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Weather is the state of the atmosphere at a given place or time. Changes in weather are causes by air masses interacting to form weather fronts, such as cold or warm fronts. Because weather is so complex, weather predictions are only probabilities and not definite outcomes. Created by Khan Academy.

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Video transcript

- [Narrator] Have you ever gotten up in a good mood, put on your absolute favorite outfit, went outside to have a fun day in the sun, and then ended up soaking wet, caught in a sudden rainstorm? Yeah, same. Unfortunately, we can't control the weather. Weather changes from day to day and from place to place, so scientists have to consider a lot of different factors to understand it. Weather can refer to humidity, or amount of water vapor in the air, temperature, air pressure, wind, and, of course, precipitation like rain or snow. When heat from the sun enters Earth's lower atmosphere, it affects the temperature and humidity of the air. As this National Aeronautics and Space Administration map shows, air masses with certain characteristics tend to form in similar places. There's many ways to classify these air masses, but to keep it simple, these blue blobs represent colder air masses, which tend to be closer to the poles, and the red blobs represent warm air masses, which are often found near the equator, where the sun's energy is strongest. The qualities of an air mass may also depend on where it forms. Air masses that form over land tend to have less humidity than those that form over an ocean. Air masses can even change characteristics as they move. For example, when a very cold and dry air mass moves over the ocean, it will likely pick up warmth and moisture, transforming it into a slightly warmer and more humid air mass. Weather results from these air masses moving from areas with high air pressure to areas with low air pressure. When this happens, a high- or low-pressure system can form. A high-pressure system has higher pressure at its center than the areas around it, so air moves out from the center. High-pressure systems typically bring clear skies. A low-pressure system has lower pressure at its center than the areas around it, so air is sucked into the center of the system. Low-pressure systems typically bring more intense weather. Why is this? Well, low-pressure systems are often associated with what we call fronts. A front is a boundary that forms when air masses of different temperatures collide near Earth's surface. Different types of fronts can then form depending on how those air masses meet. A warm front occurs when a warm air mass moves in to a replace a cold air mass. Since warm air is less dense than cold air, it rises above the colder air, causing a lot of overcast skies, moderate rain or snow, and warm temperatures. A cold front occurs when a cold air mass moves in to replace a warm air mass. The dense cold air pushes the warmer air mass upward, causing larger clouds to build overhead and bringing heavy rain or thunderstorms, strong winds, and cool temperatures with them. Weather is so complex that it can be difficult to predict what will happen next. Luckily, scientists have developed technologies that help model weather patterns so we can all make more informed choices about what to expect when we go outside. To do that, they set up tools like satellites and Doppler radar to observe atmospheric conditions such as wind speed, temperature, air pressure, and nearby geographic features. They can input the data they record into computer models, which can use that information to predict future weather. Have you ever seen a map like this used in a weather forecast? This National Oceanic and Atmospheric Administration map uses data from a Doppler radar station that measured the progression of a severe storm over the Northeastern United States. The most intense central parts of the storm are shown in red, with colors down the rainbow representing progressively less intense parts of the storm. As technology has advanced, these weather reports have gotten more accurate, but with so many factors to consider weather is still only somewhat predictable. That's why you'll always hear probabilistic weather predictions, providing a percentage that refers to how confident the model is that the prediction will happen. If your local weather forecast says there's a 30% chance of rain, it's less likely, but still possible that you're going to get rain. If it says there's a 90% chance of snow on another day, you're most likely going to get snow. Even if weather predictions aren't perfect, it's amazing that scientists figured out how to understand such a complicated phenomenon. Without modern tools and models, it would be much more difficult for us to plan ahead and be prepared for possible weather hazards. Next time you have a fancy day out planned, be sure to check the weather forecast before you go. If it says there's a decent chance of rain, don't forget your umbrella.