Weather and Snow Observations for Avalanche Forcasting: an Evaluation of Errors in Measurement and Interpretation

143

WEATHER AND SNOW OBSERVATIONS FOR AVALANCHE FORCASTING: AN EVALUATION OF ERRORS IN MEASUREMENT AND INTERPRETATION

R.T. Marriottl and M.B. Moorel

Abstract.--Measurements of weather and snow parameters for snow stability forecasting may frequently contain false or misleading information. Such error~ can be attributed primarily to poor selection of the measuring sites and to inconsistent response of the sensors to changing weather conditions. These problems are examined in detail and some remedies are suggested.

INTRODUCTION SOURCES OF ERROR A basic premise of snow stability analysis for Errors which arise in instrumented snow and avalanche forecasting is that point measurements of weather measurements can be broken into two, if snow and weather parameters can be used to infer the somewhat overlapping, parts: those associated with snow and weather conditions over a large area. Due the representativeness of the site where the to the complexity of this process in the mountain measurements are to be taken, and those associated environment, this "extrapolation" of data has with the response of the instrument to its largely been accomplished subjectively by an environment. individual experienced with the area in question. This experience was usually gained by visiting the The first source of error is associated with areas of concern, during many differing types of the site chosen for measurements. The topography of conditions, allowing a qualitative correlation mountains results in dramatic variations in between the measured point data and variations in conditions over short distances and often times the snow and weather conditions over the area. these variations are not easily predictable. For example, temperature, which may often be In many instances today, the forecast area has extrapolated to other elevations using approximate expanded, largely due to increased putlic use of lapse rates, may on some occasions be complicated by avalanche-prone terrain (e.g. increased backcountry inversions generated by mesoscale or synoptic scale skiing in developed areas, large use areas for weather conditions, undetectable from a valley site. helicopter skiing operations, or a regional Thus measurements must be taken at a site or sites avalanche forecasting center). The ultimate effect that provide information that is unambiguous of this expanded area of concern is less direct regardless of the weather conditions or they must be contact with conditions by forecasters. This has taken at enough sites that sufficient information is resulted in greater reliance on both data gathered available to sort out any ambiguities that might by instruments and on the extrapolation of these exist. data based on physical principles rather than direct subjective experience. The second source of errors is caused by the wide variation in sensors available to measure each In this paper, several basic problems parameter. Each type of sensor has a different type associated with this increased dependence on of response to the same environmental conditions instrument measurement and its interpretation are which can result in markedly differing readings at examined. Specifically, errors in the measurement of the same location. Often times instruments are precipitation, wind, and air temperature introduced chosen without consideration of their differing by sensor site selection are considered, as well as, traits, resulting in frustration and/or confusion in limitations on the sensors' responses to the interpreting the data. environment. Errors introduced by poor equipment maintenance, line noise, and calibration problems, Finally, all of the above is further although frequently serious, will not be considered. complicated by the fact that each of t~e major weather parameters (precipitation, wind, and temperature) must be combined to provide meaningful information on snowpack stability. As the best site lAvalanche-Meteorologist, Northwest for one type of measurement may not be the best for AValanche Center, 7600 Sandpoint Way NE, Box another, this results in the merging of data from C-15700, Seattle, Wa. 98115. several different areas and environments. Thus 144

1(J Stampede Pass '- ..... ~ 30 ------Snoqualmie Pass ....Cll C ...;;> ~ A 0- I\ Greatest kl 20 I \ (.. J \ .....Cll c \ ~ 10

Noy Dec Jan Feb Mar Apr

Figure l.--Comparison of weekly precipitation totals in water equivalent between Stampede and Snoqualmie Passes, Washington, 1931-1965. Data are from Climatological Handbook, Columbia Basin States, Precipitation, Vol. 2, septerrber 1969. errors introduced by either poor or unrepresentative specific avalanche starting zone requires site selection or instrument peculiarities can be establishing a proportionality between the amounts additive, further confusing snow stability analysis. received at a sensor site and that at the site in This further emphasizes the importance of question. This proportion will be affected by winds knowledgable selection of both measuring sites and at the starting zones, which may bear little instruments. resemblance to the winds at the measuring site (see below). Thus in order to be accurate under all conditions, measurements should be made at a site which is sufficiently protected to receive snow PRECIPITATION independent of wind speed or direction. The ideal site is usually protected by a combination of Site Selection topographic features and local vegetation Marriott (1984) Although it is possible to use data from less The primary information desired from suitable sites, this requires estimating the precipitation data are the amount and rate of loading magnitude of the effects of the wind at the of the snowpack and the density of new snow. It is measuring site and adds more uncertainty to the well accepted that the areal variation of these data. quantities is affected by the interaction of wind with the topography. On the small scale (e.g. If the area of concern is greater than meters to kilometers) this is by wind scouring and l02-3km2, variations due to orographically deposition of snow and associated crystal breakage, induced lifting must be considered in selecting while on the large scale (kilometers to thousands of measuring sites. Many general variations in kilometers) it is caused primarily by topographi precipitation can be estimated from climatological cally forced lifting and altitudinal effects on information (fig. 1) and/or simple orographic temperature. precipitation models. However, often, mesoscale effects of topography on the synoptic scale air flow Concerns with these effects depend on the size may produce mesoscale effects which become very of the forecasting area. For an area the size of sensitive to small changes in the synoptic scale 2 most developed ski operations «lOkm ) this wind patterns undetectable by current measurements. only requires consideration of the immediate terrain around the sampling site. On this scale, the An example of this is shown in figure 2, which assumption can be made that an approximately equal shows the differences in precipitation between amount of precipitation falls over the area, but is Paradise at 2599m (on the south side of Mt. subsequently redistributed by wind interacting with Rainier) and Crystal Mountain located at 2079m about the terrain. Determination of snow loading for a Bkm to the northwest. Synoptic scale winds 145

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-8 8 12 16 20 24 28

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Figure 2.--Comparison of daily (12OT and our) 850-rob free air wind direction and speed from Quillayute, Washington versus daily precipitation differences between Paradise (Mt Rainier) and Crystal Mountain, Washington. Winds are plotted 0-360 degrees and rounded to the nearest 5 m/sec, and water equivalents (D) indicate Paradise minus Crystal Mt. data. interacting with Mt. Rainier and the Cascade Crest Sensor Errors strongly affect the mesoscale effects of rainshadbwing and convergence in the area around Mt. Rainier. As can be seen from figure 2, there is In snow stability analysis, precipitation data little correlation between the measured synoptic is largely used to give an indication of the amount scale winds (taken at the radiosonde station near and rate of loading of avalanche starting zones. Quillayute, vlashington) and precipitation Historically, this has been accomplished by using a differences between the two stations. This shows snowboard: measuring the depth of new snow, taking a that measurements of synoptic scale winds are too snow core from the board, and subsequently weighing infrequent and too sparse to infer the location and or melting the sample to obtain the water magnitude of this type of effects: Detection of this equivalent. Increasingly, snowboard measurements type of mesoscale effect which is sensitive to have been supplemented or replaced by recording synoptic scale winds can only be found by using a precipitation instruments, almost exclusively "dense" grid of stations or potentially through the measuring water equivalent. A general review of the use of realistic orographic precipitation models types of sensors in current use and their operation (Speers-Hayes 1984). is given in Marriott and Moore (1984). 146

All of the current methods of water equival~nt Gauge Capping.-- Often during sustained moderate medsurement are subject to errors under certain to heavy snowfalls, unheated gauges will accumulate conditions. In some instances these errors can be snow along the rim of the collection cylinder, detected and allowed for, however, this is often especially for orifices of 30cm or less. As this not the case, unless information from more than one accuIDJlation grows, the effective orifice size type of sensor is available fr~n the site. decreases, reducing the measured precipitation, sometimes to zero when complete capping occurs. Gauge Sensors Additionally, following a capping episode, warming temperatures usually result in the eventual melting The most widely used sensors for water of the accuillJlated snow into the gauge, often equivalent measurements are gauge type devices. resulting in an overestimate of the current However, these sensors suffer from a number of precipitation amounts or indicating the occurrence i~herent problems including missed catch(blowover), of precipitation when none is occurring. capping (for unheated gauges) and evaporation (for heated gauges). Figure 4 shows the typical results of measurements taken at the same location from both a Missed Catch.--Wind effects on precipitation heated and an unheated precipitation gauge during measuring sites can introduce serious errors, and January 1984. The unheated gauge SUCCessfully this is particuarly true if the measurements are measured light snowfall on the 20th (confirmed by being taken using a gauge type device. Work by snowboard measurements). However, increasing Larson and Peck (1974) and Goodison (1978) have snowfall at cold temperatures on the 21st thru the shown that wind effects can introduce substantial 23rd capped the orifice of the unheated gauge errors in gauge measurements. The magnitude of the stopping almost all indication of precipitation errors is related to three factors: wind speed, wind during that time, although 7 to 9 cm of water speed vertical profile, and the obstruction to the equivalent were indicated by the heated gauge and air flow presented by the gauge. Figure 3 prepared the snowboard. On the 24th, warming temperatures by Larson and Peck (1974) displays the catch accompanied by rain caused the snowcap to melt deficiency compared to snowboard measurements as a causing an overestimate of the precipitation on the function of wind speed. This shows that even 24th. Obviously, any snow stability analysis moderate winds at a measuring site can cause prepared using the unheated gauge would be in substantial errors in gauge measurements. However, serious error. This type of error can make unheated figure 3 and additional work by Goodison (1977) have gauges virtually useless for measurements of shown that good site selection and proper shielding moderate to heavy snowfalls at temperatures below can reduce or eliminate errors in the measurement freezing. due to missed catch. It is possible to develop a correction factor for the lost catch (Larson and Evaporation.--As can be seen in figure 4 on Peck 1974), however, this requires information on the 20th and 26th and 27th, the unheated gauge wind conditions during precipitation, complicating indicated more precipitation than the heated gauge. ·iata extraction whil~ still producing questionable This effect can be attributed to evaporation within data. the heated gauge. The magnitude of this effect has not yet been quantified, however, the effects appear Wind speed (m/s) to be largest for dry snow at low snowfall rates, especially with propane heated precipitation gauges. PO 2 3 4 6 Almost all heated precipitation gauges use 0.1 tipping bucket mechanisms, which require that the snow must first melt and then drop into a movable bucket inside the heated gauge. This can result in 0.2 evaporation at the external melting surface and, to >- u c a greater extent, within the gauge itself. Q) 03 :~ ~ At low snowfall rates, thG accumulating liquid "0 0.4 .£ ,3 exposed to the internal environment of the gauge U for a relatively long period of time. As the gauge iii 0.5 u is heated above the ambient air temperature, Q) Cl :> resulting humidities inside the gauge are low. For til 0.6 C) example, an ambient air temperature of -lOoC and 100% relative humidity will result in an 0.7 Unshielded gauge internal relative humidity of 23% if the internal temperature is kept at +100 C. Thus the 08 environment inside of the gauge is quite dry and can lead to substantial evaporation. In the case of dry 0.9 continental type snowfalls, the outside relative humidities can be well below 100% resulting in even drier conditions and more evaporation. Fi~ure 3.--Mean gauge catch deficiency of shielded and unshielded United States gauges as a This effect is seen in figure 5 which shows the function of wind speed (Larson and Peck 1974). nifference between an electrically heated precipitation gauge and snowboard water equivalents 147

11 1\ !\ 10 I\ 9 /~\ ~ \\ '-f 8 i ., I \\ '; l! Ii" 6 j iJl ~ ...QI 5 ,; \ ...t 4 ~ II \; 3 I 2 I I 1 J '~ 0 18 20 22 24- 26 28 Da.te--JANUARY Figure 4.--Comparison of daily (24-hr) precipitation totals for Stevens Pass, Washington, during February, 1984. Water equivalent data are derived from electrically heated tipping bucket precipitation gauge (D) and unheated precipitation storage gauge (+) measurements. at Stevens Pass, Washington. Although Stevens Pass amount of heat delivered to the gauge is constant has a strongly maritime climate, it is obvious that regardless of external changes. This requires that at low precipitation rates, the heated precipitation a large enough rise over ambient be selected to keep gauge reads routinely lower than the snowboard. the gauge from dropping below freezing at low Although these differences may not always be temperatures or capping over at high precipitation significant, at unmanned locations, where theSe rates, while still not being so warm as to cause errors can be cumulative, this can lead to serious large amounts of evaporation at low precipitation mistakes in judging snowpack stability. rates. TJ:lis necessary compromise normally results in a loss of accuracy at both extremes. The evaporation problem is particuarly acute for gauges heated by propane as opposed to electricity. Snow Pillows In electrically heated gauges a thermostat is usually provided allowing a certain internal The only non-gauge type water equivalent sensor temperature to be maintained independent of the in use on a wide scale is the snow pillow. This cooling by the external enviroment. Thus a low device essentially measures the weight of the temperature can be selected (5-100 C) minimizing overlying snowpack which is then converted directly the evaporation. In the case of most propane gauges to the water content of the snowpack. Taking the on the market today, a set rise in temperature over difference between successive readings can then give the ambient temperaturel must be selected as the information on the water equivalent of precipitation falling on the surface of the snowpack. Although these devices have proven successful for hydrologi lRecently a new propane heated precipitation cal purposes (Bartee 1978), the snow pillow suffers gauge was introduced which uses a simple thermostat from several inherent problems which may yield to maintain the gauge funnel at or above 150 C, inaccurate or misleading measurements (Beaumont however, no information on the evaporative effects 1965) and limits their resolution especially for of this gauge are available. short term changes (i.e. one day or less). Figure 6 148

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0 1 5 9 13 17 21 25 29 Date--FEBRUARY

Figure 5.-~omparison of daily water equivalent amounts between 24-hr snow board (0) and electrically heated tipping bucket precipitation gauge (+) measurements at Stevens Pass, Washington, during February, 1984. illustrates the significant short term differences a snow pillow. Thus, although they may be used to that can exist between snowboard measurements and a estimate the magnitude of precipitation events, even Snotel (Barton and Burke 1977) snow pillow located this use is difficult unless information on local at the same site at Stevens Pass, Washington. It is temperature changes and/or other independent obvious that the snow pillow can read signficantly indications of precipitation are available. above and below the measurements taken from a Despite their lack of short term reliability, the snowboard. These differences are associated with snow pillow's widespread distribution throughout the the layered nature of the snowpack and the complex mountains for hydrological purposes makes them a way it is supported by the underlying ground. useful source of information on precipitation, but information that can only be used to substantiate The interpretation of the snow pillow information from other types of nearby precipitation data would be easier if it measured consistently sensors. greater or less than the snow board, however, due to the differing sources of error it can vary either Summary way. In deep snowpacks, the response time of snow pillows to heavy snowfalls can be slow, ranging from Thus selection of representative Bites for 5 hours to as much as 10 days (Tarble 1968). precipitation measurement depends partly on the size Warming or cooling of the snowpack may result in of the area of concern and partly on the complexity erroneous indications of increasing or decreasing of that area. In all events, the site should be water =ntent, respectively. Formation of crusts protected from wind effects either by the local and ice lenses within the snowpack may result in topography, vegetation, and/or artificial shielding bridging of the pillow, giving erroneously low so that the precipitation measured at the site is values of water equivalent. In addition, in shallow independent of local wind speed or direction. In snowpacks diurnal variations in the temperature of addition, for larger forecast areas, a sufficent the snow pillow itself may give erroneous values. nurrber of sites must be measured to detect mesoscale variations in orographically induced precipitation The net effect of these combined errors is to caused by effects such as channelling or limit the accuracy of short term changes measured by convergence. 149

2 1.9 1.8 1.7 1.6 1.6 1.4- 1'- 1.3 .. 1.2 E '0 1.1 -i 1 , 0.9 .. 0.8 ..t 0.7 ~ 0.6 0.6 0.4- 0.3 0.2 0.1 0 18 20 22 24- 26 28 Da.te--FEBRUARY Figure 6.--Comparison of daily water equivalent amounts between 24-hr snow board ([J) and 24-hr snow pillow (+) data at Stevens Pass, Washington, during February, 1984.

Additionally, all of the available. methods of Site Selection measuring water equivalent produce significant errors under certain conditions. Realistically, Selection of sites for wind measurement most measurement sites suffer from some type of requires evaluating the potential effects of all of deficiency which may cause these errors to become t~ above winds on the avalanche starting zones of manifest.' The practical solution to this appears to interest. The main consideration in choosing a be the use of one or more types of sensors at a location(s) for measuring winds is finding a site site. This may not be simply another precipitation which gives wind information from which starting sensor, but perhaps information on temperature or zone winds may be inferred. wind. With several pieces of information, it may become possible to recognize and correct errors when For small scale forecasting concerns t~y occur. «lOkmf) this often means locating the wind system close to the main starting zones minimizing the amount of inference necessary. This is particuarly WIND effective for areas whose starting zones are at approximately the same elevation with the same aspect. A wind system which gives accurate results In snow stability forecasting, wind information for directicos which would tend to load these slopes is primarily important for estimating the degree and is usually sufficient. However, for areas with a nature of snow depostion over the topography of variety of starting zone elevations and aspects, it concern. Synoptic scale free air winds interacting is necessary to choose a site which is likely to with the mountain topography create a large variety give a reasonable estimate of the local of local winds depending on the orientation of the free air wind speed and direction. Adding local free air wind to the terrain. However, this problem topography to this information then allows is often further complicated by winds driven by estimating the winds for any aspect, but not mesoscale pressure differences across a mountain necessarily any elevation (see below). A site which range, small scale channeling effects, and/or could satisfy this criteria would be an isloated, drainage winds. symmetrical peak. More often wind sites are located 150

on ridgelines or on peaks located along ridgelines ~ll but the most extended heavy riming conditions. which then provide a varying response depending on wind direction. This type of variation can be The most difficult interpretive problem roughly accounted for, except in the extreme case associated with rime formation occurs when an where the ridgeline or a local obstacle blocks wind anemometer is only partially rimed. In total r1m1ng from certain directions. In some cases this may situations, it is frequently obvious that zero wind require more than one measuring location. speed is not physically reasonable and that the anemometer is rimed up. However, in partial riming Small scale wind channeling, drainage winds, or situations, the anemometer may continue to turn, but winds wi thin inversions may produce ·dramatic at a reduced speed, giving low wino Ej)eed readings variations in winds at different elevations not that may mislead the forecaster, leading to a large derivable from measurements at any single underestimate of wind transport of snow. Similar elevations. Figure 7 illustrates the types of pr:dJlems of interpretation occur when the wind vane variation that can occur in a complex wind-terrain rimes, especially if the anemometer melts out first. situation. In this example, winds at Denny Mountain (1696m), Stampede Pass (1204m), and Snoqualmie Pass Summary (llS8m) are shown. Snoqualmie Pass and Stampede Pass are located about 1.Skm and 8km southeast of Denny The selection of wind sensor sites must be Mountain, respectively. The dramatic differences determined based on the size and complexity of the between wims measured at the three sites is area of concern. For forecasting situations obvious, both in directions and speeds. involving small areas «lOkm2 ) it is frequently Importantly, notice that there is little consistency possible to select a representative measuring site in the differences between the stations. Thus which can be used to infer winds affecting all significant variations in winds which may be loading starting zones of interest. However, in situations avalanche stating zones are not. easily inferable where wim-terrain interaction is complex, several frOm the measurements taken at anyone of these sites may be necessary to achieve the required sites. Although this is an extreme case, it resolution. Sites which are used to estimate emphasizes the potential for major differences in comitions over a large area (> lOl-3km2 ) must the wims which are loading starting zones in a be placed in positions which are as free as possible relatively small area. from the small scale local effects, accepting a consequent loss in resolution of starting zone For snow stability analysis for larger scale winds. Errors associated with the sensor areas (>lOl-3km2 ) of concern, it is necessary measurements themselves are primarily limited to to measure wims which are representative of the rime deposition on the sensors, inhibiting their la~est area possible. Since local effects have such motions, and producing false readings. Riming a strong influence on measured winds, site selection problems can be overcome by heating the sensors or, becomes critical. This usually requires placing the in some cases, by locating the sensors at a wind site at a location which measures the mesoscale relatively rime free site. free air winds. Although this often requires sacrificing important information on local effects, it is required by the need to apply the information AIR TEMPERATURE to areas beyom the immediate vicinity of the wind site. In this case, it is usually necessary to infer Air temperature measurements supply significant significant local effects from experiential information to snow stability forecasters, allowing knowledge of local behavior (when available) with a judgements to be made about such things as: the com"mensurate increase in possible errors. likely types and rates of metamor:phism in the snowpack, crystal types and densities of snowfalls, Sensor Errors surface melt and crust formation, and occurrence of rainfall, etc., all of which relate intimately to Although a variety of wind sensors are the strength of the snowpack. commerically available (Marriott and Moore 1984), the most commonly used are variations on the cup Site Selection anemometer and wind vane or the single unit "bird" which uses a plane-like body with a F~opellor and a Although temperatures in the mountains are tail. The only serious problem with wind sensors la~ely controlled by a combination of free air themselves occurs during periods of rime formation. freezing levels and diurnal variations, these In light riming situations heat lamp deriming has factors are sometimes overpowered by more localized shown some effectiveness, however, moderate to heavy effects such as terrain induced inversions. riming areas require the use of a conduction heated Temperature variations caused by topographic effects anemometer (Taylor 1984). The only alternative to may act on the scale of a few meters and be of only this is to locate the anemometer downwind of a minor significance or, in some cases, may operate on terrain feature which may remove most of the the scale of hundreds of kilometers. supercooled water droplets minimizing the riming of the instrument, but at the cost of less A single air temperature sensor does not representative measurements. Some wind vane designs provide sufficient temperature data for either local also suffer from riming problems requiring heating, or regional snow stability analysis, particuarly in however, it appears that wind vanes with a the the case of inversions. Inversions of lOoe to streamlined profile and a mean length to width ratio 200e between the top and bottom of a ski area are of about 4:1 or higher, maintain enough of their not unusual, am in some instances, may be initial shape to remain oriented into the wind under maintained for days. A single measurement taken at 151

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SOO ~ ! 260 ~ e 200 ~ q 160 ~ ~ 100 ---- - yariab1e - - ---

2 6 10 14 18 TillE (PST)

Figure 7.--Comparison of hourly wind speed and direction data from three different elevations in the Washington Cascades. Data are derived from wind sensors at snoqualmie Pass (1158m, D), Stampede Pass (1204m, +) and Denny Mountain (1696m, <», Decerrber 30, 1983. 152

either the top or the bottom of the area could be levels may exist for extended periods. misleading, however, measurements at both can show the existence and the magnitude of the inversion. Long lasting inversions (hours to days) can Measurements at intermediate levels may be of have a significant effect on the timing of warming interest when the top of the inversion lies within induced avalanches. Figure 8 illustrates the the elevation range of the forecast area, as this complexity that can result within n eingle ski area. will help to locate the top of the inversion, which The time variation of temperature sensors located at is often a relatively, sharp boundary and can lead 914m, 1341m , and 1646m in the Alpental ski area are to rapid localized temperature variations. shown. Initially, a relatively normal lapse rate exists with temperatures cooling with increasing Inversions are especially important for elevation. At about 1300 PST, warming at higher temperatures near freezing as variations in elevations produces an inversion somewhere between temperature in this range can have strong and the stations at l341m and 1646m, although, a normal immediate effects on snowpack stability. However, lapse rate continues to exist between the two lower even at temperatures well below freezing inversions sensors. Around 2100 PST the sensor at 1341m begins may have significant effects on snowpack to warm as the top of the inversion gradually metamorphism and the formation of surface hoar. lowers. Finally around 0100-0200 PST, the inversion drops completely below the middle sensor, while a For snow stability forecastin~ for areas on the normal lapse rate develops between the two upper scale of less than about 10km , temperatures sensors. In this example it is obvious that one or should be measured near the elevation of the even two temperature sensors in this situation could starting zones of concern as a minimum, or in the result in an erroneous impression of the case of starting zones at multiple elevations, temperatures in the area and consequently could lead temperatures should be taken both at the highest and to a poor estimate of avalanche stability, if lowest elevations. Temperature measurements at temperature sensors were not located near the additional elevations will improve the vertical elevation of the starting zones. Thus for stability resolution of temperature but are probably only analysis , the vertical temperature resolution essential in areas where inversions at intermediate required will depend on the distribution of

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Selection of temperature sensor location de ACKNOWLEDGEMENTS pends both on the scale of the area under consid eration and on the complexity of the associated terrain. Snow stability analysis for a small area The authors wish to extend their appreciation to the many observers who contributed data which was (~lOkm2) may be accomplished with a single used to develop this paper, only a small fraction of temperature sensor if inversions tend to be weak or which is shown. Special thanks to the Washington short lived. However, if inversions are strong and Department of Transportation Avalanche Control Crew persistent, it may be necessary to measure the at Stevens Pass, the Stevens Pass Pro Patrol, the temperat~re near the elevation of all major groups Soil Conservation Service, the Crystal Mountain Pro of start~ng zones to obtain sufficient vertical temperature resolution. Patrol, the Snow Rangers at Stevens Pass and at Paradise on Mt. Rainier, and the National Weather For larger scale areas of concern (lOl-3km2), Service Forecast Office in Seattle. for their extra sensors should be located on ridges or at sites assistance. unaffected by local temperature effects such as drainage winds or trapped cold air, allowing extrapolation of the information over a wide area. ~hen geographical conditions can produce regional ~nversions, however, it is desirable to obtain 154

LITERATURE CITED Larson, L.W. and E.L. Peck. 1974. A~curacy of precipitation measurements for hydrological Bartee, D.L. 1978. Snow sensor evaluation report. modeling. Water Resources Res., Vol. 10, pp. USDA Soil Conservation Service Special Report. 857-863. West Technical Service Center, Portland, Oregon. Marriott, R.T. 1984. Snow study sites. Avalanche Review, Vol. 2, No.4, pp.4. Barton, M. and M. Burke. 1977. SNOTEL: An operational data acquisiton system using meteor Marriott, R.T. and M. B. Moore. 1984. Recent burst technology. Proceedings 45th Annual advances in the collection and transmission of Meeting Western Snow Conference, pp. 82-87. mountain weather and snowpack data for avalanche forecasting. Northwest Avalanche Beaumont, R. T. 1965. Mt Hood pressure pillow snow Center Technical Memorandum No.5, Seattle, gage and application to forecasting avalanche Washington. hazard. lASH Publication No. 69, pp. 341-349. International Symposium on Aspects of Snow and Speers Hayes, Pamela. 1984. Diagnosis of Ice Avalanches, Davos, Switzerland precipitation in mountainous terrain: part I, rain shadow effects. Proceedings of Goodison, B.E. 1977. Snowfall and snowcover in International Snow Science Workshop, Aspen, southern Ontario: principles and techniques of Colorado. assessment. Ph.D. thesis, University of Toronto, Toronto, Ontario. Tarble, R.D. 1968. California federal-state snow sensor investigations, problems, and rewards. Goodison, B.E. 1978. Accuracy of Canadian snow Proceedings 36th Annual Meeting Western Snow gage measurements. Journal of Applied Conference, pp. 106-109. Meteorology, Vol. 27, pp. 1542-1548. Taylor, P.L. 1984. Measurement problems in mountain weather. American W<'\te::- Resources Association Symposium, Seattle, Washington.