Lightning is a good place to begin, because we know what conditions make it, yet scientists still debate exactly how a cloud builds up its electric charge and how lightning begins. Inside a thunderstorm cloud, ice crystals and hail crash into each other, and electrons are knocked from some pieces and added to others; the top of the cloud becomes positively charged and the base negatively charged. Scientists think the main charge regions come from graupel, small hail particles, bumping into smaller ice particles. Graupel carries negative charge in the middle of the storm, and the ice particles carry positive charge in the upper part.
Once the charge has built up, the flash itself follows a pattern. A path of negative charge called a stepped leader reaches down toward the ground, positive charge flows up to meet it, and a strong current surges back up into the cloud as the bright return stroke that we see.
Lightning heats the air in its channel to over 50,000 degrees Fahrenheit in a few millionths of a second. The air has no time to expand and its pressure spikes. It blasts outward as a shock wave, and within about ten yards that wave becomes an ordinary sound wave, which we hear as thunder. The rumble goes on because each part of the long, bending channel makes its own sound, and the sounds reach you at different times.
As of November 2026, the United States gets about 25 million lightning flashes in a year, and lightning does strike the same place more than once; the Empire State Building is hit about 23 times a year.
Forecasters can say when lightning is likely to be intense, but it is impossible to forecast individual strikes, because lightning is so widespread, frequent, and random. A bolt can even strike 10 to 15 miles from the storm, where the sky is clear. So take shelter early. Run to a substantial building or a hard-topped vehicle, keep moving toward shelter, and do not stand under a tree.
Scientists know the conditions that make tornadoes likely, but they do not completely understand how one forms, and they still cannot predict in advance exactly when and where a tornado will develop, how strong it will be, or what path it will follow.
The best account runs through the supercell, a huge thunderstorm with a deep, rotating updraft in which the air rises as fast as 280 km (175 miles) per hour. Wind shear makes a horizontal spin, and the updraft tilts it upright, and cold downdrafts then help to concentrate the spin at lower levels until a narrow column of fast-spinning air forms.
The strongest tornadoes come from supercells, yet most supercells never make a tornado at all. A tornado holds only a tiny fraction of the energy of its thunderstorm, but that energy is squeezed into a small area, and science does not yet completely understand how a storm focuses its energy into something so small.
Even naming a tornado can be harder than it sounds. A gustnado is a small, usually weak whirlwind that forms in a thunderstorm's outflow. It is not connected to rotation in the cloud base, so it is not a tornado, though it is sometimes wrongly reported as one.
Ratings carry their own uncertainty; the Enhanced Fujita scale rates tornadoes from EF0 to EF5 after the storm, and its wind speeds are estimates, not measurements, worked out from damage to 28 kinds of structures and trees. Over two-thirds of all tornadoes are weak, rated EF0 or EF1, and they usually last less than 10 minutes with winds under 100 mph.
Official tornado records in the US only go back to 1950, and spotting and reporting methods have changed a lot since then. Scientists do not know the true yearly average. About 75 percent of the world's tornadoes happen in the United States.
Forecasters handle uncertainty by measuring it. Each day they collect millions of observations from satellites, land stations, ocean sensors, instruments high in the air, radar, and thunderstorm locators. The computer models begin with those observations, including weather balloons launched around the globe twice a day, and use physics to calculate how the atmosphere will change. In ensemble forecasting a supercomputer runs several models at once, or one model many times from slightly different starts. If the runs agree, the forecast is trusted more; if they differ, forecasters know the weather is hard to predict.
Even so, because the atmosphere is chaotic, it will probably always be impossible to predict the weather more than about two weeks ahead, even though forecasts keep getting better.
Words to know
- graupel: icy snow pellet
- supercell: giant rotating storm
- gustnado: small whirlwind
- ensemble: group of runs
