Wednesday’s total solar eclipse will change weather conditions along its path from the North Pole to Western Europe.
During a solar eclipse, the moon passes in front of the sun and casts a shadow on the Earth’s surface, causing changes in temperature, wind speed, and humidity. This reduces the amount of solar radiation, or sunlight and energy, reaching the Earth’s surface.
The more solar radiation is blocked, no matter how brief, the more dramatically the weather changes. This effect is comparable to a shady area being much cooler on a hot day than an area in direct sunlight.
Wednesday’s solar eclipse will completely block the sun along a narrow path over parts of northern Russia, Greenland, Iceland, Portugal and Spain, providing an opportunity for more pronounced changes in the spots to occur.
However, not all eclipse weather changes occur equally. The exact drop in temperature will vary greatly depending on other factors such as cloud cover and time of year.
Typically, the greatest temperature drop occurs on sunny summer afternoons when the sun is high in the sky. As the sun angle increases, the sunlight becomes stronger and the temperature rises, making the thermometer more likely to crack due to the loss of solar radiation.
According to the National Oceanic and Atmospheric Administration, this temperature drop can be as much as 8 to 14 degrees Fahrenheit in areas with low humidity.
Once the eclipse begins, temperatures begin to drop slowly, but because changes in temperature lag behind changes in solar radiation, they do not completely bottom out until after totality.
During the August 2017 U.S. solar eclipse, temperatures in the lower Ohio Valley did not reach their lowest point until about 5 to 10 minutes after totality.
Wednesday’s solar eclipse will occur in the afternoon in Iceland and Greenland, but cloud cover will determine how much temperatures will drop, and those regions could see quite a bit of cloud cover. Clouds already block some of the solar radiation, which reduces the drop in temperature.
Totality will occur in Portugal and Spain shortly before sunset. This means that while temperatures may drop somewhat earlier than usual before the sun dips below the horizon, they are unlikely to drop as quickly as they do in the early afternoon.
Reduced solar radiation and cooler temperatures can also affect wind, cloud cover, and humidity.
Rapid cooling during a solar eclipse temporarily reduces the amount of heat stored in the atmosphere. The heat causes the air to rise, making the atmosphere unstable. The atmosphere then creates clouds, storms, and winds to release thermal energy and try to restore balance.
As the eclipse cools the air, the atmosphere becomes calmer and wind speeds decrease. This is because they don’t work as hard to maintain balance. Scientists took numerous weather measurements in Wyoming and New York during the 2017 total solar eclipse and found that wind speeds decreased by an average of 6 miles per hour as a result of the eclipse.
If temperatures drop significantly, cloud cover may also change.
The 2024 total solar eclipse reduced cloud cover in parts of North Texas because it lost the fuel, or heat, that causes the air to rise and form clouds.
How you feel humidity is also closely related to temperature. When the air temperature and the dew point, which measures the amount of moisture in the air, approach the same temperature, humidity increases. Therefore, when the temperature temporarily drops during the eclipse, the temperature tends to approach the dew point, making the air feel a little more humid.
The next total solar eclipse will occur about a year from now, on August 2, 2027. Parts of southern Spain, northern Africa, Saudi Arabia, and Yemen will be in the middle of a total solar eclipse.
The next totality event in the United States will be in Alaska on March 30, 2033.
