Author: Matthew Cappucci / Source: Science News for Students
On August 21, 2017, the moon cast a temporary shadow across a wide swath of the United States. The event spanned 14 states and lasted for up to two minutes 41 seconds in any one spot.
Many areas were plunged into brief, total daytime darkness. Others experienced a subtle, hour-long partial dimming of the sun.As millions of people along the eclipse path watched in awe, cadres of scientists went to work. The moon reduced — and in some places blotted out — warming rays of the roughly midday sun. Some scientists were looking to log local weather changes. Not surprisingly, scientists now report, the eclipse produced a localized short-term cooling along its path.
But that was not all.
Wind speeds changed. Some clouds evaporated in the abrupt cooling. In all, the rare spectacle provided loads of useful data to researchers who are trying to better understand how the globe responds when the sun’s light briefly disappears.
The most easy-to-measure changes were drops in air temperatures at Earth’s surface.
The Kentucky State Mesonet is a network of 68 automated weather stations. They span the state. These stations measured temperature, winds and intensity of sunlight every three seconds throughout the eclipse.
The station in Hopkinsville fell in the center of the shadow’s path. It got to feel the eclipse’s full impacts. Totality is the brief, spectacular event where the sun’s rays are fully blocked and no sunlight reaches the ground. Ten minutes after totality had ended in Hopkinsville, the air had cooled by 4.4 degrees Celsius (8 degrees Fahrenheit). It would take another 75 minutes to warm back up again.
As these data show, that drop in temperature tends to lag the moon’s shadowing by several minutes. That’s because of the way air is warmed. The sun doesn’t heat the air. Instead, it radiates energy that warms the ground. That heated soil soon warms the air immediately above it. Shortly after that, the surface air will move the heat — through a process known as convection — up into the atmosphere. This entire process takes a few minutes. That’s why the maximum temperature drop does not occur at the point of maximum shade.
In dry climates, the sudden cool-down from an eclipse can be even more dramatic.
Dan Satterfield is the chief meteorologist at WBOC-TV in Salisbury, Md. He traveled to Alliance, Neb., to watch the eclipse. There, he recorded a 5 degree C (9 degree F) decline in air temps. “It was a bit more than I expected actually,” he now recalls. “But then I realized that we were at over 4,000 feet [1,219 meters] elevation.”
Air tends to be drier at higher altitudes. That cooler air, higher up, can hold less water. Water takes a long time to gain or lose heat, Satterfield points out. Air on its own, though, can’t hold onto heat very effectively. Dry air loses a…
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