Here’s a kind of weather you may not know about:

Every few days, the sun emits electrically charged particles, which stream to Earth in what we call solar wind. And when solar wind meets Earth’s magnetic field, we sometimes get…a magnetic storm.

Magnetic storms can interfere with satellite, GPS, and radio signals. But their most serious effect is when they create electric fields in Earth’s interior.

These electric fields have melted power-line transformers and even caused a power grid in Canada to fail, leaving 6 million people without electricity.

Magnetic storms are more common at higher latitudes, where they create the famous Northern Lights. During intense storms, these auroras can light up the sky much farther south, into the central United States and beyond.

How much a magnetic storm could affect the power grid depends on geology. Sedimentary rocks, like sandstone and limestone, are good conductors. In areas with these types of rocks, the natural currents tend to stay in the ground.

But igneous rocks, like granite, are poor conductors. Here, electric fields look for more conductive paths, which tend to be manmade networks, like power grids—which may not be able to withstand that unpredictable extra current.

To help utility companies safeguard against these effects, scientists are combining ground-conductivity data with magnetic-storm history to produce a geoelectric hazard map.

It’s a new way of weather forecasting that just may help keep the lights on.