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. Bulletin American Meteorological Society 781 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 (usu- ally 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 pres- sure. 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 barom- eter of this type is given in Fig. 3. Devices used to measure atmospheric pressure are termed barometers. The first liquid-in-tube barom- eters 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. 782 Vol. 73, No. 6, June 1992 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.