Changes to temperature, wind speed and humidity occur as the moon crosses in front of the sun during a solar eclipse and casts a shadow on Earth’s surface.
The more sunlight blocked, the more dramatic the weather changes. The effect is comparable to how shaded areas end up much cooler on a hot day than any place in direct sunlight.
Saturday’s annular eclipse will block up to 90% of the sun In a narrow path from Oregon to Texas. Annular eclipses allow slightly more solar radiation – sunlight and energy – to make it to the Earth’s surface than a total eclipse, which entirely blocks the sun.
But a reduction in solar radiation, no matter how brief, can affect temperatures and other weather.
Not all eclipse weather changes are created equal, though. The exact drop in temperature can vary widely based on other factors like the time of year and cloud cover.
An October annular eclipse is going to have a less dramatic effect on temperatures than an August total eclipse, not just because of the amount of sun blocked by the moon, but also the lower angle at which the sun strikes the Earth in fall compared to summer.
A higher sun angle produces more intense sunshine and elevated temperatures, and the angle starts to drop in fall.
2017’s total solar eclipse took place during a summer afternoon in August, so temperatures were already high, making them more prone to crater in some locations along the path of totality. Temperatures fell 11 degrees over just one hour in Douglas, Wyoming, and widespread temperature drops of 4 to 8 degrees happened across the South.
Temperature drops during Saturday’s eclipse are not expected to be as drastic, but could still dip a few degrees in the path of annularity.
Areas only experiencing a partial solar eclipse could see a slower rise in temperatures from late morning to the early afternoon, Juan Hernandez, a meteorologist with the National Weather Service in Fort Worth, Texas, told CNN.
A slower rise in temperatures could limit how far high temperatures can climb in the afternoon hours and leave Saturday a bit cooler than if there were no eclipse. This may happen in Dallas and Phoenix, where the moon will block 80% of the sun.
An eclipse affects more than just temperature. Less solar radiation and reduced temperatures can also affect wind, humidity and cloud cover.
The quick cooldown during an eclipse briefly reduces the amount of heat stored in the atmosphere. Heat forces air to rise and makes the atmosphere unstable. The atmosphere then creates clouds, storms and wind to let out heat energy in an attempt to bring itself back into balance.
So as the eclipse cools the air, the atmosphere calms down and wind speeds drop because the atmosphere isn’t working as hard to balance itself out. Scientists took a number of weather measurements in Wyoming and New York during 2017’s total solar eclipse and found wind speeds dropped by an average of 6 mph as a result of the eclipse.
How humid it feels is tied closely to temperature. Humidity rises when the air temperature and the dew point, which measures how much moisture is in the air, approach the same temperature. So when air temperatures briefly dip during an eclipse, they trend closer to the dew point and make the air feel a bit more humid.
A significant temperature drop can also alter cloud cover.
Clouds over parts of South Carolina disappeared during 2017’s total solar eclipse because they lost their fuel – heat that forces air to rise and form clouds. It’s possible a few clouds could also dissipate in Saturday’s eclipse, even with less drastic temperature drops.