Scientists who study the weather are called meteorologists. They study how heat from the Sun, atmospheric pressure, wind and the water cycle interact with each other to create our weather. With the help of technology such as satellites and supercomputers, weather forecasts—predictions of future weather conditions in a particular place—are becoming more and more accurate. This type of weather forecasting is called numerical weather prediction (NWP). But the weather can often be unpredictable. Meteorologists can only make predictions for five or six days ahead with any detail or accuracy. Longer-term forecasts are usually based on average weather conditions for the time of year. Today, we can find out the weather forecast from radio and television bulletins, newspapers, websites, weather apps and public screens and boards.

Why forecast the weather?

People have tried to predict the weather since prehistoric times, when they first understood that the availability of food—for example, the arrival of migrating animals and the ripening of fruit—as well as their need for particular shelter and clothing, were closely linked to the weather. Today most of us still like to know what the weather will be so that we can plan what to wear and whether to take a trip to the seaside.

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Many businesses and organizations are dependent on the weather to some extent. Farmers want to know the weather over the coming seasons, so they can plan for planting, harvesting and herding. Businesses such as supermarkets, leisure services, airlines and other transport companies, and gas and electricity suppliers need to know the forecast: will everyone want to buy barbecue sausages, will storms cause transport chaos, or will people stay at home and turn on the heating? Governments and emergency services need to put plans in place if severe weather is on the way.

Thanks to new inventions from the mid-17th century onwards, weather forecasting began to develop into a science. The barometer, which measures atmospheric pressure, was invented by Evangelista Torricelli in 1643. The hygrometer, measuring humidity, was developed by Francesco Folli in 1664. The mercury thermometer was invented by Daniel Fahrenheit in 1714. In 1831, when the electric telegraph was invented by Samuel Morse, it became possible to send observations and warnings from one place to another instantly.

Weather stations

Thousands of local weather stations—on land and at sea—collect information about atmospheric conditions at their location. Some weather stations are staffed by people, while others are automated: they rely on sensors to collect and transmit information. In each country, the national weather service runs a network of weather stations: for example, the Met Office runs them in the UK, and the National Oceanic and Atmospheric Administration (NOAA) in the USA. Many companies, universities and individuals also run stations. Since the information collected is shared globally, weather maps use internationally agreed symbols, and reports are given in Universal Time. The World Meteorological Organization (WMO) oversees their coordination.

Weather stations contain some, or all of, these instruments: thermometer (to measure temperature), rain gauge and snow stake (to measure precipitation), barometer (to measure atmospheric pressure), hygrometer (to measure humidity), dewcell (for measuring dew point), anemometer (to measure wind speed and direction), psychrometer (to measure cooling caused by evaporation), disdrometer (to measure precipitation drop sizes), transmissometer (to measure visibility) and ceilometer (to measure cloud height). Measurements are taken at set times.

Weather satellites

The first weather satellite, Vanguard 2, was launched by NASA in 1959. Today there are more than a dozen weather satellites in orbit around the Earth, operated by Europe, the USA, Russia, China, Japan and India. Most of these satellites are geostationary, which means that they orbit the Equator at the same rate that the Earth spins. This allows them to constantly monitor the same large area, around 40% of the Earth’s surface. In order to get a better view of polar regions, some weather satellites are polar orbiting: they pass over the north and south poles with each revolution. The Coordination Group for Meteorological Satellites (CGMS) helps with sharing information between different countries.
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Visible light spectrum images (“normal” camera images) from satellites can show clouds, tropical storms, dust storms, pollution, ice floes and snow cover. By examining a series of images, it is possible to follow the movements of warm and cold fronts and to assess wind speed and direction. Satellites can also capture thermal or infrared images. These allow meteorologists to determine cloud heights and types, land and water temperatures, and to follow ocean currents.

Weather balloons

The first simple weather balloons were launched in France in 1892. Today, around 2000 balloons are launched every day. A weather balloon is 2 metres (6 feet) in diameter and filled with lighter-than-air hydrogen or helium. Each balloon is equipped with a package of instruments called a radiosonde. The instruments measure wind speed and direction, air pressure, humidity and temperature in the upper atmosphere, a region that other forms of weather technology cannot reach. The radiosonde transmits the data by radio. International agreements mean that most countries share information from their radiosondes. If a balloon goes so high that the temperature falls to less than -90°C (-130°F), it will pop and the radiosonde is parachuted back to Earth so that it does not cause injury.

Supercomputers

The major national weather services use supercomputers, which are powerful enough to make more than 1000 trillion calculations each second. Every day these computers are fed thousands of weather observations from weather stations, balloons, radar and satellites. This data is used to work out how the atmosphere and oceans may behave in the future. A computerized weather model contains more than a million lines of code. Today, supercomputers give very accurate short-term forecasts, up to about four days in advance, and can also predict longer-term weather and climate patterns—for example, the arrival and intensity of the hurricane season in the USA.
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During warm weather, forecasts may include the “pollen count” (the number of grains of pollen in a cubic metre of air), which is useful information for people with hay fever; and the “solar UV index”, which is the likelihood of sunburn, dependent on the position of the sun and cloud cover. The “air quality index” is a measure of pollutants in the atmosphere and is useful for people with asthma or other breathing problems.
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Weather maps

Many weather maps on television and the internet are simplified, but you may come across a traditional weather map, which shows patterns of low and high pressure. Lines called isobars link points with equal atmospheric pressure. High-pressure systems are often marked with an H and low-pressure systems with an L. Fronts are represented by lines marking the boundary of the front. Cold fronts have triangles along the length of the line while warm fronts have semicircles. Winds tend to blow along the line of the isobars, turning slightly towards a low and away from a high in the northern hemisphere (and the opposite in the southern hemisphere). The closer together the isobars, the stronger the winds.