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Unit 4: Temperature-Moisture Relationship

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Temperature & Heat   ::   Temperature & Altitude   ::   Atmospheric Moisture   ::   Relative Humidity   ::   Clouds   ::   Exercises

Determining Relative Humidity

Instruments to Measure Temperature and Relative Humidity

Instrument Shelter

Instrument Shelter

Psychrometer : Our next objective is to teach you how to determine the relative humidity of the air. You will need an instrument called a psychrometer. The instrument pictured below is one kind of psychrometer. It has two thermometers, one with a cotton wick that is wetted. The thermometer with the wet cotton wick will give wet bulb readings; the other will give dry bulb readings. The wet bulb must be ventilated with a small fan or other means to cause rapid evaporation of the water in the wick. Heat is required to change water to vapor. Thus the wet bulb will be cooled as it is ventilated and evaporation occurs. When the lowest reading is obtained on the wet bulb thermometer, read and record temperatures from both thermometers. These readings will be entered into a special table to obtain relative humidity.

Psychrometer

The psychrometer shown is battery powered but requires some manual operation. There also are automatic ways of determining and recording temperatures and relative humidity. The hygrothermograph (pictured below) measures and records both temperature and relative humidity on a continuous recording graph. This instrument is utilized by placing it in a specially designed field instrument shelter like the one shown at the top of the page. Certain standards have been established to insure uniformity of measurements, for example, the shelter will be positioned 4 feet above the ground in a non-shaded, open area that has good air ventilation. Such shelters and weather instruments are found at every weather station.

Hygro-Thermograph

Hygro-Thermograph

Psychrometric Table

Determining relative humidity and dew point temperature from a psychrometric table:

  1. Select the proper table for your elevation.
  2. Obtain dry and wet bulb temperature readings from a psychrometer.
  3. Find the wet bulb temperature at top of table, then move down that column.
  4. Find the dry bulb temperature at left side of table, then move horizontally to right.
  5. Find the intersection of wet bulb column and dry bulb line, which is a block with two numbers.
  6. The lower number in block is relative humidity, the upper is dew point.

Dew Point Temperature

Use dew point temperature to determine relative humidity at various temperatures within a fixed or stationary air mass:

  1. Locate wet and dry bulb temperature readings on psychrometric table.
  2. At intersection of these entries, determine the relative humidity and dew point.
  3. Although relative humidity will change with changes in temperature, the dew point will remain nearly the same.
  4. Select a new dry bulb temperature and locate its line in the table.
  5. Move horizontally to the right until you find the block with the same dew point as 2 above.
  6. The relative humidity number in the block with that dew point is your new relative humidity.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 27). Unit 4: Temperature-Moisture Relationship. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_4__Temperature-Moisture_Relationship_5.html. This work is licensed under a Creative Commons License Creative Commons License