Weather is what the air around us is doing right now: how hot or cold it is, whether it is still or windy, and whether rain, snow or hail is falling. It is made from a handful of ingredients. The Sun heats the Earth unevenly. That uneven heat changes the air pressure. Differences in pressure push air around as wind. Wind carries water vapour, which cools into clouds, and clouds drop rain, snow and hail, sometimes inside huge storms. Weather scientists, called meteorologists, measure all of this with thermometers, barometers, wind gauges and satellites, so they can tell us what tomorrow will bring.
Weather is about today and this week. Climate is the pattern of weather in one place over 30 years or more. If you want the difference explained in full, read our guide to weather vs climate. Here, we will take each ingredient of weather one at a time, and visit the real places on Earth where that ingredient goes to the extreme.
Sunshine: Why Death Valley Bakes and Vostok Station Freezes
Every bit of weather starts with the Sun. Its light takes about 8.3 minutes to reach us. When sunlight hits the ground, the ground warms up, and the warm ground heats the air just above it.
But the Sun does not heat the planet evenly. Near the Equator, sunlight strikes the ground almost straight down, so its energy is packed into a small patch. Near the poles, the same sunlight arrives at a low slant and is spread over a much bigger area, so each patch gets less heat. Land also warms faster than water, and dark rock warms faster than white snow, which reflects most of the light back into space.
Look at what happens at the two ends of the scale. The hottest air temperature ever officially recorded on Earth is 56.7 °C (134 °F). It was measured at Greenland Ranch, now called Furnace Creek, in Death Valley, California, on 10 July 1913. Some weather experts doubt that old reading, but the World Meteorological Organization (WMO), which keeps the official records, still accepts it.
The coldest air temperature ever measured at ground level is −89.2 °C (−128.6 °F). That was at Vostok Station, a Soviet research base high on the ice of Antarctica, on 21 July 1983. July is winter in the Southern Hemisphere, so the Sun barely rose at all. The difference between those two readings is almost 146 degrees, and the Sun's uneven heating is the reason for all of it.
Air Pressure, from Torricelli's Glass Tube to Typhoon Tip
Air feels like nothing, but it has weight. All the air above your head presses down on you. Scientists call this push air pressure. At sea level it averages about 1,013 hectopascals (hPa), the unit forecasters use. That is enough push to hold up a column of water about 10.3 metres tall.
The Italian scientist Evangelista Torricelli is usually credited with proving this. In 1643 or 1644 he filled a glass tube with mercury, turned it upside down in a dish, and watched the mercury stop at a certain height. The weight of the air on the dish was holding it up. His invention was the first barometer, the tool that still measures air pressure today.
Why does pressure matter for weather? Warm air is lighter, so it rises, leaving lower pressure at the ground. Cool air is heavier and sinks, making higher pressure. High pressure usually means calm, clear skies. Low pressure usually means rising air, clouds and rain. When a barometer reading falls quickly, a storm is often on its way.
The lowest air pressure ever measured at sea level was 870 hPa, inside Typhoon Tip over the western Pacific Ocean in October 1979. Tip was the largest and most intense tropical cyclone on record, with winds of 305 km/h (190 mph).
Wind at Commonwealth Bay and Barrow Island
Wind is simply air moving from a place of high pressure to a place of low pressure. The bigger the difference, the faster it blows.
The windiest place on Earth, according to Guinness World Records, is Commonwealth Bay in Antarctica. The explorer Douglas Mawson set up his base there in 1912. The wind averages about 80 km/h (50 mph) over the whole year, and winds regularly top 240 km/h (150 mph). This is a katabatic wind: very cold, heavy air from the high middle of Antarctica slides downhill over the ice, speeding up all the way to the sea.
The fastest wind gust ever measured, outside a tornado, was 408 km/h (253 mph). It was recorded on Barrow Island, off the coast of Western Australia, during Tropical Cyclone Olivia on 10 April 1996. The WMO checked the instrument and accepted the record in 2010. Before that, the record belonged to Mount Washington in New Hampshire, USA, where a gust of 372 km/h (231 mph) was measured on 12 April 1934.
In 1805, a British Royal Navy officer named Francis Beaufort created a simple scale for judging wind by what it does. Force 0 is calm, with smoke rising straight up. Force 12 is hurricane force. Sailors still use the Beaufort scale. You can find more record-breakers in our list of the windiest places in the world.
Clouds Named by Luke Howard in 1802
Air always holds some water as an invisible gas called water vapour. When air rises, it cools. Cool air cannot hold as much vapour, so some of it turns into tiny droplets of liquid water or tiny ice crystals. Billions of these together make a cloud.
For a long time, clouds had no proper names. Then, in 1802, a British chemist and amateur weather-watcher named Luke Howard gave a talk to a London science club, the Askesian Society, proposing Latin names for them. His system is still used today:
- Cumulus means "heap". These are the puffy, cotton-wool clouds of a fine day.
- Stratus means "layer". These flat grey sheets can cover the whole sky.
- Cirrus means "curl" or "fringe". These thin, wispy clouds are made of ice, high in the sky.
- Nimbus means "rain cloud".
Put two names together and you get a cumulonimbus, the giant thunderstorm cloud. Its top often reaches 12,000 metres (39,000 ft), about as high as airliners cruise, and a few tower to 20,000 metres. The top spreads out into a flat shape like a blacksmith's anvil. An average cumulonimbus holds about 500,000 kilograms of water, all floating in the air.
Rain and Snow at Mawsynram, Mount Baker and Vivian, South Dakota
Inside a cloud, droplets bump into each other and join up. When they become too heavy for rising air to hold, they fall as rain. If the air is cold enough all the way down, they fall as snow instead.
The town of Mawsynram, in the Khasi Hills of Meghalaya, India, is often called the wettest place on Earth. It gets an average of about 11,872 millimetres (467 inches) of rain a year. During the summer monsoon, warm, wet winds blow in from the Bay of Bengal, hit the hills, rise, cool, and drop their water. On 17 June 2022 alone, Mawsynram received 1,003.2 mm. Two towns in Colombia, Lloró and López de Micay, have reported even higher yearly averages, so the title is disputed. See how cities compare in our list of the rainiest cities in the world.
The record for snow in one winter belongs to the Mount Baker Ski Area in Washington State, USA. In the 1998–99 season, 1,140 inches fell. That is about 29 metres of snow, taller than a seven-storey building.
Hail is different. It forms when strong winds inside a thunderstorm toss frozen raindrops up and down, adding layer after layer of ice like an onion. The largest hailstone ever officially measured fell at Vivian, South Dakota, on 23 July 2010. It was 20 centimetres (7.9 inches) across, about the size of a volleyball.
Thunderstorms, Tornadoes and the Catatumbo Lightning
When warm, wet air rises very fast, a cumulonimbus cloud grows into a thunderstorm. Ice and water bump into each other inside it, building up electric charges. When the charge is big enough, it jumps as lightning. Lightning heats the air so suddenly that it bursts outwards: that bang is thunder.
The place with the most lightning on Earth is Lake Maracaibo in Venezuela, where the Catatumbo River flows into the lake. Storms there happen on 140 to 160 nights a year, for about nine hours a night, with 16 to 40 flashes a minute. Each square kilometre gets about 250 lightning strikes a year. In the language of the Barí people, the name Catatumbo means "House of Thunder".
Some thunderstorms spin. Out of the most powerful ones, a tornado can reach down: a narrow, whirling column of air that touches the ground. The United States has far more tornadoes than any other country, most of them in the central plains known as Tornado Alley. The Tri-State Tornado of 18 March 1925 crossed Missouri, Illinois and Indiana and killed 695 people, the most of any tornado in US history. A 2013 study found that at least 243 km, and probably 280 km, of its track was one continuous tornado, which would be the longest track on record. A tornado near El Reno, Oklahoma, on 31 May 2013 was about 4.2 km wide, the widest ever measured. Find out step by step how tornadoes form.
Measuring the Weather, from Fahrenheit's Mercury Thermometer to TIROS-1
Forecasts are only as good as the measurements behind them. Here are the main tools, and when they arrived:
- Thermometer measures temperature. In the 1710s Daniel Gabriel Fahrenheit made the first successful mercury-in-glass thermometers, and in 1724 he proposed his own scale. In 1742 Anders Celsius, a Swedish astronomer, proposed the 100-degree scale that most of the world uses now. Official thermometers sit in the shade, between 1.25 and 2 metres above the ground.
- Barometer measures air pressure, as Torricelli first did in the 1640s.
- Anemometer measures wind speed. The Barrow Island record gust came from one mounted 10 metres above the ground.
- Rain gauge collects falling rain so it can be measured in millimetres.
- Radar bounces radio waves off raindrops and hail to show where storms are and how they are moving.
- Weather satellites watch clouds from space. The first, TIROS-1, was launched from Cape Canaveral, Florida, on 1 April 1960, and sent back television pictures of clouds.
Thousands of weather stations, ships, balloons and satellites send readings to computers, which work out how the air is likely to move next. Next time you check it, you will know the ingredients behind the numbers, from the Sun heating Death Valley to the wind racing down the ice at Commonwealth Bay.