TCS Daily

Between The Stars

By Sallie Baliunas - September 28, 2001 12:00 AM

(Editor's note: This article, co-authored by Tech Central Station Science for the Earth Host Sallie Baliunas appears in the latest issue of Weatherwise, and includes information relevant to the ongoing debate about global warming.)

By Randy Cerveny, Darlene A. Periconi, Robert C. Balling Jr., and Sallie Baliunas

Hubble. Shapely. Michelson. The names of these celebrated astronomers conjure up images of distant stars and galaxies and revolutionary ideas about our expanding universe. Those scientists, however, were not only illustrious astronomers but also diligent weather observers at the world famous Mount Wilson Observatory in southern California, located just a few miles from the smog choked freeways of Los Angeles.

The observatory houses one of the most unique West Coast weather records of the 20th Century. In addition, the observatory is one of the few long term weather sites in the world that is located next to a major urban center but above the marine inversion layer.

After decades of neglect, Mount Wilson's weather records have only recently been rediscovered. These long forgotten records have proved to be an invaluable tool in studying the weather and climate history of California and may also be useful in the ongoing debate about global warming.

Beyond the value of a long term weather record, the Mount Wilson Observatory is also renowned in the scientific community for its research into the remarkable effects of solar activity on Earths weather. It may soon help reveal secrets about the weather on worlds far beyond our own.

Close but Not Too Close

The observatory, considered one of the premier observing sites in North America, is located a short 45 minute drive from downtown Los Angeles. It sits near the top of Mount Wilson's 5,693 foot peak in the San Gabriel Mountains, which overlook the Los Angeles coastal plain to the south.

Astronomer George Ellery Hale founded the observatory in 1904; in fact, many of the observatory's telescopes were hauled to the top of the mountain in mule drawn wagons. The site was selected over many other locations because of its exceptional atmospheric conditions for making ground based astronomical observations. One of the primary reasons for constructing a telescope so close to metropolitan Los Angeles -- which, even by 1904, was showing signs of becoming heavily populated -- was to take advantage of its location above the marine inversion layer.

Coastal locations are frequently marked by a distinct boundary between the moist air associated with the sea breeze and the drier air above. Above this inversion boundary, the thinner, drier air aloft can produce truly exceptional sky viewing. This inversion layer is the rationale for the location of many maritime observatories, such as Mauna Loa in Hawaii. Mount Wilson rises above the inversion layer that traps most turbulent air and pollutants in the Los Angeles basin below.

Because of its proximity to the Pacific Ocean, the Mount Wilson Observatory also receives a smooth flow of air that has traveled long distances across the open ocean. The air that reaches the observatory is often distinctly separate from the air below in Los Angeles, and the general lack of turbulence and atmospheric pollutants provides superb "seeing" at the site.

Consequently, even though Mount Wilson is located north of one of the most populated (and polluted) regions of the country, the astronomical observatory has remained in operation and even flourished for nearly a century because of the high quality of its visibility. Frequently, the potentially blinding glare from L.A.'s multitude of city lights is hidden below the observatory under a layer of dense, often cloudy, maritime air.

The Stars Come First

Unlike its meteorological cousin to the east, the Mount Washington Observatory in New Hampshire, the Mount Wilson Observatory never formalized a partnership with the National Weather Service to distribute its weather reports because its observations were secondary to its astronomical duties.

Yet even without such a partnership, after each work shift (around 6:00 a.m.) one of the astronomers or other observatory members collected a variety of weather observations that included the date and time of the observation, sky condition, wind direction and speed (with the highest gust of the past 24 hours), the barometric pressure, and total precipitation (including snowfall). Yes, Mount Wilson receives a fairly significant amount of snow in the winter, despite its proximity to the mild climate of coastal California. Indeed, Mount Wilson astronomers have recorded as much as 10 feet of snow at the observatory.

These weather records were collected on daily log sheets entitled "Mount Wilson Weather Bulletins." With only a few exceptions, a complete set of weather observations was recorded from November 1918 through December 1984, with a few records found from as early as 1904.

Until very recently, the original bulletins were filed away in a cabinet in the observatory's library, unknown to meteorologists and climatologists. However, two years ago, we combined forces to save this potentially important weather record. Three of us from the Arizona State University Office of Climatology, along with Sallie L. Baliunas, assistant director of the Mount Wilson Institute, have begun the laborious task of digitizing and analyzing the relevant (and less relevant) weather information in each observation.

Each bulletin form provides space for the observer to enter any general remarks or observations. Normally these remarks relate to the "seeing" quality of the night before -- for example, the obscuring nature of smoke from nearby forest fires or the amount of snow (and its hindering of astronomers' getting up to or down from the observatory) -- or to which astronomer was posted at what specific telescope.

However, a few comments have historical importance. For example, the notation from Nov. 11, 1918, is "News of signing of armistice ending the World War came at 12:55 a.m. Made some noise!" and that of March 29, 1929, "Col. Char. A. Lindbergh visited the observatory yesterday afternoon and last night." Other illustrious visitors to the observatory have included Albert Einstein and Stephen Hawking.

Our research uncovered no evidence that the station had been moved over the collection period, and based on historical photographs of the observatory, the forest setting of the station remained relatively unchanged during the entire data collection period. This ensures the integrity and consistency of the weather record.

In addition to these unique observations, a daily weather record from July 1948 to the present is available from an official NOAA cooperative station established at a post office at Mount Wilson about one half mile from the observatory. Unfortunately for scientists wishing to study the data from that site, the weather station at the post office is located on the roof of the building and close to the potentially contaminating effects of a variety of mechanical units. Furthermore, the post office is located within 500 yards of a series of communication towers built on the mountain over the period of observations potentially changing the microclimate of the area.


This detailed, long term record of weather observations has been extremely useful in studying West Coast weather. In a study recently published in the technical journal Geophysical Research Letters, we analyzed the record statistically and discovered a large and significant decline in maximum temperatures measured at the observatory from 1918 to the present, coupled with only a small upward trend in minimum temperatures and a substantial decline in the diurnal temperature range (DTR) -- one of the key variables often discussed with global warming.

Increased atmospheric carbon dioxide has often been cited as having a potentially larger effect on nighttime temperatures than on daytime temperatures. If increased carbon dioxide raises nighttime temperatures more than daytime temperatures, the daily temperature range will decrease. While we found that for Mount Wilson, the DTR is indeed decreasing, that change appears to be the result of a decline in daily temperatures rather than a marked increase in nighttime temperature. At the present time, we are not sure why this may be occurring.

We also identified a very strong relationship between temperatures recorded at Mount Wilson and precipitation during El Niño and El Niña events.

Despite the relative obscurity of the station as a resource for weather data, it has been well known for the research it supports into the effects of solar activity on Earth's weather. Baliunas and her colleagues at the observatory have published several scientific papers about the relationship. Over the last decade or so, the researchers have addressed three major solar influences on climate:

  • Changes in the overall amount of solar radiation received on Earth (i.e., the solar "constant").

  • An 11- year solar cycle change in the amount of ultraviolet radiation, which affects the amount of stratospheric ozone and therefore, eventually, surface temperatures.

  • Changes in the number of solar charged particles that interact with the Earth's magnetic field and eventually affect the Earth's atmospheric circulation and temperatures.

  • Through these studies, these "climatological astronomers" have shown that the sun may have dimmed slightly in the mid twentieth century, which could be the cause of cooler temperatures on Earth. They base this conclusion on studies of regions of intense magnetic activity (sunspots) on nearly 1,000 stars similar to our sun. The sun and other stars brighten during times of heightened magnetic activity and dim when magnetic activity wanes. Together these stars are like time machines because they help us to understand the sun's history and future.

    The research, which links the age, magnetic activity, and brightness of stars, suggests that the sun may decrease in brightness by about 0.4% within the next 50 years. That dip could cool the Earth by as much as 2 degrees F. A similar decline in sunspot numbers ("the Maunder Minimum") occurred in the 17th Century and was apparently related to the Little Ice Age on Earth.

    Otherworldly Weather

    Where will the Mount Wilson Observatory and its weather connection eventually lead? Perhaps even to the discovery of weather on planets beyond our own solar system. A National Science Foundation sponsored project, the Center for High Angular Resolution Astronomy (CHARA), involves the development of a new stellar interferometer that will allow scientists to see astronomical objects approximately 100 times more sharply than traditional telescopes can. Its precision is equal to seeing the edge of a nickel from 10,000 miles away.

    The new equipment will be used in almost all areas of contemporary astronomy. It is particularly suited to stellar astrophysics, where it will be used to measure the diameters, distances, masses, and luminosities of stars as well as spots and flares on their surfaces. With the development of CHARA and similar imaging technologies, the journey toward understanding solar/terrestrial relationships -- and perhaps eventually visualizing the weather of worlds beyond our own solar system -- has begun.

    Already using sophisticated imagery tools, scientists at Arizona State University are relating dust devil and dust storm generation on Mars to similar kinds of weather in the desert southwest of the United States. Similar extraterrestrial imaging studies at NASA are comparing the structure of major storms on other planets (such as the Great Red Spot on Jupiter) with similar vortices like hurricanes on the Earth. CHARA and other projects undoubtedly will allow even more detailed study of these important links between terrestrial and extraterrestrial weather.

    Weatherwise Contributing Editor Randy Cervany, Darlene A. Perconi and Robert C. Balling Jr., are members of the Department of Geography and Office of Climatology at Arizona State University in Tempe, Ariz. Sallie L. Baliunas is assistant director of the Mount Wilson Institute in Pasadena, Calif., and an associate of the Harvard College Observatory.


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