Avoiding Clouds at the August 2017 Solar Eclipse, with Google Earth Engine
I still vividly recall the eerie light of the partial solar eclipse that I saw growing up in Minnesota in 1994. Total solar eclipses are far more spectacular, though, and far more rare: we haven’t seen one here in the continental US since two of them grazed the edges of the country back in the 1970s. You don’t want to miss the eclipse that’s coming on August 21st, 2017, but where should you go to view it best?
It doesn’t matter how great your viewing location is if it’s cloudy when you get there! There are lots of maps online showing the “path of totality”, the thin strip across the country where for just a few minutes the moon will completely block the sun in the sky. Let’s use Google Earth Engine to figure out where to go in that path to best avoid the clouds.
The total eclipse will make landfall on the west coast of Oregon at around 10:15am in the morning, and it will exit South Carolina at around 2:45pm in the afternoon. How cloudy will it be? There are lots of tools we can use to investigate cloud cover, but one of the easiest is NASA’s moderate-resolution imaging spectroradiometer, or MODIS. MODIS flies on two spacecraft (called Terra and Aqua) to collect imagery of the globe every day, once in the late morning and once in the early afternoon. We can combine the data from these two sources to get a picture of cloud cover during the days around August 21st since the year 2000:
In this image, blue means there’s a low chance of clouds and dark red means it’s always cloudy. As you can see, it makes a big difference where you decide to go! In late August, your best bet is to go to the northwest.
Let’s look at how to make that map. The Earth Engine public data catalog puts a wide variety of Earth data at your fingertips, which makes it very easy to do this sort of analysis. The NASA MODIS global daily composite images, known as MOD09GA for Terra and MYD09GA for Aqua, include cloudiness data at a spatial resolution of one kilometer per pixel. Here’s a snippet of JavaScript code that loads these datasets and computes the average cloudiness around August 21st:
You can run that code in the Earth Engine Code Editor, an interactive environment for data analysis and visualization. I’ve put the complete script that generates all the visualizations in this article here on GitHub. The complete script also shows how to make the interactive map, shown below, which you can use to explore the likelihood of clouds in the path of totality in more detail. (Click on the image to load the interactive map.)
If you do go view the eclipse, don’t forget to bring proper eyewear so you can safely look at the sun before and after the period of totality. If you miss this one, don’t worry, you won’t have to wait as long as some people have been saying: there will be another total eclipse passing over just the eastern US in 2024. The next one like this year’s, though, won’t happen until 2045.
Finally, if you’re interested in learning more about using MODIS data and Google Earth Engine to understand cloud cover, check out the Global 1-km Cloud Cover interactive tool by the EarthEnv project. They’ve been using these same techniques to study the impact of cloud cover on global ecosystems and biodiversity!
