Satellite imagery will prove invaluable in making eclipse-day decisions.

Annular solar eclipse behind clouds
S&T's Dennis DiCicco photographed 1994's annular eclipse through a beautiful haze of clouds.

Just days from now, it’ll be October 13th, and the annular solar eclipse will be only a day away. Eclipse glasses are in hand, and your family is excited; neighbors are invited to come by and watch.

But the weather forecast is unpromising and everyone is getting anxious. Now what?

Do you hope the forecasts are wrong and stay put?  Or, do you put everyone in the car and make a last-minute change to a new viewing site? And how do you find that site?

With a day to go, short-range computer forecasts of cloud cover are valuable, but now more accurate weather information is needed. That’s where satellite pictures come to the fore. Satellite images reveal open and thin spots in cloud cover and, when animated, show how they are moving and evolving. The fine details of cloud cover, usually too small to be caught by the models, may provide an opportunity when a cloudy gloom is predicted.

Two geostationary satellites — GOES East and GOES West — keep watch over North America. Each one observes the globe in a mixture of visible and infrared wavelengths, serving up a view of the clouds throughout day and night. Visible-light images, useful only in daylight, will show their first views of the eclipse track after the Sun has risen (before 8 a.m. at this time of year). That offers precious little time to find your way to an opening in the cloud cover, as the annular eclipse begins at 9:15 a.m. on the Pacific Coast, 10:30 a.m. at Four Corners, and just before noon on the Gulf Coast. 

Visible-light cloud map for eclipse day
A visible-light image taken at 10:41 a.m. CDT, October 8th, shows the early growth of convective clouds on the dark-colored forested mountain slopes in Arizona and New Mexico. A few hours later, these clouds expanded to cover a much larger area. The cooling that comes with the eclipse shadow will slow the development of such clouds until after the eclipse is over.
College of DuPage / NASA

To see clouds at night, you must turn to infrared images. There are several wavelength bands available, but only a few sites provide images of them all. One that does is the College of DuPage site where 14 IR bands are available. The most useful (which you see on TV weather reports) is the Long-Wave IR (LWIR), a thermal infrared band at a wavelength around 11 microns. This image captures thermal emission from clouds and ground, a signal that is available day and night. In a typical LWIR scene, cold objects are presented in white and light-gray shades, while warm objects are dark gray to black. This makes clouds stand out against the ground, imitating their daytime appearance.

Infrared cloud map for eclipse day
A LWIR image taken at 8:00 a.m. CDT, October 8th, showing early-morning clouds along the eclipse track. Some clouds in the image are colored in shades of purple and blue to highlight the coldest temperatures and thus the highest cloud tops. New Mexico is almost completely clear; the pattern in the IR image is due to the different temperatures on the ground.
College of DuPage / NASA

Sometimes it’s hard to distinguish between warm, low clouds and the similar-temperature ground, but animating the satellite images will show you what is moving and what is stationary. The ground cools overnight, sometimes giving the appearance of movement, so you’ll have to pay attention to the details.

Fog is often difficult to distinguish from ground when the two have nearly the same temperature, and that’s where the second IR band comes in handy. This is the shortwave IR band on the College of DuPage site, which shows areas of fog and low cloud more distinctly than the LWIR. You’ll only want to use this band at night, as it’s contaminated by solar radiation in the daytime.  It’s most likely to be needed for sites in Texas and along the Pacific Coast, where fog is more common at this time of year.

Private weather sites such as Windy and Meteologix don’t give you a choice of all of the wavelengths like the College of DuPage does, but they have a clever way of blending IR emission to simulate a daytime image during the night. That blending loses some of the warm low-level clouds, but they also offer a pure LWIR image that will bring it back. DuPage allows you to overlay state highways and Interstates on top of the satellite images, a feature that may prove valuable in a rush to find an opening in the clouds.

Combined with the short-range numerical models that I mentioned in my last note (the NAM, ICON, and RDPS, for instance), you may find the critical information in satellite imagery helps make the final decision on whether to go or stay on eclipse morning. That’s what I’ll be doing.


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