Mt.Evans Weather Station, summary 1991-1996 Summary for CSU's Colorado Climate Newsletter 12/20/96 R.E.Stencel, J.W.Williams, D.I.Klebe, J.R.Starkey, M.Read, R.Boyce et al. University of Denver, Dept. Physics & Astronomy, Denver 80208 Email contacts: rstencel@du.edu, mread@du.edu Summary: In 1991, a remotely accessible weather station was installed to support renewed use and development of the University of Denver's Mt.Evans Observatory, located near the summit of this 14,268 ft Front Range peak. We report on weather observations made between 1991-1996. ----------- Introduction: ----------- Astronomers are hostage to weather conditions, and Denver University's Mt.Evans Observatory is no exception. Because of its extreme high altitude (14,125 ft), it was considered important to characterize the site weather extremes, with respect to human and mechanical limits Our weather station equipment, originally installed in January 1991, includes a Campbell Scienctific model CR210 datalogger, powered with three photovoltaic panels that feed three marine deep-cycle storage batteries. A Motorola cell phone and Campbell Scientific modem are interfaced for remote downloading of data files. On a mast approximately 1 meter above our summit A-frame building, are housed a temperature, relative humidity and barometric pressure sensors. Above these, on the same mast, is an R.M.Young propeller anemometer and wind vane. The original temperature and RH sensor was a Campbell model XN217 probe which includes a Hygrometrix RH sensor and a Fenwal Electronics UUT51J1 thermistor. A special barometer (Campbell model PTA 427A) was also installed to be able to sense as low as 550 mb pressure (ambient at Mt. Evans summit is approximately 600 mb). The anemometer and windvane suffer ice collisions and has been repaired annually. In June 1996, a model LI200-SZ pyranometer, manufactured by LI-COR, Inc., of Lincoln, Neb. was added. It measures global solar plus sky radiation, converting it into a current. However, our Campbell Scientific datalogger requires us to add a resistor to convert the current into a voltage for recording. The pyranometer was calibrated against an Epply precision spectral pyranometer under natural daylight conditions, exhibiting an absolute error of 5% at a maximum, 3% typically. Because of its non-linear spectral response, it is used unfiltered. The datalogger was configured to log hourly averages of data recorded each minute of the following types: date, time, RH%, temperature, standard deviation T, average wind speed (WSp), standard deviation WSp, last minute sample WSp, maximum WSp (in one minute during hour), average wind direction (WD), standard deviation WD, sample WD, internal system temperature, battery voltage, barometric pressure, pyranometer average output, maximum and standard deviation. The photovoltaic panels were roof mounted and aimed near the altitude of the winter noontime sun to maximize charging at those times. The sensors were mast mounted about 5 feet above the roofline of an A-frame structure near the summit, and this roofline is about 12 feet above the surrounding terrain. however, the mast was placed on the downwind end of the A-frame to accomodate USFS concerns about its visibility to visiting public. This placement could have affected wind direction and possibly temperature values to some extent (see below). As with any station, we suffered our share of maintenance issues. for instance, low temperatures and slow rates of charging from the photovoltaic cells caused the cell phone to drop off-line around the time of winter solstice, however, most data recovered from datalogger due to multimonth capacity. Some sensors invariably failed from time to time and were replaced, so coverage was not 100%. These data in raw hourly averages can be provided on magnetic format, although lack of staffing may delay delivery of portions requested. Quartile statistics are being developed. --------------------- Temperatures Observed: --------------------- Initially, we have were pleasantly surprised to find the average wintertime temperatures in 1991 and 1992 were not severe, except briefly during passage of a front. However, increasingly cold temperatures have been recorded since, including the record (so far) of minus 32F on 12/18/96, during the arrival of a strong arctic high. Table 1: SUMMARY MT.EVANS SUMMIT TEMPERATURE OBSERVATIONS Winter: Jan-Feb-Mar 91 92 93 94 95 96 Min(F) +1.4 +5.7 -5.0 -5.9 -13.8 -23.8 Max(F) 36.0 43.7 37.0 39.7 36.3 33.8 Spring: Apr-May-Jun 91 92 93 94 95 96 Min(F) 6.3 14.1 8.7 10.2 0.0 -6.1 Max(F) 58.9 56.5 58.2 54.9 47.7 61.0 Summer: Jul-Aug-Sep 91 92 93 94 95 96 Min(F) 22.5 23.2 17.2 9.9 6.9 18.5 Max(F) 60.7 60.4 63.1 51.0 55.8 64.3 Fall: Oct-Nov-Dec 91 92 93 94 95 96* Min(F) -1.5 +1.0 -6.6 -20.3 -12.1 -32.0* Max(F) 52.3 49.3 51.3 37.3 43.7 51.3* *based on partial coverage (thru day 340) of final quarter 1996 Some entertaining "trends" emerge from this sample of extremes, assuming sample completeness is sufficient and instrument calibration has remained stable. For example, we see a 10 degree decrease from prior year average beginning Summer of 1994 through the cold Spring of 1995. Fall and winter of 1991-1992 appear warmer than the average to date. We cannot rule out temperature probe problems as a factor in these results. ---------------- Relative Humidity: ---------------- Because the weather station measures quantities at 14,000 ft altitude, the information concerning humidity and implied partial pressure of water vapor has been of particular interest to our astronomy research efforts. When the water vapor is minimal, infrared transparency in the 1-20 micron wavelength interval can be excellent, permitting astronomical observations that are possible only at a handful of other observatories on earth. Using a hydrostatic equilibrium approximation for earth's atmosphere and the partial pressure of the water vapor with altitude (scale height 2.3 km), for 1995, we obtain a minimum precipitable water column of 1 mm during January and February, rising to 10 mm during "monsoon" periods of July and August, and then dropping again to dry December values, for 1995. ----------------------- Wind speeds and direction: ----------------------- Of keen interest for astronomers is knowledge of details of the airflow patterns around observatory sites, in order to characterize the potential for high clarity optical imaging (called "seeing" conditions). Treeless mountain ridges offer the best chance for laminar, turbulence-free airflow under certain conditions. Gusty winds of too high a speed will interfere with telescope operation and seeing. Legends concerning Mt.Evans suggested 200 mph winds were to be expected, in analogy to locations like Squaw or Milner Pass, and chinook conditions along the Front Range generally. Although jet stream visitation is plausible, to our pleasant surprise, we clocked winds no faster than 100 knots. Loss of data was generally due to the RMYoung propeller unit failing, because of shear of a plastic shaft (replaced with metal), loss of retaining nut and/or breakage due to icing. In several cases, the propeller was located a short distance downwind of the weather mast, when loss of retaining nut was the failure mode. As of this writing, the cause of the late October 1996 loss of wind speed data has not yet been determined, due to limited access during snowy autumn weather. Table 3: MT.EVANS SUMMIT WIND SPEED OBSERVATIONS Top 3 readings: Max. Windspeed per minute, in knots (Day number) 1991 71.6 (day027), 68.5 (day045), 65.9 (day047) 1992 95.1 (day105), 84.4 (day223), 82.4 (day083) 1993 93.1 (day339), 78.2 (day184), 72.9 (day326) 1994 96.3 (day112), 76.1 (day311), 73.3 (day117) 1995 85.8 (day338), 81.4 (day136), 80.9 (day133) 1996 99.5 (day353), 97.0 (day339), 85.2 (day335) There is a very clear correlation between windspeeds in excess of 30 knots and direction 250 degrees (WSW), due to topology and prevailing winds. The majority of high readings in the full sample occur prior to day 115 (mid-April) or after day 300 (early October). The preponderance of wind directions from the 250 degree azimuth is presumed to be primarily a consequence of local topology, and slightly due to mast location on the east end of the A-frame structure. On most occasions, an observer well clear of the A-frame will experience an airflow from the west-southwest. The Mt.Evans summit forms a natural wind barrier to the north and northwest. To the west lies the Abyss Lake cirque (descending 2.000 ft below Mt.Evans) and Mt.Bierstadt (14,001 ft summit). This topology is probably controlling the predominant airflow direction. Airflow from other directions is generally less frequent and much lower speed. Unobstructed exposures are found to the south and east. Not infrequently, can one observe a moonlit ocean of upslope clouds to the east, topping out at altitudes below the summit. -------------------- Irradiance/cloud cover: -------------------- Prior to the 6/96 installation of the pyranometer, it is possible to make a limited assessment of the daytime sunlight/cloud cover statistics atop Mt.Evans because the battery voltage record tracks the output of the photovoltaic cells. However, a limiting circuit was added to avoid driving the batteries to over-voltage, so the hourly record has to be examined with some care. The sunrise and sunset slopes can indicate clear or cloudy weather in a statistical sense, and if sun angles are taken into account. Line of sight morning and evening observations of the summit cloud cover have been made from the DU campus since early 1994, and these corroborate the indication that just over 50% of the evenings and following mornings have been cloud-free, and suitable for astronomical observations. ----------- Conclusions: ----------- We have remotely monitored weather conditions atop Colorado's Mt.Evans in order to better characterize conditions related to operation of DU's summit observatory. While mountain-based activities are a challenge under any condition, we find the conditions are suitable for astronomy sufficiently often to justify the investment of continued effort. Whether longer term climate changes will significantly modify these conclusions remains to be seen. We plan to continue observing real-time conditions for equipment and personnel safety, but we would welcome discussion with potential partners in the atmospheric community who would like to participate in recording and analyzing the data. Enclosures: assorted annual variations.