We continue to assemble evidence that Mt.Evans may provide one of the best INFRARED and SUB-MILLIMETER SITES available in the CONTINENTAL U.S., due to its extreme high altitude (4,303 meters), accessibility (paved road to summit), infrastructure (new 2,100 sq ft observatory building, nearby base camp at 10,600 ft) and proximity to transportation, supplies and universities in Denver. A permit from the Forest Service has *already been obtained* for the addition of a 4 meter class telescope. An FCC license for broadband 6.5 GHz line of sight communication to Denver campus, 50 km distant, is also in hand.
Copy of Cloud cover and water vapor report by Andre Erasmus available on request to rstencel@du.edu
Note: Figures referenced hereafter are available
on request.
Update: 1991-1996 remote weather station summary.
SECTION 1 includes the basic METEOROLOGICAL conditions such as temperature, wind, humidity, and precipitation, based on nearly 5 years of in-situ monitoring with a remotely accessible weather station package.
SECTION 2 compiles a list of observations in an effort to determine the CLOUD COVER STATISTICS for the mountain, using a variety of means.
SECTION 3 discusses "SEEING" measurements that have been conducted at the summit, using double stars, acoustic soundings and differential image motion monitoring.
SECTION 4 addresses SKY BRIGHTNESS issues for the site.
SECTION 1: METEOROLOGICAL CONDITIONS
=========================================================================== TABLE 1.c COMPUTED MEAN WATER COLUMNS BY MONTH FOR 1995 --------------------------------------------------------------------------- Month Computed millibars H20 Equivalent mm prec.H20 JANUARY 1.5 1.1 FEBRUARY 1.5 1.1 MARCH 2.0 1.5 APRIL 2.0 1.5 MAY 2.5 1.9 JUNE 3.5 2.6 JULY 4.3 3.3 AUGUST 5.5 (monsoon) 4.1 SEPTEMBER 3.0 2.3 OCTOBER 2.3 1.7 NOVEMBER 1.5 1.1 DECEMBER 1.5 1.1 --------------------------------------------------------------------------- Note: 1 atm = 760 mm = 1013 mb pressure at sea level, 0.75 mm/mb ratio. ---------------------------------------------------------------------------Recently, JHK region atmospheric daytime spectra were obtained with the DU ASTI spectrometer -- see Figure 1c.
A more direct measurement of the water column was done using a near infrared (1-2 micron) prism spectrometer featuring a single element pyroelectric detector. The instrument was designed to measure the 1.1 and 1.4 micron absorption features of water using the sun as a source of background continuum. Using the atmospheric transmission code RADCO, we were able to deduce the water column from the measured equivalent widths of the H2O absorption features. The results of the measurements are listed in Table 1C. Results are consistent with values derived from the temperature, barometric pressure, and relative humidity data for the same time of day and year. Unfortunately, a direct comparison was not possible due to a later discovery that the relative humidity sensor was not operating properly during this time interval. Individual p.w.v. column readings have been as low as a few hundred microns.
A one hundred page report "Water Vapor as a Factor in the Selection of Solar Observation Sites" by N.Medrud, NCAR/High Altitude Observatory, March 1970 is available from R.Stencel/DU on request. It includes Rocky Mountain sites.
D. SNOWFALL
Daily records of snowfall have not been measured directly at the summit, but such data has been acquired at our Echo Lake Lab, located approximately 15 miles to the north at an elevation of 10,600 feet, during the past several decades. This location should represent an adequate proxy to the summit for measuring snowfall. In Figure 1D we present the monthly snowfall amounts for the past six years running for the Echo Lake locale. Note that December and January are very dry months as also indicated from water vapor data. November and March are the snowiest months with year to year variability being quite large. This is consistent with experience of Colorado skiers, that there are fresh autumnal and spring snows, separated by a sometimes long, mid-winter dry spell.
SECTION 2: CLOUD COVER STATISTICS
B: Weather Bureau, and DU Line of Sight Observations
1) clear, potentially photometric skies; 2) partly cloudy but SFA (suitable for astronomy, spectroscopic); 3) mostly cloudy and not SFA; 4) upslope conditions.
During upslope conditions it not possible to ascertain conditions at the summit. Therefore in principle, mornings placed in this category, may possibly add to those in categories 1 or 2. In Figure 2B we present a month to month summary for the percentage of time that conditions atop Mt. Evans are photometric (1), SFA (1 plus 2), and undetermined (4). The basic result is that 2/3 of the days are SFA, and 1/2 of those appeared photometric. Experience with observing during summer 1998 showed that 24 of 42 nights were useable (57%) including monsoon periods of mid-July to mid-Aug. Excluding those weeks increased the percentages toward 3 out of 4.
=========================================================================== TABLE 2.b FRACTION (%) OF OVERNIGHTS AT LEAST PHOTOMETRIC & SPECTROSCOPIC --------------------------------------------------------------------------- Quarter: WINTER SPRING SUMMER FALL YEAR am pm nt am pm nt am pm nt am pm nt 1994 62 62 62 74 51 50 1995 71 62 62 56 52 52 75 65 65 70 64 58 1996 63 60 56 65 59 56 76 67 64 48* 55* 48* 1997 50* 53 50 57 45 45 62* 53* 53 52* 58 52 1998 -- -- -- -- -- -- 68** 65** 1999 52** --------------------------------------------------------------------------- Aves: 61 58 56 59 52 54 71 62 62 61 57 55 Range 10 4 6 6 7 8 9 9 6 13 7 10 --------------------------------------------------------------------------- Notes: WINTER=JanFebMar SPRING=AprMayJun SUMMER=JulAugSep FALL=OctNovDec am = morning line of sight (with hours before sunrise prob clear). pm = evening suitable for astron (workable clear evening). nt = evening and following morning BOTH indicating good astron weather. *ElNino weather pattern influence, arrived fall'96. Similar comments are reported for the VLT/Paranal, Chile site by Giacconi et al. 1999 A&A 343:L3. ** Readings after 1997 are derived from hourly photovoltaic data records. Details of the daily records that figure into this summary are available on request.
C: Thesis study on Cloud Climatology = 65% clear.
In this section we relate information reported in a 1981 Master's thesis by Roger Lee Sorensen (Colorado Sate University, Fort Collins, Colorado) entitled "Cloud and Insolation Climatology for Selected Colorado Stations". Of most interest to conditions atop Mt. Evans was Sorensen's study of cloud cover amounts for two observing stations, one in Denver and the other in Colorado Springs. Sorensen computed monthly cloud cover averages for a given hour of the day covering a 10 year time span (1952- 1961). The data is presented in Figure 1C. Cloud amounts for the morning hours are consistent with observations presented in parts A and B of this section. This data is somewhat difficult to gauge in that one cannot ascertain the percentage of totally clear days from this study. For example, the data shows that in the early morning hours of July, that the average cloud cover is roughly 35%. On one hand, this could mean that 35% of the days were totally cloudy and the others totally clear. On the other hand, this could mean that each and every day was 35% cloudy. One must also be cautious in using weather data from Denver as a proxy to conditions atop Mt. Evans. However, it does appear representative of our basic claim that 2/3 of the time is at least suitable for astronomy.
As this satellite image reveals, we occasionally see an "upslope" condition, where a southern low will force moisture up against the foothills, but the higher mountains remain in the clear.
SECTION 3: SEEING STATISTICS = subarcsecond, uncorrected.
SECTION 4: SKY BRIGHTNESS AND EXTINCTION
UBVRIJHKLMNQ extinction studies are planned for 1999, once the new scope is on-line.