Site Validation

Written by Dr. Gordon Spear

After the selection of the most promising site (see Site Selection), it was necessary to verify that the selected site would be acceptable as a site for significant astronomical research. To accomplish this task a set of instruments was developed which could independently record the necessary data at a remote site. The necessary data would include cloud cover, temperature, humidity, wind speed and direction, and barometric pressure. Because the site will need to be off-the-grid, solar radiation measurements would also be useful. Finally, astronomical seeing measurements depicting the atmospheric stability at the site would be especially important.

The instrumentation designed and assembled to make these measurements is known as the Site Survey Solar Unit, or S3U. Since Galbreath is located in an isolated and remote part of Mendocino County without electrical power, solar panels are used to obtain power and charge a small bank of batteries. This provides adequate power for the required measurements. This prototype unit was tested for a year at a site on the Sonoma State campus.

The Prototype S3U on the SSU Campus

During this time improvements and enhancements were made to the initial design. A location was prepared for installing the S3U on Observatory Ridge in the Western Highlands of the Galbreath Preserve during the spring of 2007. The S3U was moved to the Observatory Ridge site and began taking data on May 27, 2007.

The S3U Installed on Observatory Ridge

The instruments are installed on a south-facing hill just below the high point along Observatory Ridge. The weather station is in the foreground. The S3U consists of two solar panels and a charge controller which charges a bank of six 160 Ah golf cart batteries. Instrumentation consists of a Davis weather station, a Boltwood cloud sensor, and an SBIG seeing monitor to record atmospheric stability. Data from the seeing monitor and the cloud sensor are recorded in real time on a laptop computer. The weather station has on-board memory which can store approximately 90 days of data at a time. These data is manually downloaded to the computer when the instrumentation is visited. Weather data and cloud data are recorded every 30 minutes. Seeing data are recorded approximately every minute.

Front View of the S3U

The solar panels can be seen. The black cloth is sun shade to minimize heating of the batteries and instruments during the summer.

The Rear View of the S3U

The battery box is above and the instrument shelter is below.

Closeup View of the S3U Weather Station

The anemometer can be seen at the top. The small white cylinder near the top of the mounting pole is the Boltwood cloud sensor. The large black cylinder below is the rain gauge. Small solar cells provide power to run a small fan within the station to insure accurate air temperature measurements.

The data that are recorded by the S3U includes:

* temperature
* humidity
* barometric pressure
* wind speed
* wind direction
* rainfall
* incident solar radiation (W/m2)
* solar UV index
* cloud cover index
* astronomical seeing (stellar FWHM in arcsec)

The selected site can be considered to be validated if the following conditions are satisfied:

* considerable percentage of cloud-free days with conditions acceptable for obtaining quality astronomical photometry
* seeing generally better than 4 arcsec, with considerable time with the seeing approaching 2 arcsec
* temperature, humidity, and wind speed not too extreme
* adequate solar radiation for providing electrical power for the observatory


Data

Cloud Sensor

The Boltwood cloud sensor monitors cloud cover by measuring the temperature of the sky using an upward looking IR bolometer (central wavelength about 5 microns). This is compared with the temperature of the ground obtained using a downward looking temperature sensor. The downward looking sensor is essentially measuring the ambient air temperature at the observation site. The cloud index, termed SkyRI, is the simple difference between the temperature of the sky and the temperature of the ground ( SkyRI = T(sky) - T(ground) ). Temperatures are reported in degrees Fahrenheit. Since the sky always has a lower temperature than the ground, the cloud index is always a negative number. The thicker the cloud cover and the lower the clouds, the closer together the two temperature readings will become. The index will approach zero for very low thick clouds, or dense fog. Here is an example of cloud sensor data for most of the month of June, 2007.

Observatory Ridge Cloud Cover for June 2007

The upward excursions on the plot correspond to cloudy conditions. There were three brief episodes of cloudy weather each lasting about a day. Obviously, most days this month were relatively free from clouds and would have provided excellent conditions for astronomical observations. At the Observatory Ridge site at the Galbreath Preserve in June at the start of summer, under clear conditions, the sky is about 55 degrees colder than the ambient temperature near the ground.

As an example of a cloudy episode, here is a plot of the cloud index for the few days surrounding June 29.

Observatory Ridge Cloud Cover in the vicinity of June 29, 2007

In general, a SkyRI value greater than -30 corresponds to total cloud cover that would be unusable for optical astronomy. The casual observer would call such conditions CLOUDY. SkyRI values between -30 and -40 indicate partial, or thinner cloud cover. Such conditions might allow differential astronomical photometry. The casual observer might call such conditions PARTIALLY CLOUDY. SkyRI values less than -40 would correspond to perfectly clear conditions. Such conditions might allow all-sky photometry. (Note, however, that the Boltwood cloud sensor is not really sensitive to scattered cirrus clouds.) The casual observer would call such conditions CLEAR.

As an example of an episode of clear days, here is a plot of the cloud index for a few days surrounding June 25.

Note that the cloud index never exceeds -40 during this interval. All-sky photometry would likely have been possible during any of these nights. Note, however, that there is a systematic diurnal variation evident. The clearest conditions typically occur near the time of sunset in the early evening. Then the atmosphere appears to become slightly less transparent as the night progresses. This could correspond to an increase in atmospheric humidity as the ambient temperature drops. Shortly after sunrise there is also an apparent decrease in transparency of the atmosphere. This could correspond to the heating of the upper atmosphere during daylight, or could reflect the normal summer haze caused by increased aerosol concentration. Such aerosols could be dust particles, pollen, or smoke particles.

For comparison, the following plots show several of the weather station parameters for the selected cloudy days and clear days in June, 2007.

Parameter
cloudy days
clear days
temperature
humidity
solar radiation
wind speed

Cloud Sensor Plots and Summaries

The cloud sensor index, SkyRI, may be used to indicate the aproximate sky conditions based on these index values. These associated sky conditions may also be used to indicate the types of observational astronomy that would be possible. Obviously, under normal situations, astronomical observing would only occur during night time when the sun is not above the horizon.

SkyRI Index
Conditions
Observational Astronomy
>= -30
Cloudy
none
-40 < SkyRI < -30
Partly Cloudy
differential photometry, spectroscopy
<= -40
Clear
all-sky photometry, differential photometry, spectroscopy

The identification of the Partly Cloudy condition from the cloud sensor index is only approximate and merely represents an intermediate condition between "obviously clear" and "obviously cloudy." A sensor index in the partly cloudy range can indicate the presence of some clouds, but an examination of the diurnal variation of the index on days when it is "obviously clear" indicates that the index might occasionally exceed the somewhat arbitrary limit of -40 when conditions are actually clear with no indication of clouds. This diurnal variation is likely related to an increase in atmospheric aerosol concentration which appears to reach a daily maximum near mid-day (noon). Thus, the percentage of Partly Cloudy conditions as judged by the sensor index is likely to be slightly over-estimated. This will not significantly affect the assessment of the suitability of the Galbreath site for astronomical uses.

Since measurements are recorded every 30 minutes, each measurement can be used as an indicator of conditions during that 30 minute interval. Thus, we can use these measurements to determine the percentage of time during a month that conditions were Cloudy, Partly Cloudy, or Clear. Obviously, optical astronomy is not possible when conditions are Cloudy, but observational astronomy would be possible at the Galbreath site during Clear or Partly Cloudy conditions.

Month
Clouds

Measurements

Clear
(%)
Partly Cloudy
(%)
Cloudy
(%)
June 2007
1121
89.3
4.4
6.3
July 2007
1260
86.0
7.0
7.1
August 2007
plot
1439
88.3
4.4
7.4

At least in June and July it would appear that more than 90% of the night-time hours could be used for optical astronomy.

Weather Station Plots

Here is a summary of some of the environmental and atmospheric data from the Observatory Ridge site at the Galbreath Wildlands Preserve.

 

Month
Temperature
Humidity
Pressure
Wind Speed
Wind Direction
Solar Radiation
UV Index
Rain Fall
June 2007
July 2007