While you watch the eclipse, you'll also be able to feel it

On Monday, April 8, a total solar eclipse will alter weather conditions on the Earth’s surface while the astronomical marvel unfolds in the sky.

Changes to temperature, wind speed and humidity occur as the moon crosses in front of the sun and casts a shadow on Earth’s surface during a solar eclipse.

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.

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April’s total solar eclipse will block the entire sun in a 115-mile (185-kilometre) wide path from Texas to Maine, known as the path of totality. Dallas, Indianapolis, Cleveland and Buffalo, New York, are just a few cities that will experience the phenomenon.

The moon will block a significant portion of the sun and create a partial solar eclipse outside of the path of totality. The closer an area is to the path of totality, the larger the portion of the sun and its solar radiation – sunlight and energy – will be obscured by the moon.

At least 50 per cent of the sun will be blocked during the eclipse as far west as Anaheim, California, and as far east as Orlando, Florida. Only around 20 per cent of the sun will be blocked in the Pacific Northwest.

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 cloud cover and the time of year.

Time of year matters because the angle at which sunlight strikes the Earth affects temperatures, with a higher angle producing more intense sunshine and heat. The sun angle rises throughout the spring, reaches its peak in summer and starts to drop in the fall.

The last total solar eclipse took place during a summer afternoon in late August, so temperatures were already high and 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 four to eight degrees happened across the South.

April’s eclipse will unfold with a lower sun angle than August, but as the warmest part of the afternoon approaches. So, the brief, eclipse-driven cooldown will be quite noticeable before temperatures rebound back to pre-eclipse levels.

Temperatures during April’s event could drop around 10 degrees for up to an hour in the path of totality, said Andrew White, a meteorologist with the National Weather Service in Indianapolis. Temperature drops will be less pronounced in partial eclipse areas.

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The eclipse will also influence humidity levels, which limit how far temperatures can drop during totality.

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.

But air temperatures cannot fall below the dew point, so if dew points start out high during the eclipse they’ll limit how low temperatures can go.

Less solar radiation and reduced temperatures during an eclipse can also affect wind and cloud cover.

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The 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 six miles per hour as a result of the eclipse.

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 something similar happens during Monday’s eclipse.

Monday’s eclipse is the only chance to see a total solar eclipse from the Lower 48 for the next two decades. The next total solar eclipse to cross the country won’t arrive until Aug.12, 2045, but one will briefly clip portions of Montana and the Dakotas on Aug. 23, 2044.