Physical Science Activity
Relative Humidity and Barometric Pressure
- Understand the meaning of relative humidity
- Understand the connection between relative humidity and "comfort"
- Measure the relative humidity
Relative humidity is an important indicator
weather conditions and is easy to measure using a device called a psychrometer.
Relative humidity is different from absolute humidity and it
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
One thermometer's bulb is surrounded with a cotton wick which is kept
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
Evaporation from the wick surrounding the wet-bulb thermometer cools
thermometer. The lower the relative humidity in the air, the more
evaporation from the wick, and the lower the temperature of the
The relative humidity is found from a table.
- What do you think the relative humidity is today?
- What do you expect the relative humidity is outdoors today?
higher, lower,... as the indoors relative humidity
- What do expect the relative humidity value in the early morning
is often fog?
Construct a psychrometer.
- Two glass/mercury thermometers
- Apparatus stand and clamp
- Cotton gauze to wrap one thermometer's bulb
- room-temperature water to wet the wick
- notebook or cardboard
- Relative humidity table/chart
- Wrap the cotton gauze around one the bulb of one
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
- Support both thermometers with the apparatus stand and clamps.
- Dip the wick-coated thermometer into a jar of water momentarily
the wick. Do not leave the wet thermometer in the water, but
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
twin thermometer set-up.
- Vigorously fan the twin thermometers with a thin notebook,
other device and read the two thermometers. Continue fanning
the wet-bulb thermometer ceases to become any cooler. Record both
temperatures in your notes.
- Look up in the table
the value of the relative humidity from the wet and dry thermometer
Compare with your predictions.
- Measure the relative humidity outdoors and compare with your
Be careful to keep your thermometers out of the direct sunlight or you
will get erroneous results.
- Observe the haze on this day. Note the visibility of
and estimate the visibility. The instructor will help.
- Explain on the basis of energy why the wet bulb thermometer
even when the initial water temperature is the same as the ambient
See the text.
- What is meant by 100 percent relative humidity?
- What is meant by 100 percent absolute humidity?
- Why does one feel so uncomfortable on days of high relative
- How does the outdoor relative humidity compare with the indoor RH?
If you miss the regular class, you are expected to
the experiment by measuring the the relative humidity on several
days of varying haze or visibility and/or at different times of day
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
Part II. Barometric Pressure
The barometric pressure is a very important part of understanding
The changes in barometric pressure indicate the next day's weather as
as global air circulation. The pressure is caused by the weight
air above us. The weight of air depends on two primary factors:
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
as the major weather systems change. We will observe both.
The barometric pressure is measured by in instrument called a
A barometer consists of an evacuated spring-loaded diaphragm. See
drawing in textbooks and in class. As the air pressure on the
of the chamber changes, the diaphragm shrinks or expands. This
change is either magnified by an indicator needle (pure mechanical) or
measured electronically by means of a strain gauge. We will use
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
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
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.
- Record the voltage reading of the instrument just outside the
physics lab on the ground.
- Take it to the 2nd floor fire escape, keeping it shaded, and make
a reading there.
- 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.
- Predict the altitude change as you go from the physics lab to the
on the Swannanoa River, and predict the new voltage.
- Carry the instrument to the Swannanoa River bridge and record the
calculate the difference in elevation between the Swannanoa River
and the physics lab and compare with your prediction.
- Carry the instrument to Kittridge (the intersection of campus
WWC road) and record the voltage.
- Calculate the elevation difference between the river and
- Compare your calculated elevation change between the river and
and compare with the readings from a USGS map. (Maps in Morse
Pressure vs. day of week and weather pattern.
- Set-up a recording barometer (called a barograph) to begin on
The recording barometer uses a graph paper on a drum that rotates once
- Predict the daily diurnal pattern in the barograph (if any) that
daily variation. Remember the temperature warms in the daylight
cools at night and decide if this influences the atmospheric pressure.
- Predict the variation in barograph if we get a rainstorm or
- Note today's weather in your notes, and note the weather pattern
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
the weather and the barograph reading.