|Earth Science Laboratory
Except in a few isolated situations, the atmosphere always contains moisture in the form of water vapor. This water vapor has a direct effect upon our well-being and comfort. In the summer, often a feeling of uncomfortable warmth is experienced, even though the temperature is not very high. Such terms as "muggy," "close," or "humid" are applied to such days. The atmosphere may be considered a solution with water vapor as a solute or dissolved substance. As with most solutes, there is a limit to the amount that can be dissolved in a given volume of solvent. When this limit is reached with regard to the water vapor in the atmosphere, the air becomes saturated. Further addition of water vapor or any drop in temperature will result in condensation of the excess above the amount of moisture required for saturation. Low concentration of moisture, as is often found indoors in the winter, produces an uncomfortable feeling of coolness, drying of the skin, irritation of the nasal and throat passages, and unpleasant manifestation of statis electricity.
From this brief explanation, it can be seen that the moisture content of the air,or humidity, is important to the health and comfort of an individual. It is also of importance in forecasting the weather. This exercise is an attempt to provide an opportunity to make some quantitative measurement of humidity. The amount of water vapor in a particular atmosphere is most usually measured in terms of relative humidity, that is, the ratio of the amount of water vapor actually present to the amount that would be present if the air were saturated (100% relative humidity) at that temperature. The method of such measurement is known as hygrometry. Any device that is used to make this determination is called a hygrometer. Some of the common pieces of apparatus for this purpose are the hair hygrometer, the wet and dry bulb thermometers, the sling psychrometer, and an apparatus for determining the dew point (temperature at which the air is saturated with water vapor, i.e., relative humidity is equal to 100%).
|THE SLING PSYCHROMETER
Your instructor will demonstrate the correct and safe usage of the sling psychrometer, an instrument that indirectly measures the relative humidity of the room, consisting of two identical thermometers mounted on a frame with a pivoting handle that allows the thermometers to be twirled. One thermometer has a cloth "sock" or wick wrapped on the low end - this is designed to be wetted - therefore, this is called the wet bulb thermometer. The other thermometer has no wick on the end; it records the room's air temperature and is called the dry bulb thermometer.
Record the dry bulb temperature on your lab sheet below before proceeding. Also check to make sure that the wet bulb thermometer (not yet wetted) has a similar temperature. Take care not to touch the bottom end of either thermometer, as the heat from your hands will give you an incorrect reading. Then, go to the laboratory sink and wet the wick on the end of the wet bulb thermometer. It is not necessary to drench the thermometer.
The thermometers contain mercury, which is a hazardous
substance, and proper technique for handling the sling psychrometers must
be used to prevent breakage of the glass and spillage of mercury. The correct
and safe technique for handling the sling psychrometer is to stand
at least three feet away from any object or person, grasp the handle
of the sling psychrometer with your arm extended straight outward from
your body (NOT over your head!), and twirl the thermometers with a moderately
rapid circular motion. The twirling causes the water in the wick to evaporate,
which removes heat from the wet bulb thermometer, causing the temperature
of the wet bulb to decrease. Continue to twirl the device until the wet
bulb temperature stabilizes. Then record the final wet bulb temperature
below. You should note that the time of year and the existing humidity
of the room influences the rate and amount of evaporation from the wet
bulb wick, and thus, the amount of depression of the wet bulb temperature.
FINDING RELATIVE HUMIDITY FROM SLING PSYCHROMETER READINGS
Go to the "Percent Relative Humidity Table" and match up your dry bulb temperature (horizontal axis) against the wet-bulb temperature (vertical axis). Find the column that best matches your dry bulb temperature (if it does not match exactly, choose the two closest columns). Then, find the row(s) that best match your wet bulb temperature. The numbers inside this table are relative humidity in percent. Find the number where your column and row intersects - that is your calculated relative humidity.
FINDING THE DEW POINT
The dew point is the temperature at which the existing water vapor in the air will represent saturation (that is, 100% relative humidity). Go to the "Dew Point Temperature Table." Note that on this chart, the vertical axis is the air temperature (dry bulb temperature), which the horizontal axis is the "depression of wet-bulb temperature" - that is, the drop in temperature of the wet bulb. Find the best match(es) of row and column; the number at their intersection is the dew point temperature.
READING THE MERCURY BAROMETER
One of the most important elements in data is weather forecasting is atmospheric pressure. When atmospheric pressure values are progressively increasing, they signal the approach of a high pressure center towards your location, generally indicating fair weather. Conversely, when atmospheric pressure values decrease, they signal the approach of a low pressure center, usually meaning stormy weather.
Air pressure is measured by an instrument called a barometer, which can be considered as a weighing device. Although air is very light in weight, compared to water and rocks, a column of air has weight that is responsible for the pressure that is exerted.
The standard atmospheric pressure is recorded at sea level and is 1.013 bars. Because atmospheric pressure changes associated with normal weather are usually very slight, a smaller sub-unit that is 1/1000 of a bar, called the millibar is used. Standard atmospheric pressure is then 1,013 millibars.
Another measurement unit used in association with the mercury barometer is "inches of mercury," in reference to the height of a column of mercury contained inside the glass tube of this wall-mounted device. It is this device that weather forecasters refer to when they report atmospheric pressure in terms of inches of mercury. Since liquids cannot be compressed in response to changes in atmospheric pressure, the mercury column rises when air pressure increases and presses down on the mercury reservoir at the bottom of the glass column. Conversely, lower air pressure exerts less pressure on the mercury reservoir, and the height of the mercury column decreases.
To properly read the height of the mercury column, it is necessary to
stand at eye height with the top of the mercury column. Depending upon
the height at which the mercury barometer has been mounted on the wall,
you may need to use a step stool to be at the correct height. Mounted on
the metal casing of the glass tube is a sliding metal caliper with measurement
markings. The edge of this sliding caliper is operated by a thumb screw
which raises and lowers the caliper. You must carefully lower the edge
of the caliper so that it just intersects the curved upper surface ("meniscus")
of the mercury column. This will give you the height of the mercury column
in inches, to the nearest 0.1 inch. To read to the next nearest digit (0.01),
read the left side of the caliper. Only two markings on either side of
the slide caliper will match up across; this will give you the digit in
the hundredths column.
DRY-BULB TEMPERATURE, oF.
In the above example, the dry-bulb (room) temperature
was 76 oF, and the wet-bulb temperature was 56 oF;
[Read the temperature numbers inside the unbolded matrix]
(Adapted from "Elements of Meteorology, Miller & Thompson, Merrill Publishing Co., 1970)
In the above example, at an air temperature of 75 oF., the
wet-bulb temperature was 65 oF., which is a 10 degree difference;
therefore, the dew-point temperature is 59 oF.