While meteorology is far from an exact science, meteorologists use a number of tools and calculations to help them gauge what the weather is doing and where it will go in the future. Weather satellites collect data on weather patterns as they happen, transmitting them down to Earth for meteorologists to study. This data is used in conjunction with numerical weather predictions, computer-based algorithms that can predict the various outcomes of the weather at any given time. Because these calculations become less and less accurate the further into the future they go, they are constantly updated with more recent weather data; this is why your weatherman may change his predictions for the day just one hour after making them.
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