Let us start with the sky's names. In 1802 Luke Howard, a London pharmacist and amateur meteorologist, first sorted clouds into types. His three main types, stratus, cumulus and cirrus, are still the base of the system used today.

Almost all weather happens in the troposphere, the lowest layer of the atmosphere, which holds about 75 to 80 percent of the air's mass and nearly all of its clouds. At Earth's surface the air presses down with about 14.7 pounds on every square inch. Higher up there is less air above, so the pressure is lower.

Wind blows from high pressure toward low pressure, and the Beaufort Scale rates its strength from 0 to 12. Zero is complete calm and 12 is the wind of a hurricane.

Forecasting came later. Vice-Admiral Robert FitzRoy, who had captained HMS Beagle, founded Britain's Met Office in 1854, and he wrote the first public weather forecast, for 31 July 1861. A forecast today is made in three stages: find out what the weather is doing now, feed the data into supercomputer models, and have experts check and refine the result.

Big air masses carry the weather. An air mass takes on the temperature and wetness of the place where it forms, so it is named for that, such as maritime tropical (warm, over the ocean) or continental polar (cold, over land far north). Where two air masses meet is a front. At a cold front, dense cold air pushes under lighter warm air and lifts it. The lifted air makes cumulus or cumulonimbus clouds and thunderstorms, and cold fronts move up to twice as fast as warm fronts. A warm front moves more slowly. High, thin clouds such as cirrus show up ahead of it, then clouds lower and rain is likely.

Cloud droplets are too small to fall. To become rain they must grow, by colliding with other droplets or by collecting more water from the air.

Frozen precipitation comes in more kinds than snow. Sleet, also called ice pellets, is frozen raindrops, and graupel is a snowflake covered in a layer of ice. Hailstones are usually between 5 mm and 50 mm across, and most are smaller than 25 mm. Many snowflakes have six sides, because of how water molecules line up as they freeze. Snow can fall when the air temperature is a little above freezing, and the heaviest snowfalls often happen between 0 and 2 degrees Celsius.

Scales sort the strongest storms. The Saffir-Simpson scale rates hurricanes from 1 to 5, using only the storm's top sustained wind speed. The Enhanced Fujita (EF) scale rates tornadoes from EF0 to EF5 after the storm, by studying the damage. EF0 has gusts up to 85 mph and EF5 has gusts over 200 mph.

Now let us look at storms with lightning. A thunderstorm is called severe when it can make hail an inch across or bigger, wind gusts over about 58 mph, or a tornado. Every thunderstorm has lightning, because thunder comes from lightning, but lightning can also happen without a thunderstorm. It does strike the same place more than once: the Empire State Building is hit about 23 times a year, as of November 2026. Height, pointy shape and being alone decide where lightning strikes.

Around the world there are an estimated 16 million thunderstorms a year, and about 2,000 are going on at any given moment, as of November 2026. The United States gets about 25 million lightning flashes in a year, as of November 2026.

Metal does not attract lightning, though it can carry it. Because of this, the safety rules matter. Do not shelter under a tree, and crouching does not make you safer outdoors. Run to a substantial building or a hard-topped vehicle, and keep moving toward shelter.

Words to know

• troposphere: lowest air layer
• cumulus: puffy cloud
• graupel: icy snowflake
• front: warm meets cold
• meteorologist: weather scientist