Physical Science Activity

Relative Humidity and Barometric Pressure



    Relative humidity is an important indicator of weather conditions and is easy to measure using a device called a psychrometer.  Relative humidity is different from absolute humidity and it is very important to understand the difference.  Consult the textbook to read about the differences.  Click HERE for an on-line explanation.

A psychrometer is easily constructed by means of two thermometers.  One thermometer's bulb is surrounded with a cotton wick which is kept moist by momentarily dipping the wick in water.  This is called a wet-dry-bulb psychrometer.   See diagram below.

The relative humidity is measured by reading the two thermometers.  Evaporation from the wick surrounding the wet-bulb thermometer cools the thermometer.  The lower the relative humidity in the air, the more evaporation from the wick, and the lower the temperature of the wet-bulb.  The relative humidity is found from a table.


  1. What do you think the relative humidity is today?
  2. What do you expect the relative humidity is outdoors today?  The same, higher, lower,... as the indoors relative humidity
  3. What do expect the relative humidity value in the early morning when there is often fog?


Construct a psychrometer.


  1. Wrap the cotton gauze around one  the bulb of one thermometer.  Secure the gauze with a piece of wire or bread-bag "twistee".  Be careful not to get the gauze too thick or the evaporation of water will be impeded.
  2. Support both thermometers with the apparatus stand and clamps.
  3. Dip the wick-coated thermometer into a jar of water momentarily to wet the wick.  Do not leave the wet thermometer in the water, but remove it from the water jar so that the water may evaporate.  Be careful not to wet the un-wrapped thermometer.  You now have a "wet and dry-bulb" twin thermometer set-up.
  4. Vigorously fan the twin thermometers with a thin notebook, cardboard, or other device and read the two thermometers.  Continue fanning until the wet-bulb thermometer ceases to become any cooler.  Record both temperatures in your notes.
  5. Look up in the table the value of the relative humidity from the wet and dry thermometer results.  Compare with your predictions.
  6. Measure the relative humidity outdoors and compare with your predictions.  Be careful to keep your thermometers out of the direct sunlight or you will get erroneous results.
  7. Observe the haze on this day.  Note the visibility of distant mountains and estimate the visibility.  The instructor will help.


  1. Explain on the basis of energy why the wet bulb thermometer becomes cooler, even when the initial water temperature is the same as the ambient temperature.  See the text.
  2. What is meant by 100 percent relative humidity?
  3. What is meant by 100 percent absolute humidity?
  4. Why does one feel so uncomfortable on days of high relative humidity?
  5. How does the outdoor relative humidity compare with the indoor RH?

Extra project.

    If you miss the regular class, you are expected to make-up the experiment by measuring the the relative humidity on several different days of varying haze or visibility and/or at different times of day (before 8:00 am when there is often morning fog) and mid afternoon.  It is important to correlate the relative humidity with the haze/fog/rain conditions.

Part II.  Barometric Pressure


The barometric pressure is a very important part of understanding weather.  The changes in barometric pressure indicate the next day's weather as well as global air circulation.  The pressure is caused by the weight of air above us.  The weight of air depends on two primary factors: 1) how much air is above us - which is larger or smaller depending on our altitude above sea level; and 2) whether the air is ascending or descending as the major weather systems change.  We will observe both.

The barometric pressure is measured by in instrument called a barometer.  A barometer consists of an evacuated spring-loaded diaphragm.  See drawing in textbooks and in class.  As the air pressure on the outside of the chamber changes, the diaphragm shrinks or expands.  This dimensional change is either magnified by an indicator needle (pure mechanical) or measured electronically by means of a strain gauge.  We will use both.

Pressure vs. elevation.

Take the "homebuilt" microbarometer.  The voltmeter on this device measures small changes in the barometric pressure.  The pressure gauge voltage output has been compared with a standard reference voltage, and the difference has been amplified (multiplied by about 10X).  The sensitivity of this gauges is remarkable.  If you change elevation by one floor level, the voltage reading changes measurably.  You will calibrate this instrument to find the voltage change per meter of elevation.  We will then use this instrument as a simple altimeter (airplanes use a similar device to measure altitude).  This device is also sensitive to temperature and direct sunlight, so we will keep it outdoors and shaded at all times - otherwise it will give erroneous readings.
  1. Record the voltage reading of the instrument just outside the physics lab on the ground.
  2. Take it to the 2nd floor fire escape, keeping it shaded, and make a reading there.
  3. Measure the height change for these two locations with meter sticks, and calculate the change in voltage per meter elevation.  This is the scale factor in V/m.
  4. Predict the altitude change as you go from the physics lab to the bridge on the Swannanoa River, and predict the new voltage.
  5. Carry the instrument to the Swannanoa River bridge and record the voltage, calculate the difference in elevation between the Swannanoa River bridge and the physics lab and compare with your prediction.
  6. Carry the instrument to Kittridge (the intersection of campus road with WWC road) and record the voltage.
  7. Calculate the elevation difference between the river and Kittridge.
  8. Compare your calculated elevation change between the river and Kittridge and compare with the readings from a USGS map.  (Maps in Morse basement).

Pressure vs. day of week and weather pattern.

  1. Set-up a recording barometer (called a barograph) to begin on today's date.  The recording barometer uses a graph paper on a drum that rotates once a week.
  2. Predict the daily diurnal pattern in the barograph (if any) that shows daily variation.  Remember the temperature warms in the daylight and cools at night and decide if this influences the atmospheric pressure.
  3. Predict the variation in barograph if we get a rainstorm or clear, fair weather.
  4. Note today's weather in your notes, and note the weather pattern observed for each day for severa days.  This includes weekend days and days that the class doesn't meet.  See if you can see a correlation between the weather and the barograph reading.