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February 965 Arthur V. Dodd 113

DEW POINTDISTRIBUTION NTHECONTIGUOUSUNITEDSTATESARTHUR V. DODD

United States Ar my Nati ck Laboratories Natick Mass.

ABSTRACTNew maps showing the distribu tionof average monthly dew points and their standard deviationn t hc contiguousUnited States areprcscnted. These are basedon recently available summariesof ourly psychrometric data prepared

by the U.S. Air Force for approximately 200 stations and on average dcw point data prcpared by theU.S. WcatherBureau.

1. INTRODUCTIONTheaccurate epresentation of theareal distributionof water vapor near the ground has lagged behind repre-

sentation of otherlimatic elements. In theUnitedStates the standard maps of water vapor distribution inthe air near the ground are those of Day [2] published in1917. Day'smaps showed mean 8 a.m. and 8 p.m.,75thmeridian ime,vapor pressure for January, April,July, and October. They have been included in modifiedform in more recent publications [lo, 121.

New maps showing the distribution of average monthlydew pointsnear the ground n the contiguous UnitedStates are presented n figures 2 to 13. These maps arebased on recently available summaries of hourly psychro-met,ric dat a prepared by the U.S. Air Force [I ll . Amongthe severalpsychrometric elements provided inhesesummaries are he averagemonthly dew pointand tsstandard deviation,and the average dew pointand tsstandard deviation for each of eight 3-hr. timegroupseach month. The availability of the tri-hourlysummariesmakes possible consideration of the daily range andnatureof the diurnal variation of dew point. This aspect of thework will be treated in a separate publication.

Locations of approximately 200 stations in the UnitedStates f o r which psychometric summaries are availableare shown in figure 1. Lengths of record average 10years with most records running from 1949 t o 1959,or1950 t o 1960. It would be desirable t o havea longerperiod of record, bu t a comparison of the Air Forcesummaries with hose of Day did not reveal any systematicchanges in the water vapor content of the air near heground, and the IO-yr. records are udged t o be ade-quate.* It was found th at at some sites local sources of

Rantoul, Ill, (Chanute Air Force Base) is the one station in the United States with

water vapor cause higher humidities han those repre-sentative of the general station location. This can some-times be ascertained from consideration of diurnal-varia-tion of dew point; in other cases inquiry was made of theOfficer-in-Charge of weatherstations to determine therepresentativeness of psychrometric observations. Theproblem of representativeness of observation sites is mostimportant in dry climates where local sources of moisturemust be considered. For example, several tationsnthe Southwest id ot aveepresentative umidityrecords because they were located over irrigated grass.

In addition t o the Air Force summaries, there are othersummarized llunlidity data now available which aid inthe delimitation of dew point (or equivalent vapor pres-sure)distributions n the United States. Average dewpoints have been computed by th e US . Weather Bureau[13] f o r over 200 stations n he United States f o r theperiod 1946-1955 for use in evaporationcomputations.Although the average dew points were determined from6-hourly observations and are not strictly comparable tothe Air Force summaries, theiravailability makes pos-sible a more comprehensive treatment of dew pointdistributions,particularly nmountainous reas. Com-parison of the Air Force and Weather Bureau summariesagain indicates no systematic difference in average dewpoints for the different periods of record.

2. AVERAGE MONTHLY DEW POINT MAPSBecause of the nonlinear relationship between dew

point and saturation vapor pressure, an error is introducedwhen dew pointsare averaged directly.Thiserror is afunction of the dispersion of the dew points being averaged.Because the standard deviation of the dew point distribu-tion is given in the Air Force summaries, it is possible toestimate the dew point averaging error, and the valuesin the Air Force summaries were corrected for these errors.an Air Force ummar y for a long period of record which can be compared with horter general, the are less than 1OF. and the distribu-record. At Ran toul the longer summ ary is for a 25-yr. record of observations each hourfrom July 1936 to February 1961 The average annualdewpointdurin g the 25-yr. record tiOnSOn the maps (figs. 2 to 13) are little affected bywas 0.1 F.lower than the average for a 14-yr. record from 1946 o 1959. Th e largest diffcr- consideration of the averaging error [4].ences in monthly averages were found in Septembcr (longer average 0.8 F. higher) andDecember (longer average 1.O0F. lowcr). Another problem in thereparation of theaps was

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4 MONTHLYEATHEREVIEW Vol . 9 3 No

FIGURE.-Location of stations for which psychrometric summaries are available.

the question of the reduction to sea level. In general, thepractice nNorth America has been to presentmapsrepresenting thevaporpressureor dew pointnear heearths surface a t station elevations [I, 2, 3, 51 while thepractice in Europe haseen to reduce the dat a o sea levelbyappropriate formulae [6, 7 , 91. Themaps presentedhere are not reduced to sea level for two reasons. First,it has been demonstrated that the reduction formulae donot apply where the occurrence of inversions affects thenormal decrease of dew point with elevation [SI. There-fore, it is not possible accurately to reduce dew point orvapor pressure t o sea level in all cases. Second, for mostpurposes it is more useful to know the dew point or vaporpressureas it is observed at thestation.Theamountanddistribution of watervapornear heground s re-quired by such disciplines as hydrology and agronomy,concerned with water balance problems, and physiologyconcerned with the hea t balance of the human body int,senvironment.Enowledge of watervapordistribution isalso needed by design engineers concerned with operationof equipmentanddegradation of material. In each ofthese cases the nformation required is for thestationelevation and not for a theoretical sea level.

The placement of the lines of equal dew point (isodroso-therms) in figures 2 to 13 in much of the country is fairlywell defined by hedata. n hemountainous areas,however, it is more difficult t o represent heaverage

pat terns of dew point. To aid in the interpretation of tdata inmountainousareas,graphs of average monthdew point versus elevation were prepared. Several of tgraphs are shown in figure 14 to illustrate the dew pochangewithlevation.Dewoints a t intermedilevels between station elevations can be checked on thgraphs as an aid in plott ing the isodrosotherms in moutainous areas.

An interesting eature of figure 14 is the seasoreversal in dew point lapse ra te between Reno, Nev., aBlueCanyon,Calif., In summer theprimary sourcewater vapor in this area is from the southeast, associatwith the influx of air from the Gulf of Mexico. Mosttheyear hemoisture source is from the west.BlueCanyon;4O mi. west of Reno, has ower average dew poithan Reno in summer, but mostf the year Blue Canyha.s higher average dew points than Reno even thoughis a t a higher elevation.

On a very broad scale the patterns of the isoplethsthe maps reflect the prevailing controls of water vaponear the ground. East of the Rocky Mountains the easwest alignment of the isopleths reflects thedominanlatitudinal or temperature control, in the Mountain the isoplethsaremore rregularand reflect dominaaltitudinal control; while in the Far West the alignmof the isoplethsparallel t o the coastdemonstrates hedominant control of exposure to t,he Pacific Ocean.

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February 1 9 6 5 A r t h u r V. Dodd 115

FIGURE.-Average month ly dew point (OF.) and etandard de via tionof dew point (inset), January.

FIGURE.-Average monthly dew point (OF.) and standar d devi atio nof dew point (inset), February.

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116 MONTHLYEATHEREVIEW Vol. 93 , No.

FIGURE. Average monthly dew point (OF.) and standard deviatioof dew point (inset), March.

.FIQURE . Average monthly dew point ( O F . ) and standard deviatioof dew point (inset.), April.

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Febriiary 965 A r t h u r V. Dodd 117

FIGURE.-Aversge month ly dew point O F . ) and st,andard deviationof dew point (inset), hfay.

FIGURE.-Average monthly dew point ( O F . ) and standarddeviationof dew point (inset), June.

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118 MONTHLYEATHEREVIEW Vo l . 9 3 No

FIQURE.-Average month ly dew point O F . ) and standard deviatioof dew point (inset), July.

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February 965 A r t h u r V. Dodd 119

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12 MONTHLY WEATHEREVIEW Vo l . 93 No

U Y l l t l

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February 1965 Ar t h u r V. Dodd 1I1

D E W P O I N T ( F )

- B U R B A N K , C A L . T OA N D B E R G , C A L .5

0.A

n

a0- 2z0-

BURBANK bQQ Fl \ \\\' I>

' ' l I 4 , I ' I 'L I 1 ' ' I ) I I IO 25 3 35 40 45 5 55 6D E W P O I N T ( F )

- P H O E N I XOU C S O NOO R TU A C H U C A .R I Z .

D f W P O I N T ( F )

S A C R A M E N T O , C A L . T O B L UE C A N Y O N , C A L .

D E W P O I N T ( * F ) , 'O G D E N ,U T A H T O R O C K S P R I N G S , W Y .

10 I5 20 S 30 35 40 4 fD E W P O I N T O F )

FIGURE4.-Dew point change with elevation in six selected areas of the United States.

A number of factors must be considered in the inter-pretation of thestandard deviation patterns shown ininsetmaps to t'he averagemonthly dew pointmaps.Standard deviations of dew points are arger where airmass changes are requent. A second factor which in-creases the dispersion of the dew pointdistribution sthediurnal angen dew point.Obviously, standarddeviations of monthly dew pointdistribution will be

larger where diurnal ranges are larger. Diurnal ranges indew point vary from about 1 F. in the summer monthsin the South to as much as 10' F. in the very cold areasinhe winter. A thirdactor affecting the tandarddeviation of the dew point distribution is the nature ofthe dew point itself. The amountof water vapor involvedin a given dew point change increases as the dew pointincreases. Twelve times as much water vapor is involved

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122 MONTHLYWEATHERREVIEW Vol . 93 Noin a dew point change from 7 to 7 O F. as from to 4. A. V. Dodd, Area lDis tr ibu t ionandDiurna lVariation of W a5 F. Other hings being equal, it would be expected V a p o rNear theGround in theContiguousUnitedtatesthat larger diurnal variations ana standard deviations of DoctoralDissertation,BostonUniversity,Boston, Madew point would be associated with lower dew points, (Available fromUniversi ty Microfilms, Ann Arbor,Mich.1964.andhis is the case. 5. W. D. Sellers, Distr ibu t ion of Rela t ive H umid i tyn dewo in t

in the outhwesternUnited tates University of ArizoIn st it ut e of Atmospheric Physics, (Scientific Repo rt No. 1CKNOWLEDGMENTS

This stud y s par t of the authors Doc toral Disse rtation t BostonUniversity [4] and apprec iation isexpressed to Professors RobertBatchelder and Howard Hirt, Boston University Geographers, fortheir guidan ce and helpfulsuggestions. Tha nks are also due per-sonnel of th e U.S. ArmyNatickLaboratorieswhoaided n hestudy. nparti cula r, he work of LconoraKundla npreparingthedata ormachine omputati on, nd of AubreyGreenwald,PerneleuvelinkndamesMurphy, for cartography,rerecognized.

Data for the stud y were obtained from the USAF Air WeatherService Climatic Center, and the U.S. Weath er Bureau HydrologicServicesDivision. With out hecoop erat ion of thepersonnel ofthese agencies, this stu dy could not h ave been undertaken.

REFERENCES1. F. H. Bigelow, Rep ort nheTempera tures nd Vapor

Tensions of t he United Sta tes, B u l l e t i n S Weather Bureau,Washington, D.C. 1909.

2. P. C. Day,RelativeHumiditiesand Vapor Pressures overthe United States, Including a Discussion of Dat a fro m Re-cording Hair Hygrometers for a Period of about Five Years,Month lyWeatherReview upp lemen t No. 6, U.S. WeatherBureau, Washington, D.C., 1917, 61 pp.

3. Depa rtme nt of Tran sport , Meteorological Branch, Canada),C l im a t i c u m m a r ie s f o r SelectedMeteorological Stations inC a n a d a vol. I1 (Revised), Humidity and Wind, Toronto,1959.

Tucson, Aria., 1960.6. Sir NapierShaw, M a n u a l of Meteorology vol. 11, Compative Meteorology, Ch. V, Aqueous Vapour,University

Press, Cambridge , England, 1928.7. J. SzBva-KovBts, Verteilung derLuftfeuchtigkeit auf

Erde, A n n a len d er H yd ro g ra p hieundMari t imenM eteo ro -logie vol. 66, No. 6, 1938, pp 373-378.

8. G. A. Tunnel, Reduc tion of Averages of Vapour Pressure Sea Level, The Meteoro log ica lMagazine vol.82,No. 9Apr. 1953, pp. 103-112.

9. G. A. T unnel, WorldDistr ibutio n of Atmos phericWateVapour, Geophysica l Memoir No. 1.00 Great Britain, Metorological Office, London, 1958, 61 pp.

10. S S Visher, Climat ic A t las of theUni tedS ta tes Harvard Uversity Press, Cambridge, Mass., 1954.

11. U.S. Air Force, UniformSummary of SurfaceWeather Oservations, Part E, PsychrometricSummary, Air Weat hService, Data Cont rol Uni t, Asheville, N.C.

12. U.S. Depa rtm ent of Agriculture, A t la s of American Agricu l tuClimate, (Section on Precipi tation and Humidit y), Wasington, D.C., 1936.

13. U.S. WeatherBureau,Unpub lished abulat ions of dew poinfor irstorderWeatherBureau tatio ns or period 1941955, Hydrologic Services Division, Washington, D.C.

[ReceivedAugust 6 964