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22 June 2017

How a Total Solar Eclipse Impacts upon Local Temperatures

It stands to reason that when the sun is blocked out during a total solar eclipse, temperatures will drop to some extent; that’s just basic logic. However what is less clear is just how substantial this effect may be, and the specifics of the variables involved.

Certain regions of the world will witness a total solar eclipse later this very year, on August 21st to be exact, with the phenomenon recurring approximately once every 18 months. During such celestial events the moon completely covers the disk of the sun as seen from Earth, and many report experiencing a somewhat sudden drop in temperature during these periods of ‘totality’, but just how severe is this cooling effect?

If we look back as far as the total solar eclipse which occurred on December 9th 1834, some regions reportedly experienced a drop of as much as 28°F/15°C, plunging from 78°F to 50°F, or from 25°C to 10°C if you prefer. However during a later total solar eclipse witnessed on the Norwegian Island of Svalbard in March 2015, the drop in temperature was less severe at around 15°F/8°C; from a starting temperature of 8°F/-13°C, the region dropped to a decidedly chilly -7°F/-21°C over the course of the event.

Looking at the above data, it seems that ascertaining the exact drop in temperature resulting from such eclipses isn’t quite so straight forward as you may initially think; additional variables are certainly involved besides the loss of direct sunlight, such as location and time of year. This stands to reason, given the inconsistent climate experienced across the globe and throughout the various seasons.

While the exact expected drop can be difficult to calculate before the event occurs and measurements are taken, we can make a pretty close estimate. This is because blocking the sun during a total solar eclipse is in principle no different to what happens following the sunset; the sun hasn’t disappeared, but we no longer feel its warming effects. The reason as to why the drop in temperature resulting from a total solar eclipse may feel more severe than that caused by nightfall is that it happens at a much faster rate. Going from basking in the sun’s rays to sitting in the dark in such a short length of time makes the difference much more pronounced and detectable to the human body.

The aforementioned information allows us to make a pretty decent estimate in regards to the temperature drop, which according to Rick Feinberg of the American Astronomical Society averages out at around 10°F/5°C.

What is interesting is that the lowest daytime temperature experienced due to a total solar eclipse actually occurs approximately 2 minutes after the end of totality. This is according to Jay Pasachoff, an astronomer at Williams College who has been studying the local effects of eclipses alongside his colleague Marcos Peñaloza-Murillo. This is believed to be a result of the insulating properties of the atmosphere which means that sunlight doesn’t heat the air directly. Instead, the sun heats the floor which in turn heats the air, resulting in the slight delay in cooling.

At present there is still a significant lack of data regarding the local effects of total solar eclipses, and given how the scientific fields tend to advance I’m sure there’s some surprises waiting to be revealed, but for now take comfort in the fact that a solar eclipse is no more threatening than nightfall, at least in regards to temperature.

On the subject of safety, those planning to view the total solar eclipse this year, which will be visible across a 70-mile-wide strip of the US during August, should take care not to do so directly with the naked eye. The damaging effects of this are widely known at this point, but a little reminder can never hurt.

The path of totality across the US for the August 2017 eclipse is displayed in the graphic below:

Img: NASA/Goddard/SVS/Ernie Wright
Sam Bonson

Sam is an aspiring novelist with a passion for fantasy and crime thrillers. He is currently working as a content writer, journalist & editor in an attempt to expand his horizons.