Every cloud starts with rising air. As air rises it cools, and the water vapor in it turns into tiny drops that make a cloud. Air can be lifted in several ways: sunshine heating the ground, wind pushed up a mountain or slope, air rising at a low-pressure center, and air forced up at weather fronts.
In 1802 a London pharmacist and amateur meteorologist named Luke Howard first sorted clouds into types. His three main types, stratus, cumulus and cirrus, are still the base of the system used today. Clouds get their names from their shapes and from how high they float.
Some clouds warn of a coming change. High, thin cirrus clouds show up ahead of a warm front, and then the clouds lower and rain is likely. Cumulus or cumulonimbus clouds build at a cold front, where dense cold air pushes under lighter warm air and lifts it.
A cloud does not become rain at once. Cloud droplets are too small to fall, so they must grow, either by colliding with other droplets or by collecting more water from the air. Rain is sorted by how the air was lifted: frontal rain at weather fronts, orographic rain over hills and mountains, and convective rain, the showers that come when the Sun heats the ground.
In clouds colder than freezing, snow crystals start to form on tiny bits of dust or dirt carried up by the wind, and they fall as they grow heavier. Many snowflakes have six sides, because of how water molecules line up as they freeze. Snow can fall when the air is a little above freezing, and the heaviest snowfalls often happen between 0 and 2 degrees Celsius.
The National Weather Service calls a storm a blizzard when it has lots of snow or blowing snow, winds over 35 mph, and visibility under a quarter mile for at least three hours.
Not all frozen precipitation is snow. Sleet 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.
Meteorologists like numbers, so they rate winds and storms on scales. The Beaufort Scale rates the strength of wind from 0 to 12. Zero is complete calm, and 12 is the wind of a hurricane. The Saffir-Simpson scale rates hurricanes from 1 to 5. A Category 1 hurricane has winds of 74 to 95 mph, and a Category 5 has winds of 157 mph or higher. Category 3 and up are called major hurricanes.
Tornadoes have a scale of their own. The Enhanced Fujita 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. The wind speeds are estimates, not measurements. They are worked out from damage to 28 kinds of structures and trees. Over two-thirds of all tornadoes are weak, rated EF0 or EF1, and weak tornadoes usually last less than 10 minutes.
About 75 percent of the world's tornadoes happen in the United States, most of them in a broad area of the central US that the media nicknamed Tornado Alley. In spring and summer, warm air from the Gulf of Mexico meets cool air from Canada there.
Forecasts bring the numbers together. A forecaster first finds out what the weather is doing now. The first public weather forecast was written for 31 July 1861 by Vice-Admiral Robert FitzRoy, who had captained HMS Beagle. He founded Britain's Met Office in 1854.
A forecast 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. Forecasters collect millions of observations each day from satellites, land stations, ocean sensors, instruments high in the air, radar, and thunderstorm locators. Computer models start with weather balloons launched around the globe twice a day and use physics to calculate how the atmosphere will change.
Words to know
- cirrus: high thin cloud
- sleet: frozen raindrops
- blizzard: snowstorm with wind
- graupel: icy snow pellet
