio of actual water vapor pressure to at a given temperature, expressed as a
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
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 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 () 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
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
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
relates saturation vapor pressure (es) to temperature (T)
es=e0exp(RvLv(T01−T1))
e0: reference vapor pressure at temperature T0
Lv: latent heat of vaporization
Rv: gas constant for water vapor
Relative humidity (RH) calculated from actual vapor pressure (e) and saturation vapor pressure (es)
RH=ese×100%
Specific humidity (q) determined by vapor pressure (e) and total atmospheric pressure (p)
q=p−0.378e0.622e
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