Lightning, Explained

In short: lightning is a giant spark that forms when collisions between ice and soft hail inside a storm separate electric charge. Most of it stays inside the cloud. Satellites now map lightning across the whole country, in-cloud and to the ground, with new data about every 20 seconds, and a sudden jump in a storm’s flash rate is often one of the first signs it is strengthening. The safety rules are simple: if you hear thunder, you are in range, and wait 30 minutes after the last thunder before going back out.

The NWS estimates about 25 million cloud-to-ground flashes a year in the United States, and several times that many inside clouds. Lightning kills about 20 people a year in the country and injures hundreds more.

How a storm builds charge

In the middle of a thunderstorm, the updraft carries small ice crystals upward while heavier pellets of soft hail, called graupel, fall or hang in the air. When they collide, charge moves between them. The light crystals, now positive, ride the updraft to the top of the storm. The graupel, now negative, stays in the middle. A small pocket of positive charge often forms near the base.

Air is a good insulator, until it is not. When the charge difference grows too large, the air breaks down in a sudden discharge: lightning. The NWS says the channel can heat the air to about 50,000 °F. The air around it expands so fast it makes a shock wave, which is thunder.

In-cloud and cloud-to-ground

• In-cloud (IC) lightning runs between charge layers inside the storm, or from cloud to cloud. It is most of the lightning in most storms.
• Cloud-to-ground (CG) lightning reaches the ground. Most CG flashes lower negative charge from the middle of the storm. A smaller share lower positive charge from the upper storm and anvil. Some flashes leave the side or top of the storm, travel sideways for miles and strike under sky that looks clear. People call that a bolt from the blue.

Mapping lightning from space

The Geostationary Lightning Mapper (GLM) flies on the GOES-East and GOES-West weather satellites, about 22,000 miles above the equator. It is an optical sensor: it watches the tops of clouds for the brief flashes of light that every kind of lightning makes, day and night, over land and ocean.

What that means when you read it:

• It is total lightning. In-cloud and cloud-to-ground, together. GLM cannot tell them apart, so its counts are much higher than a ground-strike network’s.
• It sees the cloud top. A flash is placed where its light came out of the cloud, so positions can be a few miles off where the bolt reached the ground, more so toward the edge of the satellite’s view.
• It covers what radar cannot. Oceans, mountains, and the gaps between radars.
• It updates fast. GLM sends new data about every 20 seconds, while a radar volume takes about 5 minutes.

In MesoRadar: Live lightning, in the Layers sheet, draws GLM flashes from the mapper on GOES-East over the map, coloring each flash by age. Its settings sheet, Lightning Detection, describes it as satellite total lightning, in-cloud and ground, updated every 30 seconds, and has Sync with radar loop to rewind lightning with the radar (free). Flash density, a soft glow brightest where lightning is most frequent, is Pro. What the marks mean.

What flash rate tells you

Lightning follows the updraft. More graupel and ice colliding in a stronger updraft means more charge and more flashes. That is why a storm’s lightning can tell you about its strength before the radar shows it.

• A sudden jump in total flash rate often comes before a storm produces severe hail, damaging wind or a tornado. NOAA notes the increase can appear on GLM minutes before radar shows signs of severe weather.
• A drop can mean a weakening updraft, or that the storm has moved into a different stage.
• It is a nudge, not a warning. Plenty of storms with heavy lightning are not severe, and some severe storms, especially in tropical air, produce little lightning.

In MesoRadar: Lightning forecast, in the Layers sheet, maps the MRMS chance of lightning in the next 30 or 60 minutes (Pro).

Lightning safety

The NWS rules, in short:

• When thunder roars, go indoors. If you can hear thunder, you can be struck. Lightning can reach about 10 miles from a storm.
• There is no safe place outside in a thunderstorm. Safe options are a substantial building with wiring and plumbing, or a hard-topped metal vehicle with the windows up.
• Not safe: rain shelters, small sheds, tents, open vehicles, and convertibles.
• Caught outside with no shelter: these lower the risk but do not make you safe. Get off hills and ridges, never shelter under a lone tree, stay away from water and metal fences, and never lie flat on the ground. Keep moving toward a building or a hard-topped vehicle.
• Wait 30 minutes after the last lightning or thunder before going back outside. Many people are struck when they head out as the rain eases.
• Indoors, stay off corded phones and away from plumbing and anything plugged into the wall. Cell phones and cordless phones are safe.