Earth Science Laboratory

METEOROLOGICAL INSTRUMENTS


Objectives:
  1. To become familiar with some of the instruments used to collect weather data.
  2. To collect weather data using instruments provided.
  3. To interpret the data collected.
Questions to prepare you:
  1. We know there has to be a cloud before it can rain or snow, but what is a cloud?
  2. Why does ice form inside your home freezer?
  3. Following a shower of rain, why do the puddles of water dry up eventually?
  4. What does air pressure have to do with the inflation of a tire on your car or bicycle?
  5. Why our eardrums "pop" when we take an elevator to the top of a high building?
  6. Does air pressure change when we move horizontally in the atmosphere, just as it changes when we move vertically?

RELATIVE HUMIDITY

    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.
 
Dry bulb temperature: Final 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.
 
 
 
 
 
 
 
 
 
 
 
 
 

 


Percent Relative Humidity Table
at a Barometric Pressure of 30 inches Mercury


  40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104    
W 100 85 71 58 47 38 29 22 16 10 5                                             40 W
  100 86 72 60 49 40 32 25 18 13 8 4                                         42 E
T     100 86 73 61 51 42 34 27 21 16 11 7 3                                     44 T
        100 86 74 63 53 44 37 30 24 18 14 10 6 3                                 46  
        100 87 75 64 55 46 39 32 26 21 16 12 9 5 3                             48 B
          100 87 76 65 56 48 41 34 29 23 19 15 11 8 5 3                         50 U
            100 88 76 66 58 50 43 36 31 25 21  17 13 10 7 5 3                     52 L
              100 88 77 68 59 51 44 38 33  28 23 19 16 12 10 7 5 3 1               54 B
                  100 88 78 69 60 53 46 40 34 29 25 21 18 14 12 9 7 5 3             56  
                  100 89 79 70 61 54 48 42 36 31 27 23 20 16 14 11 9 7 5 3         58 T
                    100 89 79 71 62 55 49 43 38 33 29 25 21 18 15 13 11 9 7 5       60  E
                      100 90 80 71 64 57 50 44 39 35 30 26 23 20 17 15 12 10 8 7 5   62 M
                        100 90 80 72 65 58 51 46 41 36 32 28 25 22 19 16 14 12 10 8 7 64 P
                          100 90 81 73 65 59 53 47 42 37 33 30  26 23 20 18 15 13 11 10 66 E
                            100 90 82 74 66 60 54 48 43 39 35  31 28 24 22 19 17 15 13 68 R
                              100 91 82 74 67 61 55 49 44 40  36 32 29 26 23 21 18 16 70 A
                                100 91 82 75 68 61 56 50 46 41 37 33 30 27 24 22 20 72 T
                                  100 91 83 75 69 62 57 51 47 42 38 35 32 28 26  23 74 U
                                    100 91 83 76 69 63 57 52 48 43 39 36 33 30 27 76 R
                                      100 91 84 76 70 64 58 53 49 44 40 37 34 31 78  E
                                          100 92 84 77 70 66 59 54 50 45 41 38 35 80  
d                                           100 92 84 77 71 65 60 55 50 46 42 39 82 d
e                                             100 92 84 78 72  66 61 55 51 47 43 84 e
g.                                               100 92 85 78 72 66 61 56 52 48 86 g
                                                  100 92 85 79 73 67 62 57 53 88  
F                                                   100 92 85 79 73 68 62 68 90 F
  40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104    

DRY-BULB TEMPERATURE, oF.

In the above example, the dry-bulb (room) temperature was 76 oF, and the wet-bulb temperature was 56 oF;
the matrix above shows that the relative humidity of the room was 25%.

 

DEW-POINT TEMPERATURE TABLE
(at barometric pressure of 30 inches of mercury)

[Read the temperature numbers inside the unbolded matrix]
Depression of the Wet-Bulb Thermometer, oF. (difference from air temp.)


1 2 3 4 5 6 7 8 9 10 15 20 25 30 35
A 20 16 12 8 2 -7 -21
I 25 22 19 15 19 5 -3 -15 -51
R 30 27 25 21 18 14 8 2 -7 -25
35 33 30 28 25 21 17 13 7 0 -11
T 40 38 35 33 30 28 25 21 18 13 7
E 45 43 41 38 36 34 31 28 25 22 18
M 50 48 46 44 42 40 37 34 32 29 26 0
P 55 53 51 50 48 45 43 41 38 36 33 15
E 60 58 57 55 53 51 49 47 45 43 40 25 -8
R 65 63 62 60 59 57 55 53 51 49 47 34 14
A 70 69 67 65 64 62 61 59 57 55 53 42 26 -11
T 75 74 72 71 69 68 66 64 63 61 59 49 36 15
U 80 79 77 76 74 73 72 70 68 67 65 56 44 28 -7
R 85 84 82 81 80 78 77 75 74 72 71 62 52 39 19
E 90 89 87 86 85 83 82 81 79 78 76 69 59 48 32 1
95 94 93 91 90 89 87 86 85 83 82 74 66 56 43 24
o 100 99 98 96 95 94 93 91 90 89 87 80 72 63 52 37
F 105 104 103 101 100 99 98 96 95 94 93 86 78 70 61 48

(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.