| Show J I Y B. B v i i FIELD SURVEYING I WITH BAROMETER By J. J Cecil Alter United States Weather Bureau IB pocket aneroid barometer is as Important In my iny travel equipment Ti-IE Ti as the speedometer is to the automobile I In the first place I cannot sep separate rate the weather changes from the fluctuations of the barometer and the weather is a big fact factor r in any Journey In the tho second place th the barometer Indicates altitudes and no locality Is completely described I until Its elevation is noted The higher elevations are cooler at night and pleasanter by day than the lower regions generally And the rate and amount of ascent or descent I enter as a dominant factor in the performance of an automobile on the tho highway How fast is it climbing H How w high is the locality or How much does the geological uplift amount to are answered by the barometer with about the same satisfaction as the I answers questions concerning concerning concerning con con- the distance SURVEYING WITH WITH ANEROID Surveys are made with the pocket aneroid barometer that show with fair success the general height and surface configuration of any oil structure I IJ roadway scenic or outing region and i with the aid of a recording barometer J or barograph preliminary surveys are made for railroad lines irrigation systems systems terns highways pipelines and other work with satisfactory ory accuracy and great expediency The recording barometer in such work is kept stationary stationary sta sta- as f for r Instance in camp for showing fluctuations due duo to the weather er while the pocket Instrument Is carried carried car car- ried forward on the line or over the region Preliminary surveys with a companion companion com com- panion or extra pocket barometer are are I 1 I similarly conducted A reliable instrument instrument ment is kept in camp or at a 1 headquarters headquarters head head- I quarters and read at sufficiently frequent frequent frequent fre fre- fre- fre quent intervals to provide a check on the surveying aneroid The necessary additions or subtractions due to weather er changes as shown on the stationary instrument are to the readings of the other Instrument They should be compared together each day and not separated more than seventy-five seventy or 1100 miles in ordinary weather less If it itI I the weather Is unsettled I INSTRUMENT OFTEN MISUNDERSTOOD I Many owners of are finding but a meager satisfaction in the use of the instrument because they do not I fully understand its operation while many others who could greatly profit prof t I by the use of a good barometer barometer barometer eter are deterred through a 3 misapprehension misapprehension hension henion of its capabilities and its degree degree degree de de- de- de gree of reliability Many a barometer in Salt Lake City hangs on a wall or oris oris oris is stored in an obscure drawer that could very well be carried as a part of ot the traveling camping or exploring equipment and render a big service I to its owner Some older types of barometers have havea a rather limited range within which the I I needle oscillates oscUlates and these require e re resetting resetting re- re setting occasionally as the trip progresses progresses progresses re-I re toward unusually high or low altitudes Such instruments however can be made to serve very well es especially especially especially es- es for anticipating weather changes in any location Barometers having an ample range are often mistrusted mistrusted mistrusted mis mis- trusted by their owners because of a misapprehension as to the need for Since having it set like a timepiece only differences in altitude or air pressure pressure pressure pres pres- sure are usually measured starting with some known elevation the barometer barometer barometer eter may indicate primary or face values values values val val- ues that are widely different from the standard mercurial barometer at the nearest weather bureau station and yet et show differences in altitude and rising or falling failing pressure with change of altitude or with the weather very faithfully faithfully- It is however best to keep them adjusted to standard values If the scales will wiH permit germit ermit at this altitude GOVERNMENT AL ALTITUDE SCALES Few of the older have foot scales for indicating altitude that are more than grossly or very approximately approximately approximately correct through the newer patterns patterns patterns pat pat- terns containing valuable improvements improvements improve improve- ments have have- adjustable outer or foot scales that spare the user much calculating calculating calculating cal- cal in determining altitudes and much doubt as to the probable error The zero of the foot scale or the value representing the correct altitude of the starting place may be orient oriented d to the tip of the hand or needle then excepting corrections due dua to weather changes the change in or the actual altitude will be shown on reaching destination destination des des- or at any intervening location des I whenever the Instrument is read This gives ves a great satisfaction in the use of the in instrument at al all all times MEASURING THE rHE AL ALTITUDE For more accurate results' results in determining determining determining de de- de- de differences in altitude a number of determinations on a number of trips should be made if the distance be great and the record important In Incase I case a grade line or other considerable enterprise depends on the record a I simple calculation that takes tales into consideration consideration consideration con con- the air temperature may be made In this case Caiet a good aneroid I should show values within a a. very small percentage of error depending mainly upon the errors In the system of calculating calculating calculating cal cal- such as the nicety with which the needle are read rad from the scales The corrections for temperature t are vital because an inch on the barometer may mean a thousand feet at tit say 50 degrees but It would mean 1020 feet at 60 degrees because of the expansion of the air Barometers are usually scaled for some median temperature I say 50 degrees For each 10 degrees es above or below helow this value 2 feet per I hundred feet should be added or subI subtracted subtracted sub sub- respectively to the Indicated I altitude FORMULA FOR ACCURACY I For still closer work the difference difference differ differ- ence between two places Is equal to the tho I difference In pressure in inches divided b by their sum and multiplied by where the mean temperature of the two I places is 60 degrees If the mean temperature temperature temperature tem tem- is above 60 degrees the multiplier multiplier multiplier multi multi- must be increased by for ever every degree degree- by which the mean exceeds 60 degrees or decreased for temperatures temperatures temperatures tem tem- lower than 60 degrees As an example If the pressure readIngs readings readings read- read Ings at two places are and 25 inches the the fraction fraction to be multiplied by at 60 degrees Is 05 over giving a difference In elevation of feet DAILY BAROMETER VARIATIONS There is a slight daily variation In Inthe Inthe inthe the barometer being highest between 9 0 and 11 o'clock a a. a m. m and p. p p.- p. pm p.m. m. m and lowest between 3 Z and 5 a a. a m. m and p. p m. m the range being equivalent to 75 or I feet as shown on the altitude scale Accidental pressure variations Indicated Indicated in indicated In- In faithfully by by the barometer and due to advancing areas of high and low barometer of great extent as charted on the daily weather maps will cause falling pressure from from one to three days and then rising pressure pressure pressure pres pres- sure for a similar time The extreme range rang e from highest to lowest as shown on the altitude scale may be as much muchas as or 1000 feet though the barometer barome barome- ter be left stationary If the pressure is falling or rising fast as might also be evidenced b by a steady stead southerly or northerly wind respectively the change may be as much as the equivalent of fifty feet an hour Such changes can be eliminated only by making altitude difference readings with a minimum of elapsed time or by checking with known altitudes more frequently NORMAL L. BAROMETER READINGS Having at hand handa a known elevation and the average air pressure for such an altitude the observer may discern I at a glance whether the pressure is i running low or high with reference to i normal due to weather influences The average barometer reading in inches i the natural and only absolutely accurate accurate accurate rate scale than can be fitted to such an instrument has been found at a i i number of weather bureau stations to be about as follows feet above I Isea Isea sea level inches feet i inches feet feet 26 0 feet feet fe I feet feet feet feet ANTICIPATING WEATHER CHANGES Readings an hour or more apart at the same location will indicate any important important important im im- approach of low or high pressure pressure pressure pres pres- sure areas The areas of fair warm weather are usually in front of an advancing advancing advancing ad ad- low hence high temperatures strong southerly winds and falling barometer usually precede stormy weather The precipitation In Utah usually begins about the time the tho barometer begins to rise However without access to the daily weather maps of the weather bureau hureau the lone observer cannot well determine how low or how high the barometer will go before turning or how soon the tho change will occur A familiarity with the Instrument however and a casual watching of it from time to time will enable the ob observer observer observer ob- ob server to associate its variations very I closely with the weather conditions and changes to come as shown In the j i newspaper and other weather bureau reports I |
