Abstract
Accurate weather forecasts are very useful, because they tell people what kind of weather to expect in the next few days. But how can we predict what will happen in the future? This article will explain how meteorologists, the people who study and work with the conditions that happen in the atmosphere, predict tomorrow’s weather. To help you understand weather predictions, we will first explain how changes in the atmosphere result in various types of weather. In this article, we will tell you how meteorologists turn knowledge of the conditions in the atmosphere into a weather forecast, just like you see in the news or on a weather app!
What Information Can a Weather Forecast Provide?
Every day in the news, we can see the weather forecast for the next few days, telling us if it will be sunny, cloudy, windy, rainy, or cold. Many people rely on weather forecasts every day—for more than just knowing whether they should bring a raincoat! For example, pilots must be able to fly airplanes safely, farmers need to look after their crops, and local governments need to know whether they should prepare roads for icy conditions. Usually, a weather forecast tells us about a few weather characteristics that are most important to us, for example, temperature, cloudiness, precipitation (such as rain or snow) and wind. To understand how meteorologists predict these conditions, we must first explain how these weather characteristics occur based on what is happening in the atmosphere.
Temperature
The temperature depends on many things, including cloudiness and which direction the wind blows. In the daytime when the sky is clear, the sun can heat up the air more than it can in cloudy weather. However, during the night the opposite is true. Just as you cover yourself with a blanket to keep warm when you go to sleep, cloud cover helps to keep the air closer to the ground warm. If the sky is clear at night, it will be quite chilly. Temperatures can also be affected by air moving from hotter or colder places. For example, when warm air travels north from northern Africa, Europe can experience very high temperatures.
Clouds
Clouds form when there is enough water vapor in the atmosphere. Air can hold a certain amount of water vapor, depending on the air’s temperature. The warmer the air, the more vapor it can hold. When there is more water vapor in the atmosphere than the air can hold, then it will condense. This means small water droplets will form. These droplets are so light that they float in the air, forming clouds. If it is cold enough, the little droplets of water can freeze. The clouds will then be made up of tiny ice crystals instead of droplets.
Precipitation (Rain and Snow)
As the water droplets in a cloud swirl around and bump into each other, they merge and grow bigger. Eventually they become so big that they are too heavy to float in the sky, and they fall to the ground as rain. Different types of clouds can lead to different types of rain. For example, think of a surprising, intense summer shower, compared to a light rain that lasts all day. If the cloud consists of ice crystals, snowflakes can form as the crystals bump into each other. If it is cold all the way down to Earth’s surface, then there will be snow. Otherwise, the snowflakes melt and turn into rain before they reach the ground.
Wind
To understand how windy it will be, meteorologists must look at the air pressure in the atmosphere. Even though we barely notice it, air has mass, and the air pressure tells us the weight of all the air that is above your head. You can think of this like pumping up a bike tire: as you pump more air in, the pressure in the tire increases. Now let us think of this for the whole atmosphere. If there is high air pressure, that means there is more air than normal sitting above us. This is usually associated with calm conditions and sunny days. On the other hand, if air pressure is low, then there is less air than normal above us. This is typically associated with wet and windy weather. The strength of the wind depends on how quickly the pressure changes between locations.
Components of a Weather Forecast
Now that you know a bit about the atmosphere, we can look at how to predict the weather. Weather prediction requires observations, weather models and of course, meteorologists! Figure 1 shows how information is collected and combined to make a weather forecast. The following three sections will explain the steps in more detail.
- Figure 1 - There are three main steps to creating a weather forecast.
- (A) Accurate observations of the current weather and the atmosphere are crucial. These observations can be made from the ground or from satellites. (B) Weather models are programs run on supercomputers which perform complex calculations based on equations that describe weather characteristics. (C) Meteorologists combine information from weather models with their own experience and knowledge of local effects to create a forecast that they can present to the public.
Measurements at Ground Level and From Satellites
Accurate measurements of the weather are very important for producing an accurate forecast. These measurements come from various types of equipment, which measure lots of weather characteristics including temperature, humidity, precipitation, and wind (Figure 1A). In addition, radar images are very important, particularly for precipitation [1]. Radar works by sending out a light pulse, a bit like flashing a torch, then detecting how the light is reflected back by precipitation. From radar data, it is possible to figure out where there is snowfall or rain, and how intense the precipitation is. Another very important source of information for generating weather forecasts comes from satellites. They provide “photos” of the Earth from space at regular times throughout the day, which are especially useful for collecting information about cloud cover.
Weather Prediction Models
Imagine you are watching a film. The decisions that the main character makes in the beginning will affect how the story ends. This is similar for the weather: changes in the current weather, such as changes in temperature and humidity, will affect the weather in the future [2]. By providing precise information about the current conditions in the atmosphere to a weather model (Figure 1B) meteorologists can calculate how the atmosphere will look each hour, for about a week into the future. They can do this because many processes that happen in the atmosphere are predicted by mathematical equations. For example, there are equations that describe how quickly warm air rises, how raindrops are formed, and how wind creates waves on the ocean, just to name a few examples! A weather model combines these equations and calculates them using a supercomputer. The weather model can cover a large region, like the whole world, or a smaller region, such as one specific country. In a film, we cannot change the character’s decisions and, similarly, we cannot make a weather model always predict sunny weather, or snow on Christmas!
Producing a Forecast
Meteorologists combine the measurements and the output from the weather model, along with their own experience, to produce a weather forecast for the public (Figure 1C). The model only gives numbers for certain weather characteristics, for example the temperature and pressure at certain locations, so it is the job of the meteorologist to interpret what these numbers mean, and how they can be expressed and explained. Often, the meteorologist can also improve the forecast further, with local knowledge and experience. The meteorologist then communicates the information to the public, for example during a weather report on the radio or TV, or via a weather app.
Can We Be Certain About the Weather?
Sometimes weather predictions are spot on, but other times they miss by a little. To get a measure of how reliable the forecast is, meteorologists run many weather models to get suggestions about how the weather could evolve. This is called an ensemble method [3]. Think of this like throwing a paper plane many times and looking at the path the plane takes each time (Figure 2). The first part of the path will probably look very similar for almost every throw, but the plane could still land in quite different places. If all suggestions from the weather models are similar, then meteorologists can be quite certain of the future weather, but if the models tell different stories, they know that the forecast is uncertain. Meteorologists also get information about what could happen in the most extreme situation, which allows society to prepare for the worst-case scenario and helps keep everyone safe.
- Figure 2 - If you throw a paper airplane many times, the paths of the plane could represent ways that weather could evolve.
- Short-term weather forecasts are usually more certain than longer-term forecasts, just like it is difficult to predict exactly where the plane will land, even if you throw it the same way each time. The difference between the forecasts created by various weather models gives a good estimate of how likely various types of weather will be. In this case, we see the most paths going toward the partly cloudy symbol, so the weather is most likely to be partly cloudy next weekend.
Summary
Weather forecasts provide vital information to the public, helping societies to run smoothly. Forecasts are much better now than they were just a couple of decades ago [4], and today we have access to lots of information and knowledge that helps meteorologists understand and model the atmosphere. There is still ongoing research on the physics of the atmosphere and weather modeling, but for such a complicated system, we can forecast the weather incredibly well. So, the next time you hear a weather forecast that says that there might be rain, you should probably pack your raincoat!
Glossary
Weather Characteristics: ↑ Things that can be measured in the atmosphere, for example, temperature, pressure, or humidity.
Precipitation: ↑ Any form of water, liquid or solid, that falls to the Earth’s surface, for example, rain, sleet, or snow.
Meteorologist: ↑ A person who works with understanding or predicting the weather.
Water Vapor: ↑ Water in the form of a gas.
Condense: ↑ The process of a substance (like water) changing from gas to liquid form.
Air Pressure: ↑ The pressure caused by the weight of the air (which has mass) above a certain location.
Weather Model: ↑ A program usually run on a super-computer which simulates the atmosphere and weather.
Ensemble Method: ↑ A weather model that creates many suggestions for how the weather could evolve, which gives meteorologists information on the chances of certain events, like precipitation, occurring.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
EH and GM were funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 956396 (ITN project EDIPI—European weather extremes: drivers, predictability and impacts). CH was funded by the Energimyndigheten (Swedish Energy Agency) VindEl program, Grant Number 47054-1, and his work forms part of the Swedish Strategic Research Program StandUp for Wind. The authors would like to thank the reviewers and editor for their support in improving the quality of this study.
References
[1] ↑ Serafin, R. J., and Wilson, J. W. 2000. Operational weather radar in the United States: progress and opportunity. Bull. Ame. Meteorol. Soc. 81:501–8. doi: 10.2307/26215121
[2] ↑ Bauer, P., Thorpe, A., and Brunet, G. 2015. The quiet revolution of numerical weather prediction. Nature 525:47–55. doi: 10.1038/nature14956
[3] ↑ Gneiting, T., and Raftery, A. E. 2005. Weather forecasting with ensemble methods. Science 310:248–9. doi: 10.1126/science.1115255
[4] ↑ Jung, T., Balsamo, G., Bechtold, P., Beljaars, A. C. M., Kohler, M., Miller, M. J., et al. 2010. The ECMWF model climate: recent progress through improved physical parametrizations. Q. J. R. Meteorol. Soc. 136:1145–60. doi: 10.1002/qj.634