Just How ‘Cool’ Was The Eclipse? Temperature Data From NASA Langley
Anyone who witnessed the total solar eclipse on August 21st, 2017 would agree--it was an awe-inspiring event. The Sun was blotted out by the Moon, a 360° sunset glowed from every direction. Nocturnal animals like crickets began their night-time activities while people went quiet to watch the stars come out in the middle of the day. Even for those not in the path of totality, the extra-sharp shadows cast by the partially eclipsed Sun and the crescent-shaped lights cast through pinhole projectors, interlaced fingers, gaps in leaves, and even through kitchen colanders made the event something to remember.
During a solar eclipse, the Moon comes in between the Earth and Sun. As a result the Moon blocks incoming solar radiation before it reaches the Earth. Over time these events have been studied and notes have been made describing a chilling effect accompanying eclipses throughout history. A study during a partial eclipse over the United Kingdom in 2015 recorded temperature drops up to 3 °C, and thousands of users across the Americas used GLOBE Observer to record the temperature drop during the August 2017 eclipse.
Earth gets nearly all of its energy from solar radiation. Temperatures drop at night because one side of the Earth is in shadow and is therefore no longer receiving energy from the Sun. Similarly, when clouds cover the sky, temperatures on the surface often drop because though there is radiation reaching the clouds, not as much energy reaches the ground, making it feel cooler.
GLOBE Partners at NASA Langley were among those who made measurements. The NASA Langley Research Center in Hampton, Virginia experienced an 85% partial eclipse from 13:21 until 16:06 Eastern Daylight Time (or 17:21-20:06 UTC) with intermittent cloudiness before and after the eclipse but clear skies during. A weather station on-site measured air temperature and solar radiation once per minute and was used to provide reference measurements. Figure 1. shows the drop in solar radiation (points where clouds obscured the Sun are removed from the eclipse curve, for the sake of clarity) and the corresponding drop in air temperature.
One interesting point is the lag time between the dropping sunlight and the air cooling. Peak eclipse occurred at 18:46 UTC, while the lowest temperature (29.2°C, down two degrees from 31.0 °C) wasn’t recorded until 20 minutes later. Just like a hot cup of tea, the atmosphere takes time to cool down once it’s no longer being actively heated.
Scientists at NASA Langley also made air and ground surface temperature measurements with GLOBE instruments during the partial eclipse. Figure 2. shows the same air temperature reference measurements as Figure 1. alongside measurements made using a digital thermometer.
Both thermometers measured the same temperature drop once measurement uncertainty is taken into account. Figure 3. shows the observed temperature drops as well as each thermometer’s uncertainty range. While the digital thermometer showed different numbers than the reference station for the decrease in temperature, it kept the reference station’s measurements within error bars for the majority of the observation period. The slight variance towards the end of the eclipse could be due to changing wind patterns, or perhaps differences in cloud cover between the reference station and the GLOBE observers. The measurements were not taken at the same exact location, so there were some site differences.
The atmosphere isn’t the only part of Earth affected by changes in solar radiation. Anyone who has ever walked across a hot, sandy beach or dark pavement know the ground heats up in the Sun as well. Using an infrared thermometer, the Langley researchers measured the temperature changes on both grass-covered soil and cobblestone pavement. In Figure 4., we see that both surface types experienced large temperature drops as the eclipse progressed. Note that while both graphs appear similar, the maximum temperature shown for the grass temperature plot is the minimum temperature for the cobblestone pavement plot.
It’s also interesting to note that the surface temperature changes did not lag behind the eclipse as much as the air temperature drop. A combination of factors are responsible for this difference. First, on a dry day in Hampton, Virginia, there is more water in the air than on the ground. Water has a very high heat capacity, meaning it can hold a large amount of heat energy in the absence of a heating element (like the Sun). This energy is released slowly as heat when water molecules collide with other compounds, which brings us to the second factor controlling heat dissipation in the atmosphere: circulation. Unlike the solid particles of grass, soil, or pavement, air is blown around by wind and circulated by atmospheric convection. This mixing of the gases in the air means that even while the air in a certain location is cooling, fresh air from warmer locations may be mixing in, dampening any short-term changes in overall temperature. These factors together help explain why surface temperatures change more quickly than air temperatures, and also why the fluctuations in surface temperatures may be larger.
Even if you didn’t get to witness the total solar eclipse of August 2017, the data collected from across the Americas made great use of the natural experiment on solar radiation and proved to us all: a solar eclipse is very cool.
Writer: Jill Teige, assisted by Preston Lewis