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A SHRAE JOURNAL

HVAC systems must counteract weather forces when outdoor tem-perature or humidity moves away from an acceptable range forsafety and comfort. Consequently, a clear understanding of weather

behavior is helpful for system designers and operators. Shortcomings in thatunderstanding are often at the root of problems involving poor indoor airquality and premature deterioration of buildings and HVAC equipment.

sents a significant advance over the 20to 40-year-old data contained in the 1993Handbook, some of which was obtainedthrough graphical interpolation of as littleas five years of data.

Annual Not Seasonal ExtremesData contained in 1993 and earlier edi-

tions were calculated at different timesusing different methodologies. For ex-ample, Canadian cooling data was basedon the extreme for July alone, while U.S.data was based on the extreme for thesummer season consisting of June, Julyand August. To ensure uniformity forworldwide calculations, the 1997 data isbased on annual extremes rather than sea-sonal or single-month peaks.

For example, earlier handbooksshowed the dry bulb temperature ex-ceeded for 1% of the hours during thesummer period. Now, the 1997 Handbookshows the temperature which is exceededfor 0.4% of the entire year’s observations.Likewise, the old 99% winter seasonal drybulb for heating has been replaced by thenew 99.6% annual value. The percentiles

of 0.4, 1 and 2.5% for cooling and 99.6%and 99% for heating were chosen becausethey yielded values which, for most sta-tions, corresponded closely to the earlierseasonal extremes. So the new annual val-ues are close to, but seldom the same asthe old seasonal extremes. The user canexpect that variations result more from thedifferent calculation methods rather thanfrom any significant climate change.

This new methodology is especiallyuseful in light of ASHRAE’s commit-ments to its international members inpopulation-dense tropical climates. Thesummer and winter seasons near the equa-tor occur during different months com-pared to seasons in continental locationslike Canada and Argentina. Therefore, an-nual rather than seasonal calculations aremore appropriate for a uniform worldwidemethodology.

International CoverageEarlier handbooks reflected ASHRAE’s

intense focus on U.S. and Canadian sites

About the Authors

Lewis G. Harriman IIILewis G. Harriman IIILewis G. Harriman IIILewis G. Harriman IIILewis G. Harriman III is director of research atMason-Grant, Portsmouth, N.H. He is the Chair ofthe Handbook Subcommittee of ASHRAE TechnicalCommittee 3.5, Desiccant and Sorption Technologiesand is the author of The Dehumidification Handbook.

Donald GDonald GDonald GDonald GDonald G. Co l l i ver. Co l l i ver. Co l l i ver. Co l l i ver. Co l l i ver, Ph .D, Ph .D, Ph .D, Ph .D, Ph .D. , P. , P. , P. , P. , P. E. E. E. E. E . ,. ,. ,. ,. , is professorof engineering in the Biosystems and AgriculturalEngineering Department of the University of Ken-tucky in Lexington. He was the principal investigatorfor ASHRAE Research Project 890, which producedthe new climatic design data in the 1997 ASHRAEHandbook—Fundamentals.

KKKKK. Quinn Hart, P. Quinn Hart, P. Quinn Hart, P. Quinn Hart, P. Quinn Hart, P.E.E.E.E.E. ,. ,. ,. ,. , is the chief mechanical en-gineer of the U.S. Air Force at the USAF Civil Engi-neering Support Agency at Tyndall AFB in PanamaCity, Fla. He was the project officer for the revision ofthe Department of Defense Engineering Weather Data,USAF Handbook 32-1163.

New Weather DataFor Energy Calculations

By Lewis G. Harriman III,Member ASHRAEDonald G. Colliver, Ph.D., P.E.Fellow ASHRAEandK. Quinn Hart, P.E.Member ASHRAE

Recognizing the economic benefit ofavoiding such problems, organizationssuch as ASHRAE, the U.S. Departmentof Defense and industry-funded researchinstitutions have recently invested over$1,000,000 in engineering weather dataresearch. This article describes some ofthe products of that research, a selec-tion that is necessarily limited by spaceconstraints of the magazine. Readers areencouraged to share what they considerto be important sources and uses forweather data by writing letters to the editorfor possible publication in future issues.

Peak Load DataChapter 26 of the 1997 ASHRAE Hand-

book—Fundamentals contains the re-sults of ASHRAE research project 890-RP, which defined new peak design con-ditions for sizing equipment.1, 15 The re-vised and expanded information repre-

The following article was published in ASHRAE Journal, March 1999. © Copyright 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paperform without permission of ASHRAE.

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rather than international locations. Forexample, the 1993 Handbook containeddata for two locations in Rhode Island.That data density is useful. However, atthe opposite extreme, the entire countryof China, with a population of 1.2 billionpeople and a land mass larger than theUnited States, was also covered by datafor exactly two locations.

The 1997 Handbook reflects thesociety’s intention to become a more inter-national organization. Coverage outsidethe United States and Canada has ex-panded from 243 locations to 801. Unfortu-nately, some previously listed domestic andinternational locations were dropped fromthe new handbook. Raw data available toASHRAE for those locations did not meetnew uniform standards for completeness,and/or the length of their periods-of-record.Projects are underway in some parts of theworld, notably the Indian subcontinent andAustralia, to locate better raw data and tocalculate design values for more locationsusing the internationally consistent meth-odology developed during the ASHRAEresearch project.

Correct Humidity ExtremesSomewhat surprisingly, and possibly

because of the emphasis on temperaturerather than moisture control, older hand-books did not contain data describing ex-tremes of humidity. The 1993 and earlier

handbooks showed only the average hu-midity during periods of extreme tempera-ture. Those values do not represent theextreme humidity, which occurs at mod-erate temperatures during rainstorms orduring early morning as dew evaporates.

The humidity misimpression createdby focusing on high temperature was of-ten quite significant. An example can beseen in the listing for Huntsville, Ala.,shown in Figure 2. The high dry bulbtemperature is 94°F (34.4°C) with an aver-age coincident wet bulb temperature(MWB) of 75°F (23.9°C). Those peak drybulb values create the impression that the

extreme humidity ratio is 100 gr/lb (14.3g/kg). In fact, the true peak moisture ismuch higher at 135 gr/lb (19.3 g/kg), asshown in the 0.4% dew point columns.These columns also show that the peakmoisture occurs at an average dry bulbtemperature (MDB) of 83°F (28.3°C) ratherthan at 95°F (35°C). That represents a sig-nificant reduction in the sensible heat ra-tio for a cooling coil which pre-treats out-side air, and probably suggests a differ-ent equipment selection for such appli-cations.

Having correct peak moisture datashould allow improvements in equipment

Figure 1: Design conditions are useful for sizing equipment. However, hourlyrecords are used for simulating controls and estimating energy use, becausedesign conditions occur less than 1% of the equipment’s life.13

Figure 2: New design data for sizing dehumidification and cooling equipment from the 1997 ASHRAE Handbook—Fundamentals.

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W E A T H E R D A T A

and systems for dehumidification. The chapter alsosuggests using the peak humidity condition as a“part-load check point” for cooling systems thatcontrol humidity as a secondary function.

Another potential use for the peak humidity datainvolves ventilation air, which is usually loaded withtwo to six times more moisture than sensible heat.2

Because the enthalpy is highest at the peak moisturecondition (except in desert and high-altitude cli-mates), engineers might consider sizing ventilationpreconditioning equipment at the peak dew pointcondition rather than the peak dry bulb temperature.3

Peak Loads for Extended PeriodsThe handbook describes extremes that are ex-

ceeded for 0.4, 1.0 and 2.0% of the 8,760 hours of theyear (35, 58 and 175 hours). Because those hours donot occur together, this presents a problem for sizingthermal storage and power storage systems, wherethe engineer must know the continuous duration ofthe peak load. Duration is also important for analyz-ing night setback or off-peak precooling andpredrying strategies. Those techniques use the ther-mal inertia and moisture capacitance of the building to con-serve energy or to “store” cooling or dehumidification effectahead of expected peak periods. ASHRAE research project 828-RP was completed in 1997 to address these needs.16.

The researchers identified extreme weather patterns, whichlasted for one, three, five or seven 24-hour periods in a continu-ous sequence. Separate sequences are defined for extremes ofdry bulb, dew point, enthalpy and low wet bulb depression.Those hourly observations are stored on a CD-ROM, togetherwith a computer program, which displays summaries of the dataas well as the actual hourly data for each extreme sequence.The CD-ROM includes data for 381 locations in the UnitedStates and Canada.4

Hourly Data for Troubleshooting and AnalysisAlthough extreme design data allows the engineer to size the

equipment, it does nothing to quantify how much energy thesystem will use in a year, nor does it help the operator understandhow the system will respond to changing loads. Hourly weatherobservations are used for analyzing system behavior during the99.6% of the hours in a year that the weather does not create thepeak loads. New sources for hourly data are available in threeformats: current observations, historic observations and historicsummaries.

Current Hourly Observations for TroubleshootingEngineers, service technicians and building operators con-

stantly analyze and troubleshoot normal problems of HVACsystems. Often, knowing the current and recent weather condi-tions can help explain the cause of a problem, and lead to afaster solution. Recently, the World Wide Web has become asource for such current observations.

Figure 3 shows the results of a query of the hourly weatherarchive maintained for public use by an industry-funded research

institution. 5 The archive is available at no charge, and containssimultaneous hourly observations of dry bulb temperature, hu-midity ratio, barometric pressure and wind speed for 240 locationsin the United States and Canada. The archive is updated 24 hoursa day, 365 days per year, with records beginning on Feb. 1, 1998.Any series of hourly observations since that date can be down-loaded as a tab-delimited electronic file for importing into spread-sheets or other computer analysis tools.

Historic Hourly Observations for Energy AnalysisFull-year, 8,760 hour energy analysis is becoming more im-

portant than in the past, because computing power is so inex-pensive, and because power costs can now vary throughout asingle day rather than changing only on a seasonal basis. Thereare two categories of hourly observations: actual year and typi-cal year.

Actual hourly observations are available from many sources,including the U.S. National Climatic Data Center and the Indus-trial Climatology Division of Environment Canada.6, 7

Using historic observations, system simulations can be runfor a specific year in the past. An engineer could estimate howa new system design would have behaved, for example, in theactual conditions experienced during 1979. On the other hand,actual records also reflect the normal problems encountered inlarge-scale data acquisition. Some records are missing data ele-ments for a few hours. They may also contain zero values,which reflects the fact that the data gathered during those hourswas outside the checkpoint boundaries and therefore too inac-curate to include in the record.

For many computer programs, missing data elements or un-expected zero values can cause problems. Also, the weather forone particular year is not likely to reflect the average weatherover the 15-year life of the system. So many engineers prefer touse typical hourly observations instead of records for a single

Figure 3: Current hourly weather downloaded from a web-basedweather archive located at www.gri.org/desiccant.5

3 4 A S H R A E J o u r n a l M a r c h 1 9 9 9

specific year. In the U.S. and Canada,popular formats include Typical Meteo-rological Year (TMY and TMY-2),Weather Year for Energy calculations(WYEC and WYEC-2) and CanadianWeather for Energy Calculations (CWEC).

TMY and CWEC records are con-structed by examining the weatherrecords for a series of past years, and thencalculating how “typical” are the monthsin each year. In other words, the recordsfor all Januarys are compared to eachother, and the “most average” January iscombined with the “most average” Feb-ruary, and so on until 12 typical monthsare assembled in a single 8,760-hourrecord. The “most typical” January mightbe from 1962; February may be from 1976and so forth. Mathematical smoothing isapplied to the data at the beginning andend of each month to avoid abruptchanges in values. Figure 4 shows anexample of a partial extract from a TMY-2record, imported into a spreadsheet.9

The most recent TMY-2 methodologyis well documented8 and repeatable, andthe program that examines the historicrecords can be adjusted to weight differ-ent weather parameters more strongly asit searches for “typicality.” For example,in 1995 the National Renewable EnergyLaboratory constructed 239 TMY-2records for U.S. locations.8, 9 In thosefiles, the primary selection criterion wascompleteness of the solar record, and drybulb is weighted slightly more stronglythan wind speed in the search for typical-ity. So in those records, the temperaturewill be slightly “more average” than thewind speed.

The original WYEC records were gen-erated for 56 U.S. cities by ASHRAE re-search in the 1970s.10. The methodologyreflected the resources of the society atthe time. An expert examined actualrecords. Hand adjustments were madewhen data was missing, or when just av-erages rather than observed data wereavailable for a particular variable. Conse-quently, the methodology is less welldocumented, and difficult to repeat. Thenewer WYEC-2 records10 were con-structed for 76 U.S. cities using a meth-odology similar to that used for the 239TMY-2 locations.

In Great Britain, an Example WeatherYear (EWY) data set is available for 15

Figure 4: An extract from a TMY hourlyrecord, imported into a spreadsheet.9

Figure 5: A joint frequency table forhours at each temperature and humidityratio, imported into a spreadsheet. 9

locations. A complete year of representa-tive weather data for these locations wasselected by the Chartered Institution ofBuilding Services Engineers (CIBSE).18

From the available set of weather years, asmaller group of candidate years waschosen by examining the value of severalparameters during each month. Candidateyears had values, which were within twostandard deviations of the long-termmonthly averages for those parameters.The single year selected for each loca-tion had values with the minimum totaldeviation of all parameters from theirlong-term averages.

Typical hourly weather for other Euro-pean locations are contained in the dataset of “Test Reference Years” (TRY) con-structed by the Commission of the Euro-pean Communities (CEC).19 These yearswere compiled using individual monthsthat were selected as “average” using amethodology similar to that used in theUnited States and Canada to compile theTMY and CWEC records.

Typical hourly weather data for loca-tions outside the United States, Canadaand Europe are not easily available in thepublic domain. As a substitute for typicaldata, the International Surface WeatherObservations (INSWO) set of actualhourly records for 1,500 sites is availablethrough the U.S. National Climatic DataCenter.6 In addition, some commerciallyavailable building energy analysis soft-ware includes actual 8,760-hour recordsfor Latin American and Asian locationsembedded in the program, but these pro-prietary data are not available as separate

files.12 Current ASHRAE research is ad-dressing this issue. Typical hourly recordsfor 200 non-North American locations willbe published within two years.

Summaries of Hourly DataWhen less precise estimates of energy

consumption are adequate, summaries ofhourly data are used in place of a full8,760-hour record. These are available inmany forms.

The “typical day” summary format isused in some computer programs pub-lished by a major manufacturer of HVACequipment.11 Weather data embedded inthese programs is not actually 8,760 ob-servations. It is a summary of one or more“typical 24-hour days” for each month.For example, January might be repre-sented by the same single day, repeated31 times.

Another popular format is the “BINsummary.” These summaries indicate thenumber of hours a single variable is withina specified range (the “BIN width”). Forexample, a dry bulb summary indicatesthe number of hours in which the dry bulbtemperature falls between the upper andlower boundary of each BIN (usually be-tween 2°F and 5°F [1°C and 2°C]). Often,a BIN summary will also contain the aver-age value of other variables during thehours in each BIN. For example, the drybulb summary might contain the averagewet bulb for hours in each BIN.

Traditional BIN summaries are limitedin some respects. They allow reasonablecalculation of loads, which depend on theprimary BIN variable, but they do not re-

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flect the loads dependent on any aver-age coincident variable. For example, adry bulb BIN summary with average wetbulb estimates sensible heat loads, butunderestimates moisture loads by 25% to45%. Recently, computer tools have be-come available to allow construction ofBIN summaries by variables other thandry bulb temperature,9, 17 but any BIN sum-mary will underestimate the magnitude ofloads dependent on average coincidentvariables. Joint-frequency tables do nothave this limitation and sometimes areused as an alternative to BIN summaries.

A joint frequency table sorts the hoursof the year and places the hour-countsinto a two-dimensional matrix. One canvisualize a joint frequency table for tem-perature and humidity as an overlay for apsychrometric chart. Total hours occur-ring at each combination of temperatureand humidity can be placed in a matrixover the appropriate points on the chart.Figure 5 shows such a joint frequencytable.9 The advantage of this type of sum-mary is that two variables, not one, areappropriately quantified for the year. Nei-ther value is the product of averaging.

The recent ASHRAE research projectrevising the peak design data in the Hand-book—Fundamentals involved the cre-ation of joint frequency tables for all thelocations displayed in Chapter 26 of thatvolume. The society is planning to pub-lish these monthly and annual joint fre-quency tables of dry bulb/wet bulb, drybulb/ enthalpy and dry bulb/wind speedin a CD-ROM format during the coming year.17

Both BIN summaries and joint frequency tables are limited inthat they lose the time sequence of climate conditions such asthe hour-by-hour history of temperature and solar radiation.Thus, they are not used for estimating any thermal or moisturelag effects in buildings.

Understanding Weather and Climate DynamicsOne of the greatest challenges for a designer is gaining an

overall understanding of the climate in an unfamiliar location.In both government and commercial building practice, designservices are increasingly centralized to save costs and adminis-trative overhead. But such centralization virtually guaranteesthat important “school-of-hard-knocks” knowledge about lo-cal climate behavior is less available to the remotely locateddesigner, sometimes with costly consequences. A standard pro-cedure for selecting cooling equipment for a restaurant in Chi-cago may not scale well for the same restaurant when built inPuerto Rico, with its ten-times larger latent load. A military dor-mitory design that works well in New Hampshire may not per-

form well in Alaska. Both are cold climates, but Alaska has far-larger drifting snow loads, and a foundation will crack if thebuilding melts the permafrost.

To help the designer gain a rapid, quantified understandingof local climate behavior, the U.S. Department of Defense re-cently redesigned its Tri-Service Engineering Weather DataManual, widely known as “The Blue Book” for the color of itscover. The result has been published in electronic form. Infor-mation is available for 511 U.S. locations and 292 internationalsites.13 Figure 6 shows one example of the extensive graphicsavailable for each site. By comparing graphics for a new loca-tion with the same graphics for a familiar location, a designercan gain a fast understanding of the differences between thetwo sites. The numbers behind these and other graphics can beexported from the file by “cutting and pasting” the values intoother computer programs for quantitative analysis.

Finally, HVAC designers and system operators can often ben-efit from understanding the dynamic nature of weather behavior.We design at steady-state conditions, but systems are never atthe equilibrium conditions we must assume for design. Weather

Figure 6: The new electronic version of the Department of Defense weatherdata contains graphics, as well as numerical values, allowing a designer to quicklycompare climates at different locations.13

3 6 A S H R A E J o u r n a l M a r c h 1 9 9 9

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and internal loads change minute-by-minute in highly complex ways. For thosewho would like to gain a rudimentary un-derstanding of weather dynamics, refer-ences are available at low cost to educatethe layman.14

SummaryIn summary, our industry now has bet-

ter, more extensive and less expensive en-gineering weather data than ever before.That data comes to us courtesy of expen-ditures by United States and Canadiantaxpayers, member donations to ASHRAEresearch and from industry-funded re-

search institutions. Over the next fewyears, it will be interesting to observe howsystem designers and equipment manu-facturers use this improved informationto benefit building owners and occupants.

References1. Climatic Design Information. 1997ASHRAEHandbook—Fundamentals, Chapter 26.

2. Harriman, L. G. III, D. Plager, D. Kosar.“Dehumidification and cooling loads from ven-tilation air.” ASHRAE Journal 40(11):37–45.

3. Harriman, L. G. III. 1998 “New peak mois-ture design data in the 1997 ASHRAE Hand-

book of Fundamentals.” Proceedings of theSymposium on Improving Building Systemsin Hot and Humid Climates. Energy SystemsLaboratory, Texas A&M University.

4. ASHRAE Design Weather Sequence ViewerVersion 2.1. 1998.

5. Hourly Weather Data Archive 1998–99.Chicago: The Gas Research Institute,www.gri.org/desiccant.

6. Solar and Meteorological Surface Obser-vation Network (SAMSON) 1961–1990. Na-tional Climatic Data Center of the NationalOceanographic and Atmospheric Administra-tion of the U.S. Department of Commerce,

Table 1: Common types and sources of engineering weather data.

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W E A T H E R D A T A

Please circle the appropriate number on the Reader Ser-Please circle the appropriate number on the Reader Ser-Please circle the appropriate number on the Reader Ser-Please circle the appropriate number on the Reader Ser-Please circle the appropriate number on the Reader Ser-vice Card at the back of the publication.vice Card at the back of the publication.vice Card at the back of the publication.vice Card at the back of the publication.vice Card at the back of the publication.

Extremely Helpful ....................................................... 454

Helpful ....................................................................... 455

Somewhat Helpful ..................................................... 456

Not Helpful ................................................................ 457

Asheville, N.C. (828) 271-4400.

7. Canadian Weather Energy and EngineeringData Sets (CWEEDS) 1953-1995. Environ-ment Canada, Atmospheric Environment Ser-vice 4905 Dufferin Street Downsview, ONM3H 5T4 (416) 739-4328.

8. TMY-2 Methodology. National RenewableEnergy Laboratory of the U.S. Departmentof Energy. Building Energy Technology Pro-gram 1617 Cole Blvd. Golden, CO 80401-3393 (303) 275-6026.

9. Typical Meteorological Years (239 U.S. cit-ies). (TMY-2) BinMaker PLUS CD-ROM.Batavia, Ill.: Gas Research Institute (773) 399-5414.

10. WYEC-2 Weather Year for Energy Calcu-lations (76 U.S. cities). ASHRAE.

11. Trace 600 Building Energy AnalysisProgram. The Trane Company.

12. Hourly Analysis Program (HAP) CarrierCorporation. Syracuse, N.Y.:SoftwareSystems.

13. Department of Defense EngineeringWeather Data 1999. USAF Handbook 32-1163 (I). Tyndall AFB, Fla. (850) 283-6346.

14. Williams, Jack. The Weather Book (2ndEdition) 1997 Vintage Books, New York:Random House. ISBN 0-679-77665-6.

15. Colliver, D. G., Gates, R. S., Zhang, T.F.Burkes and Priddy, T.K. 1998. Updating theTables of Design Weather Conditions in theASHRAE Handbook—Fundamentals.

16. Colliver, D.G., R. S. Gates, H. Zhang, andT.K. Priddy. 1998. Sequences of Extreme Tem-perature and Humidity for Design Conditions.ASHRAE Transactions 104(1):133-144.

17. Colliver, D. G., T. F. Burks, and R. S.Gates. 1999. ASHRAE Design Weather DataViewer. CD-ROM.

18. Irving, S. J. 1988. The CIBSE ExampleWeather Year. Weather Data and Its Applica-tions —A Symposium for Building Service En-gineers. London: Charted Institution of Build-ing Services Engineers. CIBSE 44 181 6755211.

19. 1985. Test Reference Years TRY, WeatherData Sets for Computer Simulations of SolarEnergy Systems and Energy Consumption inBuildings. Commission of the European Com-munities, Directorate General XII for Science,Research and Development, Brussells, Bel-gium 32 2 295 25 59.

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