Del, Article...20th Century Cooling at Mount Wilson, California..
Mount Wilson has a special place in the history of 20th century science. Mount Wilson Observatory was founded by George Ellery Hale in 1904. The site was carefully selected over many other candidate sites because of its exceptional “seeing”. The Observatory is located an hour's drive from downtown Los Angeles which, at first glance, would seem to be a disadvantage for astronomers. However, the world-class seeing is a natural blessing related to the fact that Mount Wilson (elevation 5,700') rises above the infamous inversion layer that traps turbulent air (and pollutants) in the Los Angeles Basin below. Because of its close proximity to the Pacific Ocean, the Observatory receives a smooth flow of relatively pristine air that has traveled thousands of miles across the open ocean. The air that reaches the Observatory is remarkably decoupled from the air in Los Angeles, below.
The exceptional seeing that results from this setting has allowed major discoveries to be made from research at Mount Wilson. Edwin Hubble essentially invented modern cosmology from his work at the Observatory. Using the 100-inch telescope, Hubble discovered that the universe is made up of billions of galaxies, each containing billions of stars. He found evidence of an expanding universe (the Big Bang), and his observations and theories revolutionized the way we view our place in the cosmos. Similarly, measurements of the Sun from atop Mount Wilson have led to many of the greatest discoveries in solar physics. New telescopes under construction on the mountain will certainly lead to equally exciting and important discoveries in the next century as scientists continue the legacy of fundamental research at the Mount Wilson Observatory.
There was another observation program at Mount Wilson of potential interest in the ongoing climate change debate. The same great astronomers who worked on Mount Wilson maintained a weather station at the site. Following each night's activities, they would go to the weather station and record the daily maximum and minimum temperatures, precipitation over the past 24 hours, snowfall and snow cover, and notes of special interest to astronomers (e.g., comments about the seeing or even comments about current visitors). They kept the written record in a library at the Observatory and these hand-written records are still there to this day. Over the past few months, we have entered all the daily records into a computer file allowing for statistical analysis of variations and trends over this century.
The nearly-continuous daily weather records at the Observatory begin in November 1918 and end in December 1984. Some earlier records were found from as early as April 18, 1904, but many gaps exist in the earliest years. In addition to these observations, there is a post office on the mountain less than a mile from the Observatory, and a daily weather record is available from that station from July 1948 to present.
A plot of the mean monthly temperature anomalies (deviations from 1951-1980) for Mount Wilson Observatory is shown in Figure 1. If we look only at the actual observations made at the Observatory (the open circles), we find that Mt. Wilson cooled at a highly statistically significant rate of 0.26¡C (0.47¡F) per decade from 1918 to 1984. Multivariate statistical techniques were used to match the post office records to the Observatory records during the 1948 to 1984 overlap period, and the resulting equations could then estimate the recent values for the Observatory. When these estimates are added to the dataset, we found that the Mount Wilson Observatory cooled by 0.20¡C (0.36¡F) per decade from 1918 to 1998.
That's another amazing observation from Mount Wilson! Here we have a relatively pristine site that receives a flow of air from the Pacific, and over this century the site has been cooling steadily. Microclimatic changes related to construction at the Observatory and near the post office should have produced a warming bias, and yet, the temperature data reveal a rather steady cooling throughout this century. The warmest temperature recorded at the Observatory was 101¡F (38.3¡C) in August of 1933. The only other days with a 100¡F (37.8¡C) temperature occurred in 1919 and 1924.
As with many other parts of the planet, Mount Wilson is experiencing a significant decline in the diurnal temperature range as maximum temperatures fall and minimum temperatures remain unchanged. At other mountain sites, similar declines in maximum temperature have been ascribed to increases in cloudiness. Although we do not have daily data on cloud cover for Mount Wilson, it is noteworthy that the post office station has recorded a significant increase in precipitation over the past 50 years, which may be an indication of increased cloudiness.
One of the scientific legacies from the Mount Wilson Observatory is the reliance on observational evidence to support or refute theoretical predictions. Obviously, the weather records atop Mount Wilson are limited in their ability to support or refute theoretical predictions about global warming. But we must wonder what the great scientists of the Observatory would say if a theory predicted warming for Mount Wilson, and a century of observations showed cooling. Would they accept the theory or accept the facts? |
