What is Relative Humidity

1
What is Relative Humidity?

• Air is said to be saturated when it reaches the
  maximum amount of water vapor it can hold at its
  current temperature and air pressure (warm air
  can hold more water vapor than cool air)
• Relative humidity is a measure of the amount of
  water vapor in an air sample relative to its
  value if the air were saturated
• When air is saturated, water droplets or ice
  particles begin to form

2
Why study Relative Humidity?

• Hydrologic Cycle
• Measures how much water is in the air
• Influences rates of evaporation and
  evapotranspiration.
• Key element of local climate
• Energy Cycle
• Allows calculation of the dew point (the
  temperature at which dew would form)
• Dew point influences daily temperature range
• Amount of water vapor in the air influences the
  formation of storms

3
How to measure Relative Humidity
Digital Hygrometer

• Digital hygrometers use ceramic and metallic
  compounds to measure electrical resistance
• Directly read relative humidity
• Works below freezing
• Needs at least 30 minutes to
  adjust to outside conditions
• Can be ruined by condensation
• Must be kept indoors or
  in a dry container (e.g., a can
  with rice and a tight lid)
  

4
How to measure Relative Humidity
Sling Psychrometer

• Use two thermometers that are whirled by hand.
• The wet bulb thermometer has a wet wick
• attached that is cooled by
  
• evaporation when the device
• is whirled.
• The amount of evaporation
• depends on the relative humidity
• and the air temperature
• Relative humidity is calculated
• using the temperatures of the
• wet bulb and dry bulb thermometers

5
Key Points

• Significant Values
• The most common range of values of relative
  humidity is 20 - 95
• When there is rain or fog 100 is reported
  (without taking a measurement)
• When and Where
• Daily, preferably between one hour before and two
  hours after local solar noon
• Beside the instrument shelter