How to spot solar eclipse events in solar energy datasets

From the solar energy modeling perspective, a solar eclipse causes a temporary drop in the irradiance for those areas where the sun and moon cast shadows on their journey through space.

On the occasion of the most recent total solar eclipse from April 8th, 2024, read how solar irradiance modeling works and how to identify solar eclipse events on solar energy datasets.

Figure 1: Global Horizontal Irradiance (GHI) map and GHI time series for three selected locations during the eclipse.

Solar eclipse shadow and clouds

An eclipse event occurs approximately every 18 months somewhere on the planet. Depending on whether the eclipse is seen as partial, total, or annular, the effect on daily irradiance profiles will be of different intensity.

It's likely that under cloud situations, the effect of a solar eclipse in the irradiance datasets is superposed with the effect of clouds, making it difficult to detect it from daily profiles of Global Horizontal and Direct Normal Irradiances (GHI and DNI).

Figure 2: Visible satellite imagery provided by GOES satellite during the eclipse.

However, we can observe something in all locations: a sudden but smooth drop in the clear-sky irradiance profile on both clear-sky Global Horizontal and clear-sky Direct Normal Irradiances (GHIc and DNIc).

We can notice this particular shape even better in locations where the eclipse happened during central hours and was seen as a total eclipse (or at least a significant part of the sun was covered).

Figure 3: Drop of clear-sky values on clear-sky GHI and DNI during a total solar eclipse. Sample for a location in Dallas, Texas.

Understanding clear-sky irradiance

Clear-sky irradiance represents the solar irradiance available at the Earth’s surface for the considered location and period if we assume that no cloud would cover the sky. In other words, clear-sky irradiance represents the theoretical maximum value on which the satellite-derived cloud attenuation signal is superimposed.

Clear-sky irradiance depends on several factors.

Firstly, it depends on the Earth-Sun distance and the Sun's position in the sky. Once this is calculated, the solar irradiance model adds the effect of solar irradiance being scattered and absorbed when crossing the atmosphere. For that, we use inputs like elevation above sea level and the composition of atmospheric gases, especially water vapor and ozone content.

Another important factor accounted for in clear-sky irradiance is the atmospheric aerosol content.

These tiny particles typically come from dust storms, industrial emissions, and other events like wildfires and volcanoes. They are transported in the air following specific patterns, which are also modeled and incorporated into the clear-sky irradiance computation chain.

Figure 4: Graphs showing GHI and DNI daily profiles of 7th, 8th, and 9th April, respectively, for Durango (Mexico), Dallas (USA), and Belleville (Canada).

Figure 5: Graphs showing GHIc and DNIc daily profiles for 7th, 8th, and 9th April, respectively, for Durango (Mexico), Dallas (USA), and Belleville (Canada).

Next solar eclipse

The next solar eclipse (this time of an annular type) will be on October 2nd, 2024. It will be seen from areas in the Pacific Ocean and the very Southern part of South America.

Understanding the nuances of eclipses, aerosols, and clouds is essential for stakeholders in the solar energy sector worldwide to optimize solar energy usage. Solar irradiance modeling synthesizes meteorological knowledge, computational modeling, and remote sensing techniques to help with this.