Atmospheric Pressure

Basic Meteorological Observations for Schools: and Shawn B Harley+ Atmospheric Pressure

Abstract and class projects in pressure measurement. This article may also be of interest to weather hobbyists and This article addresses measurement of atmospheric surface others concerned with monitoring atmospheric pres- pressure using economical instruments. It is intended to provide sure. members of the Society with a ready reference to respond to Atmospheric pressure, also called barometric pres- inquiries from earth and physical science teachers at the junior and senior high school levels. We describe various types of barometers, sure, is the force per unit area exerted by the earth's discuss observing techniques, present our experiences with a gravitational pull on the column of air above the point selection of commercially available pressure-measuring equip- in question (Huschke 1959). Because its variations ment, and discuss simple experiments illustrating effects of atmo- with distance imply a force acting on the air, pressure spheric pressure. To assist members in advising teachers inter- is the most important surface-measured variable for ested in meteorology as part of the science curriculum, a few suggestions for student and class projects are included. understanding the weather. If unopposed, this force will accelerate air from higher toward lower pressure. In the vertical, the force due to the variation of pressure 1. Introduction with height is almost always nearly balanced by the gravitational force. (The most common exception is to This article addresses the measurement of atmo- be found beneath rapidly growing cumulonimbus spheric surface pressure with economical instruments.1 clouds, where the buoyancy force must be taken into In the style of earlier articles describing temperature, account.) Due to this near balance between pressure rainfall, and surface wind measurements (Snow and and gravity, vertical accelerations are small and the Harley 1987, 1988; Snow et al. 1989), we give a resulting speeds are slow. Variations of pressure with description of techniques for measuring surface pres- horizontal distance are usually very small, but, acting sure with simple instruments, along with comments on over long distances and long time periods, they cause our experiences in implementing the techniques. We the winds to blow. Variations of pressure with time at also provide a list of appropriate references.2 Our a point on the earth's surface reflect the passage of intent is to provide members of the Society with a highs and lows that appear on surface weather maps ready reference to be used in response to inquiries and of the large waves that move through the upper from teachers of the earth and physical sciences in air. junior and senior high schools, and to aid those who are interested in pursuing educational initiatives de- Additional information on meteorological measurements in schools scribed by Walker (1984), the American Meteorologi- and further suggestions for student projects are to be found in the cal Society (1985,1989), and Snow and Smith (1990). books by Haynes (1947), Peter (1964), Smith (1966), Trowbridge To assist members in advising teachers interested in (1973), and Couchman et al. (1977), and in the pamphlets by Geer including aspects of meteorology in science curricula, (1975), Pedgley (1980), and the U.S. Department of Commerce we also provide suggestions for experiments illustrat- (1979). Berry (1961) and Monroe (1980, 1983) describe useful experiences in establishing and operating a weather station. Three ing effects of atmospheric pressure and for student particularly valuable references for the precollege teacher are the manual by Neuberger and Nicholas (1962), the field guide by Schaefer and Day (1981), and the sourcebook by Parker (1988). The article by Bohren (1983) provides a summary of the physical *Department of Earth and Atmospheric Sciences, Purdue Univer- aspects of pressure. The professional review articles by Mazzarella sity, West Lafayette, Indiana. (1985) and Brock (1985) contain useful information, but may require +National Weather Service Forecast Office, Indianapolis, Indiana. some interpretation or explanation for the nonmeteorologist. The 1The use of trade and company names is for the information and classic text by Middleton and Spilhaus (1953) and the handbooks convenience of the reader. Such use does not constitute an en- produced by the Meteorological Office (1981) and the U.S. Depart- dorsement or approval or disapproval by the authors or by AMS of ment of Agriculture (1976) are invaluable sources for the details on any product to the exclusion of others that may be suitable. the installation and maintenance of many types of conventional ©1992 American Meteorological Society meteorological equipment.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC by the early 1700s they became one of the most popular scientific instruments. The form of barometer invented by Torricelli utilized a column of liquid (usually mercury) in a vertical tube. While there have been many variations on this design, the basic principles remain in use in contemporary, high-quality laboratory barometers. An example of this type of barometer is shown in Fig. 1. Because barometers utilizing liquids in tubes are difficult to move from place to place and require manual adjustment prior to reading, they are not suitable for many applications. Although alternative designs using flexible evacuated chambers to over- come problems related to handling liquids were put forward as early as the 1650s, the technology of the time precluded actual construction. The first practical aneroid barometer (meaning "containing no liquids") was developed and patented by Lucien Vidie in 1844 using ideas expressed initially by Gottfried Leibniz in 1698. An example of the face of this form of barometer is shown in Fig. 2. Aneroid barometers, which require only infrequent manual adjustment, were soon modified by the addi- tion of a clockwork mechanism carrying a paper chart to produce a continuous record of atmospheric pressure. These recording instruments are identified as barographs. Aneroid barometers and barographs con- tinue to enjoy wide popularity in many forms, ranging from household decorations to precision units used in offices of the National Weather Service.

FIG. 1. An example of a high-quality mercurial cistern barometer of the type commonly found in many laboratories and weather stations. A schematic showing the major components of a barometer of this type is given in Fig. 3.

Devices used to measure atmospheric pressure are termed barometers. The first liquid-in-tube barometers were developed by Evangelista Torricelli in the mid-1600s ao a result of attempts to produce a vacuum. FIG. 2. An example of the face of a typical economical aneroid barometer; the interior mechanism of this barometer is shown in Fig. Their utility as instruments to monitor changes in the 5. A schematic showing the major components of a barometer of this atmosphere was quickly recognized, and consequently, type is given in Fig. 4.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC 2. Considerations in the measurement The long-term time average value of mean sea of atmospheric pressure level pressure is 1013.25 hPa (29.92 in-Hg). The normal range of mean sea level pressure in the a. Scales and units middle-to-high latitudes is 970 to 1040 hPa (28.64 to A variety of units are used in the measurement of 30.71 in-Hg) (Kotsch 1983). Occasionally, extreme atmospheric pressure. The traditional meteorological values outside this range are observed. Extreme low unit is the millibar (mb). Inches of mercury (in-Hg) and pressures are generally associated with hurricanes millimeters of mercury (mm-Hg) are other traditional and tornadoes. The record observed low pressure for units found on the scales of many barometers still in North America and its surrounding waters occurred on service and should be used if the situation requires 13 September 1988, when Hurricane Gilbert's central (World Meteorological Organization 1971). In the United pressure dropped to 888 hPa (26.22 in-Hg). Pres- States, both millibars and inches of mercury are in sures in tornadoes are suspected to be lowerthan this, common use (U.S. Department of Commerce 1988). but have never been measured. Extreme high pres- There is a worldwide movement to adopt the sures are generally associated with large, very cold Systeme International des Unites (SI) in all scientific high pressure systems originating at high latitudes. work. The SI unit of pressure is the Pascal. For The extreme high pressure for North America is 1078.6 measuring atmospheric pressure, hectoPascals (hPa) hPa (31.85 in-Hg), observed at Northway, Alaska, on are most convenient because of the ease of conver- 31 January 1989. sion between them and millibars. Some modern ba- Pressure changes at a station are often of great rometers use kiloPascals as a scale.3 interest, as they relate directly to changes in the weather, so a number of terms have come into use to b. Resolution describe them. Changes are always given with re- In the United States, weather station barometers spect to a specified interval of time, typically the 3-h are generally read to the nearest 0.01 hPa (U.S. period of time preceding an observation. The pressure Department of Commerce 1988), or 0.001 in-Hg on change is defined as the net quantitative difference in older instruments with scales marked in this unit. For the pressure between the beginning and the end of the most practical purposes, reading atmospheric pres- specified time interval. The pressure characteristic sure to the nearest 0.1 hPa(0.1 mm-Hg or 0.01 in-Hg) qualitatively describes the pattern of the pressure is satisfactory. change (usually as indicated on a barograph trace) over the same period of time. Table 1 gives the c. Related quantities different pressure characteristics that are used in A barometer reading (corrected to standard condi- standard weather observations. The term pressure tions4 of temperature and gravity, if necessary) at a tendency is used to denote a report of both the amount location is referred to as the station pressure. Since and character of pressure change over the period. atmospheric pressure decreases with height, station During frontal passages, or with a thunderstorm elevation plays a direct role in determining station overhead, pressure at a station can change rapidly pressure. with time. If the pressure oscillates with an amplitude Elevation may vary greatly from station to station. In of at least 1.0 hPa (0.03 in-Hg) about the mean trend order to compare barometric readings made at sev- over a period of a few minutes, it is said to be unsteady. eral locations, meteorologists eliminate the effect of The phrases pressure rising rapidly and pressure station elevation from each reading. For this purpose, falling rapidly are used to describe increases or de- each barometric reading is reduced to mean sea level creases, respectively, at a rate of change of 2.04 hPa pressure, the pressure value that would exist if the (0.06 in-Hg) or more per hour that yield a net change station were at mean sea level (0 meters elevation). of 0.68 hPa (0.02 in-Hg) or more. If the rate of pressure rise exceeds 0.17 hPa (0.005 in-Hg) per minute and if the total rise is 0.68 hPa (0.02 in-Hg) or more, the change is termed a pressure jump. 3Pressure unit conversion factors (under standard conditions of temperature and gravity): 1 mb = 0.0295300 in-Hg; 1 in-Hg = 33.8639 mb; 1 hPa = 1 mb; 1 kPa = 10 mb. d. Barometers and barographs Correction to standard conditions is required for readings of any 1) LIQUID-IN-TUBE BAROMETERS barometer using a liquid column. In the case of readings made using A variety of designs for liquid-in-tube barometers mercurial barometers of the type shown in Fig. 1 for routine weather have been developed over the years. Figure 1 shows observations, standard conditions are defined as a temperature of a mercurial Fortin barometer, a form of cistern barom- 0°C (where the density of mercury is 13.5951 g cm-3) and a gravitational acceleration of 980.665 cm s~2. The correction proce- eter used in many weather stations as a reference dure is discussed in section 2d1. standard. Features of cistern barometers are shown in

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC TABLE 1. Pressure characteristic. A / Rising, then falling

r~ Rising, then steady; / or rising, then rising ' more slowly Barometer now / Rising steadily, higher than / or unsteadily 3 h ago

/ Falling or steady, then . / rising; or rising, then rising ^ more quickly

Steady, same as 3 h ago

Falling, then rising, same V or lower than 3 h ago

\ Falling, then steady; or falling, then falling more slowly

Barometer now Falling steadily, or unsteadily lower than 3 h ago

Steady or rising, then falling; or falling, then falling more A quickly

FIG. 3. The major components of a cistern barometer. As discussed in the text, the length of the sealed tube depends upon the the schematic of Fig. 3. Here the scale and the tube are choice of the working fluid. If provision is not made to adjust the held fixed in a frame; the vertical position of the cistern height of the cistern, then the width of the cistern should be large can be adjusted relative to the tube by the finely compared to the size of the bore of the sealed tube. threaded screw. In a cistern barometer, the pressure due to the it. The variations in the temperature of the barometer weight of the vertical column of air directly above the can lead to expansion and contraction of the working instrument is exerted on the surface of the working fluid, the barometer tube, and other components, fluid in the cistern. This is balanced exactly by the affecting the height of the column. Therefore, a ther- pressure of the column of fluid in the tube, exerted at

the base of the tube where the column enters the 5 surface of the fluid in the cistern. Because the top of the Here, pfluid = mgM = gph, where pfluid = (pressure exerted by the column of fluid in the tube where the tube enters the fluid in the cistern tube is sealed, this pressure is due to the weight of the due to the column of working fluid); mgM = (weight of fluid in the 5 column of working fluid (together with the partial tube)/(cross-sectional area of the tube); and gph = (gravitational pressure of saturated vapor of the working fluid in the acceleration) x (density of working fluid) x (column height). Since the enclosed region above the fluid). If the pressure goes density of the working fluid varies only as a weak function of temperature and since the gravitational acceleration varies only up, fluid flows from the cistern into the tube until a new slightly with location, pressure of the fluid is determined mainly by the equilibrium is established; if the pressure goes down, column height. fluid flows from the tube into the cistern. Thus, the 6More strictly, the height of the column of fluid in the tube is directly height of the column of fluid in the tube is directly proportional to the difference between the pressure of the air on the proportional to the pressure of the atmosphere on the surface of the fluid in the cistern and that exerted by saturated vapor of the working fluid at the top of the column. In the case where surface of the fluid in the cistern.6 mercury is the working fluid, this vapor pressure contribution is very Certain corrections to the reading of a liquid-in- small and usually can be neglected, but where water is used, it is glass barometer must be made in order to standardize large and must be taken into account.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC mometer should be mounted with the barometer and read each time a barometric reading is made. The correction that should be made to the barometer reading is then obtained from a standard table. The value of gravity will also affect the height of the column of liquid. Because the earth's gravity varies with latitude, the station's latitude should be determined. Again, the correction to be made to a barometer reading is obtained from a standard table. The Smithsonian Institution (1966) gives an extensive table for each of these corrections. Abbreviated versions of these tables can be found in other meteorological resource books. (The additional correction required to reduce readings to sea level is discussed in section 3c.)

2) ANEROID BAROMETERS Aneroid barometers are available in a variety of FIG. 5. An internal view of the economical aneroid barometer designs. Figure 2 shows the face of a typical economi- shown in Fig. 2. This model has an external spring—the U-shaped cal aneroid barometer. Featuresoftheinternalworkings piece of metal partially surrounding the corrugated disk that is the evacuated chamber. To the lower left of center is the screw used to common to all aneroid barometers are shown in the occasionally adjust the reading to agree with a reference instrument. schematic of Fig. 4; the realization of this mechanism in the case of the barometer in Fig. 2 is shown Fig. 5. In an aneroid barometer, the pressure due to the and the air remaining in the chamber. The spring may weight of the vertical column of air directly above the be internal to the chamber, as sketched in Fig. 4, or it instrument is exerted on the surface of a flexible, may be external, as in the example in Fig. 5. In some partially evacuated chamber. This chamber is gener- cases, the flexible walls of the chamber provide the ally in the form of a thin disk. The pressure on the spring. In each case, the spring prevents the chamber chamber is balanced exactly by the action of a spring from collapsing under the weight of the atmosphere. When an aneroid barometer is initially assembled, the chamber is compressed slightly, causing the spring to be under tension. If pressure goes up, the chamber is compressed a bit more, thereby increasing tension in the spring until a new equilibrium is established; if pressure goes down, the chamber expands a bit, decreasing tension in the spring. The evacuated chamber is designed so that a change in its volume is directly proportional to a change in pressure. Because of the disk shape of the chamber, most of its flex is in the end walls. The motion of an end wall can be coupled through a mechanical linkage to move an indicator or pen across a previously calibrated scale. Simple barographs usually use two or three evacuated chambers, connected in series, to provide a wider range of motion for the pen across the chart. If several evacuated chambers are connected in series, the resulting compound instrument will produce significant motion for even very small changes in pressure; such an instrument is termed a microbarograph. An example is shown in Fig. 6, where eight chambers are enclosed in the central cylinder. If the scale of an aneroid is laid off in terms of height FIG. 4. The major components of an aneroid barometer shown rather than pressure,.the unit becomes an altimeter. with an internal spring. As noted in the text, the spring required to prevent the evacuated chamber from collapsing can be internal to, This presupposes some relationship between pres- external to, or part of the chamber. sure and height. In practice, the relationship that is

Bulletin American Meteorological Society 785

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC liquid-in-tube barometers. It is not usually practical to attempt to correct for wind-induced errors. Temperature errors are most common in aneroid barometers, particularly inexpensive ones, where the spring constant can change with temperature. Typi- cally, a spring will weaken with an increase in temperature, leading to pressure readings that are too high. Mechanical systems in good quality barometers are composed of several metals of different thermal properties to compensate for changes in spring constant.

3. Measurement of atmospheric pressure

FIG. 6. A typical commercial, microbarograph of the type com- a. Practical points monly found in weather stations. The central cylinder contains eight Meteorological barometers are generally indoor evacuated chambers (similar in appearance to the one in Fig. 5) connected in series. As the chambers expand and contract, the instruments, though some devices intended for use by mechanical linkage moves the pen in an arc across the chart (note surveyors in determining elevations are made to be the curved lines printed on the chart). A clock mechanism internal to used outdoors. Barometers should be mounted in the chart drum rotates the chart one time per week (other times of locations that minimize fluctuations due to the local rotation are possible by changing gears). The two small cylinders on the right third of the instrument are oil-filled dashpots used to environment. Liquid-in-tube barometers are the most suppress wind-induced oscillations. The knob on top of the central sensitive and, hence, the most demanding in their cylinder allows the indicated pressure to be adjusted to agree with mounting requirements. The best conditions are a a reference instrument. permanent mounting on a sturdy interior wall, one with no direct sun, in a room with a uniform temperature used most often is that for a U.S. Standard Atmo- from floor to ceiling. Because such barometers are sphere, 1976 (see U.S. Committee on Extension to inherently fragile, the location should be well away the U.S. Standard Atmosphere 1976). from walkways and heavily used work areas. Ensuring The advantages of the aneroid are "absence of that the tube is vertical is very important. The height of liquid, portability, low weight, and easy adaptation to fluid in both the cistern and the sealed tube must be record data" (Brock 1984). They are generally less read, so the mounting height must be selected with the accurate than a mercurial barometer. Aneroids can be height of the user in mind. Ideally, the height of the made to be very precise (i.e., capable of showing small reference level (see Fig. 3) above mean sea level is changes in pressure), but need to be compared peri- known. odically to a mercurial barometer in order to maintain It is often helpful to mount white cards behind the the accuracy of their calibration (i.e., to maintain their cistern and the tube, next to the scale. If a flashlight ability to indicate actual values of pressure). beam is then shone on the card, reflected light makes it much easier to see the meniscus. e. Errors Mounting requirements for aneroid barometers are Barometers are subject to a variety of errors. Some somewhat less demanding. Many styles can be used can be reduced to a negligible amount if care is used outdoors for short periods, if used carefully. Normally, in siting, mounting, and reading the instrument (e.g., the manufacturer will specify the orientation of the ensuring the tube of a liquid-in-tube barometer is truly barometer face for accurate readings. Most meteoro- vertical; avoiding parallax errors in reading both types logical aneroid barometers are intended for wall mount- of instruments). Other errors are nearly unavoidable— ing and so are read with their faces vertical; aneroid the most common of these are due to wind and barometers and altimeters intended for outdoor use temperature. are generally read with theirfaces horizontal. Because Wind blowing around a building housing a barom- the forces generated within the aneroid by changes in eter can induce pressure changes in the building's atmospheric pressure are quite small, friction in the interior. These fluctuations are superimposed on the mechanism often retards motion of the pointer. It is atmospheric pressure, and with a strong, gusty wind, good practice to lightly tap an aneroid just before a can be as great as 1 to 3 hPa. These "pumping" reading is taken so that the mechanism can move into fluctuations are most noticeable in the more sensitive place.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC b. Frequency of observations mean sea level. This allows a one-time tabulation of a To detect the long-term changes in atmospheric temperature-dependent reduction factor; the appro- pressure associated with passage of a front or a low priate value is selected from the table and again added pressure system, three-hourly observations are usu- to the station pressure to obtain an estimate (accurate ally sufficient. More frequent readings will reveal greater to within a few percent) of the mean sea level or levels of detail. When the barometer is unsteady, such "reduced" pressure. An example of such a tabulation as during a thunderstorm, readings should be taken (for a range of altitudes) is to be found in appendix C every 5 min. of Lutgens and Tarbuck (1989). c. Reduction to sea level Atmospheric pressure decreases with increasing 4. Barometers for school use height. In the first kilometer above sea level, the normal rate of decrease of pressure with height is a. Considerations in selecting barometers for about 0.115 hPa rrr1; in the second kilometer above school use sea level, the rate of decrease is about 0.104 hPa rrr1. While a good quality mercurial barometer gives the Compared to most pressure changes across the earth's best performance in terms of ability to measure air surface due to atmospheric processes, these are very pressure accurately, several factors should be consid- rapid rates of change with distance. As a conse- ered before an instrument of this type is procured or fabricated. Because such barometers have fragile components and contain a large quantity of mercury (a hazardous material), safety is a prime consideration. Aneroid barometers are probably For safety, and to obtain high accuracy in the readings, the most feasible for many school mercurial barometers should be permanently mounted, applications. They are inherently safe, as described in section 3a. Students generally find require no corrections to yield station mercurial barometers difficult to read and the correc- pressure, and are relatively easy for tions for temperature and gravity to be complex and difficult to follow. Mercurial barometers also represent students to read. a significant investment, as even inexpensive models will cost $250; a laboratory-quality instrument can cost more than $1500. Consequently, we recommend that quence, a mapping of station pressures will show such an instrument be procured only after careful pressure variations that are due mainly to differences consideration of its projected use. in station elevation. To reveal horizontal pressure Aneroid barometers are probably the most feasible differences associated with meteorological events, for many school applications. They are inherently the mapping must be of pressures all reduced to sea safe, require no corrections to yield station pressure, level; that is, each reported pressure must be adjusted and are relatively easy for students to read. Many for its station's height above sea level. For this reason, models double as altimeters. Serviceable aneroid weather reports usually contain reduced pressures, instruments are available over a wide range of prices, not station pressures. with several simple instruments available under $100. Various reduction schemes are in use throughout Indeed, it is feasible to consider procuring a good- the world. Each postulates some relationship for the quality aneroid (with an investment of around $500) for variation of temperature with height in a fictitious a laboratory reference and several less-expensive column of air assumed to extend from the station down instruments for hands-on projects by students. to sea level. The weight of this column is then com- A barograph, with its ability to produce a chart puted and added to the observed station pressure. A showing the variations of pressure over time, is a very particularly simple scheme assumes that the column useful classroom aid. The visual impact of the result- has the properties of the"standard atmosphere." This ing picture of pressure variations can be very impor- leads to a "reduction factor," a single number that is tant in communicating the dynamic nature of the added to the observed station pressure. Blackadar atmosphere. While it demands routine maintenance, a (1984) provides a BASIC computer program to com- barograph can document the passage of so many pute this value given the station elevation. A slightly meteorological events (including those that occur out- more complicated scheme uses the current (outdoor) side of regular school hours) that it is well worth the air temperature at the station and the long-term mean effort. The clock in a barograph can be either electric atmospheric rate-of-change of temperature with height or spring driven. We urge that only spring-driven units (a decrease of 6.5°C km-1) to extrapolate down to or electrical ones that contain their own battery be

Bulletin American Meteorological Society 787

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC ing from 983 to 1014 hPa. The dial-type barometers TABLE 2. Summary of tested barometers. were always gently tapped before reading to assure that the pointer was not sticking. Manufacturer Price Pressure Graduation Plots of the pressure reading difference (Pres- model no. ($) range Suretestitem- Pressu ^reference)versus the reference pressure reading (Pressurereference) were made to visually Downeaster 40* 966-1066 mb 2 mb Manufacturing Co. 28.5-31.5 in-Hg 0.1 in-Hg assess the performance of the tested barometers. Average error was computed by averaging the values Eschenbach 50* 800-1100 mb 1 mb of the differences, and average absolute error was Alpin 5000 computed by averaging the absolute values of the differences. Hoffritz 40* 934-1061 mb 1 mb A typical distribution of pressure differences be- 27.5-31.5 in-Hg 0.1 in-Hg tween a barometer and the reference aneroid is shown Huger 50 985-1040 mb 0.5 mb in Fig. 7. Two of the barometers, the Downeaster and the SK #69026-00, exhibited systematic trends. The SK 69026-00 25 95.3-107.4 kPa 0.1 kPa Downeaster read too low for pressures below 1005 mb, with an error reaching 6 mb for pressures below Swift Instruments 29 940-1060 mb 1 mb # Inc. 477 27.75-31.3 in-Hg 0.05 in-Hg 995 mb. The SK 69026-00 read too high for pressures 70.5-79.5 cm-Hg 0.1 cm-Hg below 1000 mb, but the error magnitude only reached 2 mb. The remaining barometers showed no system- ^Estimated cost in 1986 atic trends. For the Swift #477 and the Huger, the observed variation of errors was small, with most errors being on the order of 2 mb or less. The Hoffritz (Fig. 8) and Eschenbach (Fig. 9) showed widely scat- considered, because the building electrical service tered errors that occasionally reached magnitudes of may fail during periods of greatest interest. Baro- 6 mb. graphs are expensive; the most economical unit costs Average errors and average absolute errors7 for the $300, while "good" quality units cost $1250 or more. tested barometers are given in Table 3. The Hoffritz, There are also continuing expenses for charts and Huger, SK, and Swift barometers had average errors other supplies, but these are small. Overall, we believe of less than 1 mb; however, the Hoffritz had a large that its educational value warrants the expense of average absolute error. The Downeaster had an aver- procuring and maintaining at least one barograph as age error of over 3 mb. Both the Downeaster and the part of a school's weather program. Eschenbach had large average absolute errors. The selection of an accurate, economical barom- b. Performance testing of some commercially eter appears to be a difficult undertaking. No apparent available instruments physical feature of the tested barometers seemed to Since use of mercurial barometers in school situa- contribute to the accuracy of the instrument. The tions is limited, only aneroid barometers were tested. barometers that consistently compared best with the We chose five typical aneroid barometers with dial- type faces for testing. These models were representa- tive of aneroid barometers ranging in price from $20 to 7Average error is the algebraic sum of departures from "true" $50. A sixth model used in the testing, the German- readings divided by the number of observations. As the departures can be both positive and negative, there is a tendency to cancel. An made Eschenbach Alpin 5000, has a digital readout. instrument with no systematic difference should yield an average This model is typical of many "digital barometers" now error of zero. Errors in this case are due to random differences in on the market. A summary of the tested barometers is readings between the test item and the reference barometer. On the given in Table 2. other hand, a positive average error means the test item consistently During the test period, the selected barometers reads higher than the reference barometer, while a negative average error indicates that the test item consistently reads lower. were located on the third floor of the geosciences The average absolute error is the arithmetic sum of the magni- building at Purdue University. A laboratory-quality tudes of the departures (i.e., sign is ignored) divided by the number aneroid barometer, WeatherMeasure model number of observations. If the average absolute error and the average error BM70, was used as the reference instrument with are about the same in magnitude, this indicates a systematic which all readings were compared. Readings were difference between the test and reference items. If the average error is near zero and the average absolute error is nonzero, this last taken once a day for a 3-month period and recorded in provides a measure of the scatter to be expected in the readings of the units printed on the face of the instrument. All the test item relative to the "true values" from the reference barom- barometers were exposed to station pressures rang- eter.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC TABLE 3. Average errors and average absolute errors for the tested barometers.

Manufacturer I Average I Average model # I error (mb) I error (mb)

Downeaster -3.20 3.28

Eschenbach 5000 -1.27 3.78

Hoffritz 0.60 2.58

Huger -0.77 0.80

SK 69026-00 0.72 0.94

Swift 477 0.49 0.83 FIG. 7. A plot of the differences in pressure (Pressuretest jtem -

Pressurereference) between a Downeaster Manufacturing Co. barometer and the reference laboratory-quality aneroid barometer. This shows a systematic trend toward too low readings for pressure less than 1005 mb. and fall of atmospheric pressure and its relationship to the weather.

c. Local fabrication 1) GENERAL CONSIDERATIONS The basic principles behind constructing many simple barometers are relatively straightforward. A volume is separated from the atmosphere; at least a portion of the surface confining the volume must be flexible. As the atmospheric pressure increases, the volume is compressed; as the pressure falls, the volume expands. An indicator connected to the flexing surface can be calibrated to provide a quantitative measure of the pressure acting on the volume. In

FIG. 8. A plot of the differences in pressure (Pressuretest item -

Pressurereference) between a Hoffritz barometer and the reference laboratory-quality aneroid barometer. This shows no systematic trend, but does exhibit wide scatter in the differences. reference barometer in our test were the Huger and Swift models. If an accurate barometer is available for use as a reference, inexpensive barometers of the type tested could be adjusted to give absolute pressure values of greater accuracy. Alternatively, correction procedures could be applied to their readings to give absolute pressure. This, however, is probably more complicated than is desirable in a school setting. FIG. 9. A plot of the differences in pressure (Pressuretest jtem - One should keep in mind that in a school weather Pressurereference) between an Eschenbach Alpin 5000 digital barom- station, accurate measurement of pressure may not eter and the reference laboratory-quality aneroid barometer. The output of the test item was in whole millibars, so the readings of the be as important as noting the pressure tendency. reference barometer were rounded to whole numbers before the While an economical barometer may be off by several subtraction. This shows no systematic trend, but does exhibit wide hectoPascals, it may still allow observation of the rise scatter in the differences.

Bulletin American Meteorological Society 789

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC many cases, the position of the flexible surface serves as the indicator (e.g., the height of the meniscus in liquid-in-tube barometers); in others, the flexure is amplified mechanically (e.g., as in the aneroid cell in a barograph). The difficulty is that most simple barometers are strictly variometers; that is, they respond to changes in temperature as well as to changes in air pressure. This occurs because the confined volume contains a small amount of air or vapor (as opposed to the vacuum required for a true barometer) that exerts an internal pressure. (Changes in this internal pressure due to changes in temperature are described by the ideal gas law.) Sensitivity to temperature changes can be reduced by either minimizing the amount of trapped air or by regulating the temperature of the instrument.

2) CAPE COD-STYLE BAROMETERS The simplest barometers to construct are those of the Cape Cod style. Here the confined volume is separated from the atmosphere by a liquid, usually a light oil or colored water. The indicator is a glass tube, open to the atmosphere at one end and immersed in the fluid at the other. When the atmospheric pressure rises, the liquid level in the tube falls; when the pressure goes down, the level rises. Unfortunately, Cape Cod-style barometers, with their large volume of trapped air, are very sensitive to temperature changes. The design shown in Fig. 10 attempts to insulate the volume from temperature FIG. 10. An improved form of the Cape Cod barometer that can changes by suspending it in the center of a large glass be fabricated from locally available materials. The barometer is jug (see Neuberger and Nicholas 1962). In this case, made from a small pill bottle and an eyedropper in a rubber cork. In the volume is the air- and vapor-filled space above the assembling the barometer, the bottle is filled about two-thirds full of colored water, then the cork with the eyedropper is inserted to seal liquid surface in the small pill-bottle barometer. The air the bottle (keep a finger over the top of the eyedropper to seal it while in the jug provides an environment for the barometer it is being inserted). With the tip of the eyedropper located above the in which the temperature changes only slowly. A small bottom, about one-third the bottle's height, colored water should rise hole in the jug stopper ensures that atmospheric about two-thirds of the way up the eyedropper. The level of the pressure is felt inside the jug.8 Harbster (1988) de- colored water in the eyedropper then goes up and down as the pressure falls and rises. After a scale is taped to the exposed portion scribes two other methods for constructing Cape of the eyedropper, the barometer is suspended by a wire in the Cod-style barometers using ordinary laboratory equip- center of the gallon jug [from Neuberger and Nicholas (1962)]. ment.

3) LIQUID-IN-TUBE BAROMETERS The partial vacuum can be achieved by filling the The essential elements of a liquid-in-tube barom- sealed tube with liquid and inverting it into the cistern. eter are shown in Fig. 3. If a working fluid of low The length of the tube and the size of the cistern volatility is used, this results in a true barometer, one depend upon the selected working fluid. Water can be in which the effects of temperature changes are mini- used if a tall, indoor space, such as a stairwell or a mal. Portions of both the tube and the cistern must be gymnasium with a balcony, is available—a water transparent in order to adjust and read the barometer. barometer needs to be about 10 m tall! Walker (1987) provides details on the fabrication of a water barometer. In a water barometer, the confined volume is filled with water vapor and air that was initially dissolved in 8 This system can be used to illustrate the effect of pressure changes the liquid. To obtain accurate pressure values, it is on a barometer. Attach a piece of plastic tubing to the hole in the jug stopper. Sucking air out through the tubing causes pressure to fall necessary to monitor the volume's temperature and to within the jug; blowing into the tube causes pressure to rise. correct the readings.

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Unauthenticated | Downloaded 10/06/21 01:43 AM UTC If it is not practical to make the position of the cistern adjustable, then one must take account of the fact that when fluid rises in the sealed tube (i.e., pressure rising) the fluid level in the cistern falls, and vice versa. If the cistern is wide enough, however, the fluid level in it is barely affected by changes in atmospheric pressure, and the barometer need only be read at the top. Mercury is 13.6 times as dense as water,9 so a mercurial barometer need only be 90 cm tall. Mercury, however, has many dangerous properties, so the construction of a mercurial barometer should be un- dertaken only after safety issues have been considered and, in some cases, permission to use mercury has been obtained from local authorities. Discussion of the fabrication of a mercurial barometer is given in Laird (1948). Yates (1961) provides some safety tips for handling mercury.

4) ANEROID BAROMETERS Figure 11 illustrates a simple aneroid-type barometer made using a sealed metal can.10 This approxi- mates the aneroid cell used in many commercial barographs. The can should have a large diameter, as this instrument's sensitivity depends upon the area of the flat surface on which the pointer is mounted. The original contents of the can should be emptied through a small hole. The can should then be heated to drive out as much air as possible. With the can hot, the hole FIG. 11. A simple aneroid barometer that can be fabricated from should be sealed (preferably with solder) to maintain locally available materials. The can must be affixed to the base in such a way that flexure of the bottom of the can is minimal. The "best" a lowered pressure in the interior. The sealed can location of the cork and indicating stick must be found by trial and should be fastened to a base. A small cork should be error [from Caldwell et al. (1981)]. glued to the top of the can. A thin stick set in a slot in the cork and held firmly in place by a drop of glue serves as a pointer. A ruled card mounted on the same bly has cooled to freezer temperature there should be base as the can provides a scale with which to mea- little change in the position of the pointer. sure deflections of the pointer. As shown in Fig. 11, under high-pressure condi- If the amount of air sealed inside the can is small, tions, the top of the can is depressed and the pointer then deflections of the pointer due to changes in swings upward; under conditions of low pressure, the temperature will be small. In this case, the scale can be top bulges upward and the pointer is deflected down- calibrated in appropriate units of pressure. The effects ward. The cork should be positioned on the top of the of temperature changes can be checked by noting the can at the location that gives maximum deflections to position of the pointer and then placing the whole the pointer. Usually a position midway between the assembly in a freezer for an hour or so. If there is little center and the rim works well, but testing may be air remaining in the sealed can, then after the assem- necessary to determine the best position. (The can should be fastened permanently to its base to prevent bulging of the bottom of the can from lifting the whole assembly.) 9 -3 The density of water at 0°C is 0.99984 g cm ; the density of mercury It is possible to fabricate barographs locally using at0°C is 13.5951 gem"3. 10Many textbooks and science activity books describe a barometer the mechanism from a standard aneroid barometer. constructed using a balloon stretched across the mouth of a wide- Laird (1950) and Ford (1961) provide clues as to how mouth jar. A drinking straw glued to the balloon acts as an indicator this might be done. of pressure changes. Although inexpensive and easy to construct, it is the authors' experience that these devices are more useful in d. Calibration and comparison checks demonstrating the gas laws than they are for use as a barometer. The difficulty is that air leaks quickly through the stretched balloon Liquid-in-tube barometers are primary instruments; material. that is, they do not need calibration and, with reason-

Bulletin American Meteorological Society 791

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC able care, will give accurate readings for many years. should be asked to begin to develop "forecast rules" Aneroid barometers do require an initial calibration to for their location by generalizing from their observa- establish the scale. This is usually done against a high- tions, and to test their rules in future situations.11 quality liquid-in-tube barometer. Aneroid barometers To aid the students in their generalizations, log also require periodic comparison checks (to a more entries can be compared with what is expected from accurate instrument) and occasional adjustment. Al- simple models of traveling pressure systems. A rela- most all commercial instruments incorporate an ad- tively large drop in pressure corresponds to the ap- justment screw for making these adjustments. As proach of a low pressure system. A relatively large rise noted earlier, for most schools, the best course may be in pressure corresponds to the approach of a high to purchase one good-quality aneroid barometer, mount pressure system or the departure of low pressure it permanently in a laboratory, and then use it as a system. A useful aid to the student in their compari- reference to set students' instruments. sons is the "Weather Cycler" described in Stroud Additional checks can also be made with the high- (1985) and available from many commercial suppli- quality instruments to be found at NOAA National ers. Examples of such studies are to be found in Weather Service offices or Federal Aviation Adminis- Jameson (1923; pp. 25-29) and Monroe (1980). tration Flight Service stations. Some television station Quantitative experimental activities with pressure weather centers may also be able to supply accurate can be divided into those requiring only one barometer observations of local pressure. In performing such and those requiring two or more. The simplest (easiest checks, be sure that it is understood whether station to perform) require only one instrument and, in most pressure or reduced pressure is being used. In the cases, do require that instrument to be precisely former case, differences in elevation between the calibrated. A plot of a sequential series of readings school and the site of the reading will have to be taken from one instrument spaced roughly one-half hour to into account. In the latter, the school value will have to one hour apart (or even better, from the continuous be reduced to sea level before a comparison is made. trace produced by a barograph) allows students to see the details of passage of high and low pressure systems. In the subtropics, the approach of tropical 5. Suggested activities cyclones can be observed. On fair days, small rises and falls attributed to the atmospheric tides can be The existence of air pressure and some of its effects discerned.12 When the weather is unsettled, the oscil- can be demonstrated in many ways. The crushing of lations of pressure reflect the passage of small atmo- a heated and then sealed can is a traditional approach. spheric waves (termed gravity waves). The passage However, the "HarBottle" discussed by Harbster (1990) of a thunderstorm overhead can produce rapid rises provides a better, more direct series of demonstra- and falls (with magnitudes of 5 to 7 hPa) in the local tions. pressure; readings taken at least every 5 min are A valuable arithmetic exercise is to compute the necessary to reveal these. weights of columns of air of various sizes (for example, By comparing outdoor conditions with changes in the weight of air pressing down on a school cafeteria pressure, students can relate the rate of change of table). Blackadar (1970) provides some sample calcu- pressure to wind speed. Gusts in the wind give rise to lations and relates them to experiments that can be very rapid fluctuations in pressure. As a synoptic-scale conducted in the field. Blackadar (1984) also dis- low pressure system passes by, the greater the rate of cussed a simple BASIC computer program that can be change in pressure, the stronger the local winds will used to explore many aspects of atmospheric pres- be. Note that these observations do not require an sure. absolute measure of pressure, but only the change A particularly valuable activity that shows the im- over short periods of time. In many cases, particularly portance of the barometer as a tool for the meteorolo- when working with younger students and locally fabri- gist is the maintenance of a log of weather events and the associated changes in barometer readings. This can be done with a simple barometer, as only the 11 An interesting activity is to compare observed events with the direction of the change or tendency (pressure rising, weather words stormy, rain, change, etc., printed on many barom- steady, or pressure falling) is required. As a minimum, eter faces. In general, there will be only rough correlation. a daily log entry should show the direction of pressure 12The maxima in the atmospheric tides occur within an hour either change over the preceding 3 h or so, the direction of side of 10 A.M. and 10 P.M., local standard time. The minima occur within an hour of 4 A.M. and 4 P.M., local standard time. The amplitude the wind, current weather conditions, and comments of the tidal oscillations is strongest in the tropics and decreases on changes in the weather that have occurred since towards the poles. These tides reflect the diurnal cycle of solar the previous log entry. After a month or so, students heating of the earth's atmosphere above any one location.

792 Vol. 73, No. 6, June 1992

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC cated barometers, purely qualitative observations (e.g., wind at two locations, students can determine the "pressure rising") will suffice. magnitude of the pressure gradient from the ratio of A single instrument can also be used to obtain simple differences, namely, differences between two locations and so allows ex-

ploration of the variation of pressure with height. A field PG = (p2-p1)/(z2-z1), trip with an aneroid barometer to a tall building provides a good experimental setting. Students should where PG = pressure gradient; (p2 = pj = the differ- determine the pressure difference between the top ence in pressure readings at the two schools; and (z2 and bottom floors of the building. Near sea level, an - z:) = the distance between the two schools. ascent of five or six stories (allowing 3 m for each floor) A plot of this value against the average of the two should show a pressure change of about 1.7 hPa. wind speeds should show that the strength of the wind More demanding are those activities that require is directly related to the magnitude of the horizontal near-simultaneous readings at several locations. If pressure gradient. arrangements are made for two or more schools to The direction of the pressure gradient is from low share readings of pressure and pressure tendency, toward high pressure; the pressure gradient force is relationships between the distribution of surface pres- countergradient—that is, directed from high toward sure, wind, and the movement of pressure and frontal low. The direction of the surface wind is determined by systems can be explored. Because weather systems several factors; however, it should have a strong tend to move from west to east in the United States, it component that is normal to the gradient. The direction is best if cooperating schools are located along an of this component is given by "Buys Ballot's Law," east-west line. which says that "if one stands with one's back to the Correlation of pressure tendencies between two wind, the pressure to the left (right) is lower than to the locations can be used to measure the speed of move- right (left) in the Northern (Southern) Hemisphere." ment of frontal systems. As a front passes, students at This is a good exercise to do as a frontal system the westernmost school should watch for the occur- approaches, first with both schools to the east of the rence of the minimum pressure (so that the pressure front, and then with both to the west. In the first case, tendency for the period is a fall, then a rise). The time the wind will probably have a southerly component; in of the minimum should be relayed to the school(s) to the latter, a northerly one. the east. When the front passes an eastern school, the Interdisciplinary studies can be conducted to relate time of the pressure minimum there should be noted. pressure, the current weather, and various physiologi- The speed of the front along the line joining two cal and psychological phenomena. Correlations be- schools is the difference in these times divided by the tween pressure and "aches and pains" in people's distance between the schools. This activity is best joints, mental attitude, and the behavior of some done in the winter, when the pressure troughs in which animals and birds have all been reported. fronts lie are deepest, so that the changes in pressure associated with the passage of a frontal system are most pronounced. Acknowledgements. The assistance of C. J. Fuqua and of If the pressures at two or more schools are to be F. O. Richardson in setting up the equipment for the field comparison test is gratefully acknowledged. Giovanni Soto prepared compared, the readings should be made simulta- Figs. 3, 4, 10, and 11. Three anonymous reviewers made numer- neously and then reduced to sea level. Because ous suggestions that resulted in a much improved paper. This pressure differences in space are usually quite small, work was supported in part by the National Science Foundation, the readings must be of high absolute accuracy (that through the Division of Atmospheric Sciences and the Division of is, all the barometers must be properly calibrated and Teacher Enhancement and Informal Science Education, under Grant Nos. TEI8550912 and TPE8751668. capable of resolving the local pressure to 0.1 hPa) to avoid obscuring real changes or differences with instrument errors. Because of the limited resolution of References most simple barometers and the small magnitude of

typical horizontal pressure gradients (the ratio of pres- American Meteorological Society, 1985: Guide to Establishing sure difference between two locations to the distance School and Public Educational Activities. Amer. Meteor. Soc., 21 separating them) associated with synoptic systems PP- (about 0.5 hPa per 100 km), schools sharing data ,1989: Preprints, Second Int. Conf. School and Popular Meteo- should be at least 100 km (65 miles) apart, so that rological and Oceanographic Education, Crystal City, Virginia. pressure differences can be determined with some Amer. Meteor. Soc., 171 pp. Berry, K., 1961: A school weather station. Weather, 16, 14-17. confidence. Blackadar, A., 1970: More or less under pressure. Weatherwise, 23, From simultaneous observations of pressure and 283-285, 315.

Bulletin American Meteorological Society 793

Unauthenticated | Downloaded 10/06/21 01:43 AM UTC , 1984: Using your computer—Barometers, altimeters, and Neuberger, H., and G. Nicholas, 1962: Manual of Lecture Demon- small computers. Weatherwise, 37, 154-156. strations, Laboratory Experiments and Observational Equip- Bohren, C. F., 1983: Simple experiments in atmospheric physics— ment for Teaching Elementary Meteorology in Schools and Conceptions and misconceptions of pressure. Weatherwise, 36, Colleges. The Pennsylvania State University, 183 pp. 82-84. Parker, S.P., Ed., 1988: Meteorology Source Book. McGraw-Hill Brock, F. V., 1984: Instructor's handbook on meteorological instru- Book Company, 304 pp. mentation. National Center for Atmospheric Research Tech. Pedgley, D.E., 1980: Running a School Weather Station. Royal Note., TN-237+IA, 328 pp. Meteorological Society, 16 pp. , 1985: Ground-based observing systems. Handbook of Ap- Peter, N.L., 1964: Weatherwise, the Technique of Weather Study. plied Meteorology, D.D. Houghton, Ed., Wiley-lnterscience, Pergamon Press, 179 pp. 341-351. Schaefer, B.J., and J.A. Day, 1981: A Field Guide to the Atmo- Caldwell, W., L.H. Smith, J.E. Newman, and J.M. Macklin (Revised sphere. Houghton Mifflin, 359 pp. by E.L. Frickey), 1981: Weather II— An Indiana 4-H Science Smith, L.P., 1966: Weather Studies. Pergamon Press, 100 pp. Project. Cooperative Extension Service. Purdue University, 22 Smithsonian Institution, 1966: Smithsonian Meteorological Tables. PP- Smithsonian Institution Press, 527 pp. Couchman, J.K., J.C. MacBean, A. Stecher, and D.F. Wentworth, Snow, J.T., and S.B. Harley, 1987: Basic meteorological observa- 1977: Examining Your Environment: Mini-climates. Holt, Rinehart tions for schools: Temperature. Bull. Amer. Meteor. Soc., 68, and Winston of Canada, Ltd., 92 pp. 486-496. Ford, M.W., 1961: Home constructed instruments. Weather; 16, , and , 1988: Basic meteorological observations for schools: 407-409. Rainfall. Bull. Amer. Meteor. Soc., 69, 497-507. Geer, I.W., 1975: Weather Study. The Research Foundation of , and D.R. Smith, 1990: Report on the Second International State University of New York, 12 pp. Conference on School and Popular Meteorological and Oceano- Harbster, D.A., 1988: Air apparent. Sci. and Children, 25, 12-14. graphic Education. Bull. Amer. Meteor. Soc., 71,190-198. , 1990: Demonstrating pressure differences. Carolina Tips, , M.E. Akridge, and S.B. Harley, 1989: Basic meteorological 53(3), 9-11. observations for schools: Surface wind. Bull. Amer. Meteor. Haynes, B.C., 1947: Techniques of Observing the Weather. Wiley, Soc., 70, 493-508. 272 pp. Stroud, S., 1985: Weather news. The Earth Scientist, 2(4), 29-30. Huschke, R.E., Ed., 1959: Glossary of Meteorology. Amer. Meteor. Trowbridge, L.W., 1973: Experiments in Meteorology—Investiga- Soc., 638 pp. tions for the Amateur Scientist. Doubleday and Co., 270 pp. Jameson, P.R., 1923: Weather and Weather Instruments for the U.S. Committee on Extension to the Standard Atmosphere, 1976: Amateur. Taylor Instrument Companies, 136 pp. U.S. Standard Atmosphere, 1976. U.S. Government Printing Kotsch, W.J., 1983: Weather for the Mariner. Naval Institute Press, Office, 227 pp. 315 pp. U.S. Department of Agriculture, 1976: Fire Weather Observers Laird, C.A., 1948: Workshop for weathermen—How to construct a Handbook. Agriculture Handbook No. 494, U.S. Government mercurial barometer. Weatherwise, 1,136-137. Printing Office (Stock No. 001-000-03510-0), 152 pp. , 1950: Workshop for weathermen—How to construct a U.S. Department of Commerce, 1979: The amateur weather fore- barograph. Weatherwise, 3, 20-21. caster. NO A A, 9(4), 14-21. Lutgens, F.K., and E.J. Tarbuck, 1989: The Atmosphere—An Intro- , 1988: Surface Observations. Federal Meteorological Hand- duction to Meteorology, 4th ed. Prentice-Hall, Inc., 491 pp. book, No. 1, U.S. Government Printing Office, 265 pp. Mazzarella, D.A., 1985: Measurements today. Handbook of Applied Walker, J., 1987: The amateur scientist—Making a barometer that Meteorology. D.D. Houghton, Ed., Wiley-lnterscience, 283- works with water in place of mercury. Sci. Am., 256, 122-127. 328. Walker, J.M., 1984: Weather education. Proc. First Int. Conf. School Meteorological Office, 1981: Measurement of Surface Wind, Vol. 4 and Popular Meteorological Education, Oxford, Roy. Meteor. in Handbook of Meteorological Instruments. 2d ed. Meteorology Soc., 268 pp. Office 919d, Her Majesty's Stationary Office, 50 pp. World Meteorological Organization, 1971: Guide to Meteorological Middleton, W.E.K., and A.F. Spilhaus, 1953: Meteorological Instru- Instrument and Observing Practices (4th Ed.). WMO-No. 8. ments. University of Toronto Press, 286 pp. Tech. Paper, 3, Secretariat of the World Meteorological Organi- Monroe, M., 1980: Amateur weather observation. Weatherwise, 33, zation, 379 pp. 148-155. Yates, R.F., 1961: The Weather for a Hobby. Dodd, Mead & , 1983: For the amateur—Instruments get dirty, too. Weather- Company, 181 pp. wise, 36, 80-81.

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