When the moon throws shade, the weather can change | Science News for Students

When the moon throws shade, the weather can change

From plummeting temps to wind and cloud changes, solar eclipses can trigger wacky weather
Aug 16, 2018 — 6:45 am EST
a photo of the August 2017 solar eclipse

The August 21, 2017 solar eclipse, seen here from Oregon, triggered odd weather across the United States.

Frank Fichtmüller/iStockphoto

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.

Story continues below graph.

a graph showing the temperature and solar radiation before, during and after the eclipse in Hopkinsville, Ky
A trace of the temperature and incident solar radiation on either side of totality in Hopkinsville, Ky, shows a big dip caused by the eclipse.
Kentucky State Mesonet/Western Kentucky University

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.

an image of the sun taken 15 seconds before totality
Fifteen seconds before totality in Alliance, Neb., the moon has almost blotted out the sun.
© M.E. Cappucci

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 lot of heat as soon as the sun is gone. That means that drier air can see faster and more dramatic warming and cooling.

This explains why eclipse-driven temperature swings can be really dramatic in very dry air. Such as Lusaka in Zambia. This arid capital in South Central Africa saw a total eclipse of the sun on June 21, 2001. That event briefly brought a whopping 8.3 degree C (15 degree F) drop in temperature.

Changing winds and cloud cover

Scientists measure the amount of incoming sunlight in watts per meter squared. A typical value would be around 800 W/m². At sites in the path of last year’s total eclipse, the number briefly dipped to zero. Confused birds returned to their roosts. Crickets began to chirp as they thought night had come early.

a gif showing how solar radiation was changed as the eclipse passed over the United States
This gif, from a simulation, shows changes in solar radiation as the eclipse passed over the United States. Dark blue areas indicate darkness from both the eclipse and cloud cover.
NOAA/ESRL/GSD/CIRES Joe Olson, Michael Toy, and Jaymes Kenyon; NCAR/MMM - Wei Wang, Jimy Dudhia, Dave Gill Alex Montornes, Univ. of Barcelona

And at some sites, a cool breeze blew in from the west. That’s where the moon’s shadow was sweeping in from. This rapid false nightfall in places churned up air currents. Many researchers reported feeling an “eclipse wind” — something that had been mentioned in folklore for ages. Scientists are still analyzing data, but they noted an abrupt wind shift falling in the wake of the shadow’s passage.          

Sometimes, though, the opposite happens: A total solar eclipse can bring a calming of the winds. That’s what Suzanne Gray and Giles Harrison reported two years ago in the Proceedings of the Royal Society A. These research meteorologists work in England at the University of Reading. They studied a total eclipse over the British Isles on March 20, 2015. And their data showed that wind speeds did not rise. They dropped.

Clouds, too, may be affected.

White, puffy summertime clouds often dot the afternoon sky. But during the 2017 solar eclipse, these all but vanished over much of the United States. Scientists at many sites charted what was happening. Then they compared these data to what computer programs had been forecasting.

First, those forecasts: In the days before the eclipse, scientists had been running computer models — programs to project what might occur. Each run of the computer program had been based on the anticipated weather and eclipse shadows. Computer models know how air temperature responds to the amount of sunlight coming in. Even though scientists knew that the sun would be blocked, they had to rely on computer models to predict how this might affect temperatures.

The U.S. National Weather Service uses one computer model for this. It’s known as the HRRRX (High Resolution Rapid Refresh Experimental) model. Meteorologists run it once an hour, feeding it constantly updated data. Always reflecting the latest conditions, scientists consider it the most accurate U.S. tool for forecasting weather.

a photo of the 2017 eclipse at totality, where the moon completely covered the sun, and the sun's corona is visible
As the moon fully blocks the sun during the brief window of totality, the milky-white hair-like silvery tendrils of the corona – the sun’s atmosphere – emerge in their full glory. A solar eclipse is the only time when the corona is visible, so scientists only have perhaps a few minutes every decade to make observations and conduct experiments!
© M.E. Cappucci

Astronomers made calculations of how much the sun would be dimmed. Then they passed this information to programmers to feed into the model runs in the days before the eclipse. Indeed, short-range weather models have now been adjusted to include all eclipses through the year 2050. Before this, weather models had ignored the role of any eclipse.

One spectacular image from the Earth Systems Research Laboratory in Boulder, Colo., shows the difference in what the model had forecasted before and after the update. The original version of HRRR and other models had predicted a large thunderstorm cluster — known as a complex — to rumble through Nebraska on eclipse day. It never happened, thanks to the dimming of the sun. And the newly tweaked HRRRX model had warned this might not occur.

What seems to have happened is that the eclipse’s sharp cutoff of sunlight prevented most surface air from rising. This halted updrafts. These are pockets of air that bubble upwards. Normally, these updrafts feed the puffy cumulus clouds that can emerge in the heat of the day.

From the ground, people in many places reported seeing clouds thin somewhat before the eclipse reached totality. In other words, as the incoming sunlight shrank, so did the mass of the clouds.

This video shows the shadow of the moon sweeping across the landscape, extinguishing clouds in its track.

The National Oceanic and Atmospheric Administration captured an incredible video of this from its GOES-16 weather satellite. That video showed a white patchwork of clouds over the U.S. Gulf Coast and southeastern states. As soon as the eclipse’s shadow moved past this area, the sea of white dissolved. It left clear blue skies in its wake.

On average, a total solar eclipse occurs at any particular site only about once every 375 years. That makes it a pretty rare event. But for those that missed the 2017 eclipse, don’t despair. Americans will have another shot close to home on April 8, 2024. And if you’re more adventurous and don’t mind hiking across the globe, pack your bags. A total solar eclipse will throw some serious shade at Chile and Argentina in July 2019.

In this video-diary entry for the Washington Post, the author and meteorologist Dan Satterfield witness the 2017 eclipse. 
M. Cappucci/Washington Post

Power Words

(more about Power Words)

arid     A description of dry areas of the world, where the climate brings too little rainfall or other precipitation to support much plant growth.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

atmosphere     The envelope of gases surrounding Earth or another planet.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climatology     The study of climate over seasons, decades or millennia. Climate varies over time and this field looks at measuring all aspects of climate and using such data to better understand what factors are behind those changes. Scientists who study climatology are known as climatologists.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

cumulus     A type of especially puffy cloud that sometimes look like floating mounds of cotton. Its base may be flat and fairly close to the ground (just 1,000 meters, or 3,300 feet up). The cloud’s top crown tends to look like rounded towers. the head of a cauliflower, it is called cumulus congestus or towering cumulus. When these clouds develop into giant moisture-filled towers, they evolve into what’s known as cumulonimbus clouds, the type from which thunderstorms emerge.

current     A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.

eclipse     This occurs when two celestial bodies line up in space so that one totally or partially obscures the other. In a solar eclipse, the sun, moon and Earth line up in that order. The moon casts its shadow on the Earth. From Earth, it looks like the moon is blocking out the sun. In a lunar eclipse, the three bodies line up in a different order — sun, Earth, moon — and the Earth casts its shadow on the moon, turning the moon a deep red.

elevation     The height or altitude at which something exists.

GOES satellite     A type of weather satellite that collects images of severe storms, hurricanes, wildfires and other weather hazards in near real-time, every hour of the day. Each is “geostationary,” meaning it appears to hover permanently over the same spot on Earth’s surface. The newest version of these orbiters — GOES-16 and -17 (that last one due to launch in early 2018) — provide four times the image resolution of NOAA’s older GOES satellites, and at five times the speed.

humidity     A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)

meteorology     (adj. meteorological) The study of weather as it pertains to future projects or an understanding of long-term trends (climate). People who work in this field are called meteorologists.

model     A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

moon     The natural satellite of any planet.

National Weather Service     An agency of the U.S. National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures.

National Oceanic and Atmospheric Administration (NOAA)     A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.

network     A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

roost     A place where winged animals temporarily rest or sleep. The term also refers to the act of resting on a branch or above-ground perch.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

simulation     (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.

solar eclipse     An event in which the moon passes between the Earth and sun and obscures the sun, at least partially. In a total solar eclipse, the moon appears to cover the entire sun, revealing on the outer layer, the corona. If you were to view an eclipse from space, you would see the moon’s shadow traveling in a line across the surface of the Earth.

totality     (in astronomy) The brief period during an eclipse when one object totally obscures another. For a solar eclipse (when viewed from Earth), this would be when the moon appears to completely block out the sun’s light.

wake     An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.

water vapor     Water in its gaseous state, capable of being suspended in the air.

watt     A measure of the rate of energy use, flux (or flow) or production. It is equivalent to one joule per second. It describes the rate of energy converted from one form to another — or moved — per unit of time. For instance, a kilowatt is 1,000 watts, and household energy use is typically measured and quantified in terms of kilowatt-hours, or the number of kilowatts used per hour.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.


Discussion: NASA Temperature Change during Totality https://eclipse2017.nasa.gov/temperature-change-during-totality

Blog: Dan Satterfield. Numerical weather model adjusted for eclipse forecast. Dan’s Wild Wild Science Journal. The American Geophysical Union. August 18, 2017.

Journal: S.L. Gray and R.G. Harrison. Eclipse-induced wind over the British Isles on 20 March 2015.  Philosophical Transactions of the Royal Society A. Vol. 374, September 28, 2016, p. 20150225. doi: 10.1098/rsta.2015.0224.