Eclipse meteorology

“Plan of Observations. -The meteorological observations proposed were especially directed towards the subjects of barometric pressure, air temperature, humidity, solar radiation, and wind velocity. The instruments located on the top of the tower were in charge of Mr. Rotch, and those at its base Mr. Upton.”

―Winslow Upton and A. Lawrence Rotch, Meteorological and other observations made at Willows, California, in connection with the total solar eclipse of January 1, 1889. Annals of the Astronomical Observatory of Harvard College, Vol. XXIX, 1893.

Diagram of the solar eclipse
Drawing of the total solar eclipse of January 1, 1889

The scientific instruments used during a solar eclipse include telescopes with a protective filter to reduce the brightness of the Sun to protect an astronomer’s eyesight. The earliest known image of the Sun taken by a camera was recorded in 1845. But it was still common for scientists to draw sketches with pencil and paper as it has been done for centuries. The above image is based on a number of photos taken with different exposure times. The short exposures record bright features but leave out many of the fainter ones. The longer exposures cause the bright features to be overexposed but reveals subtle details. The sketch above is a composite of these different photographs.

Survey map of Willows, CA
U.S. Geological Survey map showing Willows, CA in 1904 with railroad tracks shown as lines with tick marks

Ladd Observatory astronomer Winslow Upton and his colleague A. Lawrence Rotch, a meteorologist from Blue Hill observatory, traveled to the small town of Willows California to observe a total solar eclipse. In addition to telescopes they also used scientific instruments that you might not expect – a weather station. This was done to record changes in the atmosphere during the time that the Moon blocks sunlight.

They traveled to Willows by train. It is north of Sacramento in the Central Valley. The weather forecast for this region was much better than along the coast which often has rain or fog.

Railway Map of Northern California

They setup their weather instruments a short distance from astronomers of the Harvard College Observatory at the western edge of the town. The party from Lick Observatory that recorded the diagram above was in Bartlett Springs to the southwest. Some of the weather instruments were at ground level but some instruments including wind speed and direction were measured at the top of a small tower that was about 30 feet tall.

Observation tower
“The Observation Tower from the South-East.”

When the eclipse began they noticed a change in animal behavior which they noted in their report. “With the rapidly increasing darkness the cocks began to crow, and a frightened bird flew close to the tower.” “Wild geese were heard passing, near or during totality, and a cock crowed during the total phase.”

Graphs of weather data during the eclipse

Some of the instruments were read and recorded manually. Others used a chart recorder that left a line on paper as a cylinder slowly rotated. There was a distinct drop in temperature, as would be expected, that occurred 10 minutes after the total phase of the eclipse. This coincided with a large increase in relative humidity. The temperature dropped by 6.3° F and then increased by 4.7° F after totality. They concluded that the change in air pressure was not that different from the previous day and couldn’t be attributed to the eclipse.

Eclipses have been described as a natural meteorological experiment. It allows scientists to study how the atmosphere changes when the sunlight that is heating the air is temporarily removed. Weather observations are still conducted during eclipses. Upton and Rotch performed pioneering work on this subject.

Partial solar eclipse of August 21, 2017 at maximum

At Ladd Observatory we tried to use a modern digital weather station to repeat the kinds of measurements done by Upton and Rotch in 1889 during a partial eclipse visible from Providence in 2017. Unfortunately the weather was partly cloudy during most of it. Because it was a partial instead of a total eclipse the sunlight was not completely blocked. We expected a smaller drop in temperature. The cloud cover prevented us from detecting any meteorological change due to the eclipse with the clouds blocking more sunlight than the Moon. We did, however, capture some images like the one above during breaks in the clouds

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