The United States will have a front-row seat to one of the most spectacular solar eclipses to sweep across the country in our lifetimes. Millions of lucky observers from coast to coast will have the chance to watch the Moon scoot in front of the Sun on the afternoon of August 21, 2017, briefly plunging cities like Salem, Oregon, Hopkinsville, Kentucky, and Columbia, South Carolina, into night-like darkness during the day. Read our field guide to the solar eclipse for tips on how to make the most of this spectacular event.
While a solar eclipse can be amazing to behold, the phenomenon has little impact on Earth. It may, however, have a small but noticeable effect on weather in the areas that experience a total eclipse.
The entire country will be able to see the Moon cover the Sun in some form, but the best viewing areas will be along a northwest-to-southeast path across the middle of the country. According to NASA, a location needs at least 90 percent coverage to notice any darkening at all, and even 99 percent coverage of the Sun only provides the same level of darkness you'd see at twilight. Areas totally covered by the Moon's relatively narrow shadow will experience conditions akin to dusk, prompting street lights to turn on and even tricking birds and bugs into thinking that the day is drawing to an end. Studies have shown that the total eclipse could also have an effect on temperatures and even winds.
Researchers who studied an eclipse across Europe in 1999 found that the event lowered air temperatures by as much as 5°F across the path of totality. This brief dip in air temperatures also affected local wind speed and direction—not by much, but it was enough for both people and instruments to take notice of the so-called "eclipse wind." The effect on the atmosphere in Europe wasn't a fluke. A weather station in Zambia recorded a temperature drop of nearly 15°F during a solar eclipse in June 2001, and there are reports through history of observers noticing a distinct cooling effect in the midst of a lunar shadow.
The duration of the eclipse and the amount of moisture in the air will determine how much the Moon's shadow will lower temperatures. Moist air has a higher heat capacity than drier air, so when it's muggy outside it takes longer for the air to warm up and cool down. This is why daily temperatures fluctuate less in Miami, Florida, than they do in Phoenix, Arizona. Communities that lie among the drier, cooler Rocky Mountains are more likely to witness a noteworthy dip in temperatures compared to states like Tennessee or South Carolina, which are typically locked in the humid doldrums of summer at the end of August.
If you're lucky enough to witness this spectacular astronomical phenomenon, make sure you bring your eclipse glasses—and a thermometer.