Everyone talks about the weather - Especially astronomers trying to see the objects in deep space! Here are a few factors that astronomers look for in a potential telescope site:

The Number of Cloud-free Nights

Clouds, fog, and haze all make observing difficult or impossible. Dry climates generally have fewer clouds. Astronomers measure the number of cloud-free nights using all-sky cameras at the site and/or satellite imagery. More cloud-free nights are generally better!

Precipitable Water Vapor (PWV)

The dryness of the air is important for both observing conditions and equipment maintenance. Infrared light is blocked by water vapor in the air, so infrared observations require a dry site. High humidity—lots of water vapor—promotes rusting of equipment and dew forming on the mirror and instruments that increases the operating costs. PWV is measured from space using satellite images, and by using detectors called radiometers on the ground at the site.

Wind Speed and Direction

Wind direction and strength are important for the design of the telescope, especially very large ones. High isolated peaks such as those that are good observational sites are also prone to high winds—at some sites exceeding hurricane speeds. Large telescope enclosures have to be very strong to withstand strong winds, so higher winds requires stronger enclosures that increase construction costs. Strong winds blowing around and into the enclosures can cause vibrations in the telescope that make the seeing worse. The direction of the prevailing winds also affects where the dome is placed on the site to minimize the wind. So fewer and slower prevailing winds are better.

Annual Weather Patterns

Annual weather patterns must also be considered. Kitt Peak has an annual monsoon season with thick clouds and heavy rains that last through the months of July and August. During the monsoon, a clear night is so unlikely that the observatory simply shuts down for maintenance every year during those two months. Site that are dry all year round are preferable, but in the case of Kitt Peak, excellent observing conditions during the rest of the year outweighed the loss of observing in the summer.

Global Climate Patterns

Although an observatory site is at one place on the surface of Earth, the local weather at that place is best understood in the context of global weather patterns. The global weather pattern is illustrated in the figure below:

Figure by Karen Masters at Cornell Univ.

The Sun heats the surface of the Earth more strongly at the equator than at the poles. As the air over the equator heats, it rises and flows north and south toward the poles. Cold air over the poles sink towards the surface and the air flows out from the poles over the surface. This would form a single cell of air rising at the equator and sinking at the poles, but the rotation of the Earth breaks the flow into three zones:

  1. The hot equatorial zone where rising air cools and drops huge amounts of rain onto the surface allowing the world’s tropical jungles to grow and thrive. Clouds are almost constant, and the prevailing surface winds flow from east to west;

  2. The cold polar zone where cold sinking air dries and cools the surface and prevailing surface winds are also flow from east to west; and

  3. The Temperate zone where temperatures are warm part of the year and cool part of the year, and prevailing surface winds flow to from west to east. Sinking air in the temperate zone warms and dries the surface causing the world’s deserts to form in a band around 30 degrees north and south of the equator:

Deserts Figure courtesy of the United States Geological survey.

Ocean currents also help form climate zones on land. As shown in the figure below, warm currents form in the equatorial regions, and cold currents in the polar regions.

Figure © National Maritime Museum, Greenwich, London

The east/west flowing currents are deflected north and south by the continents, with cold currents flowing south along the west coast of North America, and north along the west coasts of Africa and South America. Cold currents cool surface air and reduce turbulence in the winds flowing ashore. The cold air also helps confine the (reduced) turbulence to a thin layer near the ground, allowing coastal mountains to stick above the turbulence into the smoother air.

As can be seen in the figure below, the pattern of ocean temperatures reflects the excess heating at the equator, the cooling at the poles, and the movement of warm and old ocean currents.

Sea Surface Temperature Sea Surface Temperature image is from Unisys in the Image and Map Archive.

By looking at all three diagrams, one can see that the combination of dry air (deserts) and winds flowing from the sea onto the land over a cold ocean current only occurs along the west coasts of North and South America and Africa near latitude 30 degrees north and south. Some of the driest deserts in the world are fond in these locations: the Atacama in South America, the Namibia in Africa, and the American Southwest desert in North America. Some of the driest and smoothest air in the world is found in these locations, making them prime regions to locate telescopes.

Measuring Weather

Local weather variations can be found in part by looking at satellite data over the test sites. Here are some Internet sites with International weather images:

Internet Sources for Weather Satellite Images

Office of Satellite Operations: GOES Satellites. This site contains GOES satellite images for the last month. Click on “Real-time Images” and then click on image of choice.

Space Science and Engineering Data Center. This site contains a complete archive of weather satellite data, including GOES. Click on “Archive Data Information” to enter the archive.

Weather Data And Satellite Images at ESO. Current weather images focused on Northern Chile. Click on the image of choice. The “Latest Thermal IR Image over Chile” is a good one to start with.

Weather data specific to a particular site can be gathered from portable weather stations placed temporarily at the test sites. If such data are not available, satellites images can be used to get estimates of the realtived properties of the sites.

Cloud cover data can be obtained from visible or infrared satellite images. One possibility would be to select 26 bi-weekly (i.e., one years’ worth of data) night-time images of the site, say at midnight, and tabulate the cloud cover for each image. Then create the ratio of clear nights to total nights for each site. This rather crude measure can be improved by including more images: say, each week or every day at the same time, or including more years. You can be as detailed as you wish.

Similarly, Precipitable Water Vapor can be obtained from the standard GOES Water Vapor Images. Select 26 bi-weekly night-time images of the site, say at midnight, and measure the pixel value for the water vapor over the site for each image. Then average all your pixel values together for a measure of the water vapor that can be compared between sites.

Wind speeds and annual weather patterns can be obtained from local weather services like Look fro a link to Climate or Averages. If you cant get the exact site, find the nearest city for which data exist.