Abstract
The temporal variation in the direct normal irradiance, diffuse horizontal irradiance, global horizontal irradiance, and ambient temperature was quantified during the 21 August 2017 solar eclipse. The magnitude and obscuration of the eclipse at the observation location were 0.97 and 96.94%, respectively. A two-axis sun tracker equipped with a pyrheliometer, a shaded pyranometer, an unshaded pyranometer, and a pyrgeometer were utilized to obtain the solar irradiance measurements. Thermistors contained within the pyrheliometer and the pyrgeometer were used to quantify changes in temperature. Unfortunately, on the day of the eclipse, the weather conditions were not ideal and periodic cloud coverage impacted measurements of the direct normal irradiance. As such, solar irradiance and temperature data from 2 days prior, when the weather conditions were near-ideal, are presented for comparison. The temperature of the air as measured by the thermistors enclosed within the pyrheliometer and pyrgeometer recorded an average temperature drop of 8.49 °C. The minimum temperature was recorded approximately 47 min after totality. The diffuse horizontal irradiance reached a maximum value of 495.9 W/m2 shortly after the start of partial eclipse and dropped to a minimum value of 0 W/m2 near totality. Similarly, the global horizontal irradiance reached a maximum value of 1138 W/m2 and dropped to a minimum value of 1.54 W/m2 near totality. For reference, at the same time of day, but 2 days prior, the diffuse horizontal irradiance and global horizontal irradiance were at values of 89.2 W/m2 and 962 W/m2, respectively, under clear sky conditions. The rate of decrease in the temperature of the air as well as in the diffuse and global horizontal irradiances after first contact and before totality was greater than the rate of increase in these parameters after totality. The difference in the magnitude of these gradients before and after totality is largely attributed to normal diurnal variations as the solar eclipse occurred in the afternoon.
Highlights
A total solar eclipse provides a unique opportunity to study temporal changes in the direct and diffuse components of solar irradiance at the surface not associated with normal diurnal variations
The station data were accessed via the Climate Data Online (CDO) tool available at the National Centers for Environmental Information’s (NCEI) website
The temperature decreased at an average rate of 0.091 °C/min between first contact and the minimum temperature occurrence
Summary
A total solar eclipse provides a unique opportunity to study temporal changes in the direct and diffuse components of solar irradiance at the surface not associated with normal diurnal variations. In addition to changes in the magnitude of the solar irradiance, the occultation of the sun by the moon impacts various meteorological variables such as temperature, wind speed, and relative humidity. The solar eclipse on 21 August 2017 in the United States afforded researchers the opportunity to collect measurements in the path of totality. The path of totality crossed the United States from the Northwest to the Southeast. The first point of contact was near Lincoln Beach, Oregon at 16:05 UT and the last point of contact was near Charleston, South Carolina at 20:09 UT. The longest duration of totality, 2 min and 40.2 s, occurred near Carbondale, Illinois at 16:52 UT
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