How are weather forecasts made?

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How are weather forecasts made?

First, Weather Measurements are Made: Twice each day, radiosondes (weather balloons) are launched all over the world at approximately the same time. They ascend up through the troposphere (the lowest 7-10 miles of the atmosphere, where our weather occurs), and measure temperature, wind, air pressure, and wind speed and direction (through radio tracking or GPS tracking). This information is continuously transmitted back to the ground station, then sent to the National Centers for Environmental Prediction (NCEP) in Washington, D.C. These measurements constitute a global “snapshot” of our weather at one point in time, every twelve hours. Additional measurements are made at surface observing stations (there is probably one near you), on ships at sea, from commercial aircraft, and by weather radars of different types. While different forecasting centers might provide up to four updated forecasts during the day, it is this twice-daily measurement system that leads to only two significantly improved weather forecasts each day.

Next, These Measurements are Put Into Weather Forecast Models All of these measurements are also transmitted to a different weather modeling centers – in the U.S., England, Germany, Australia, and other countries. These modeling centers have very fast supercomputers that have been programmed with equations that describe the interactions between temperature, wind (both horizontal and vertical), and pressure; as well as equations for water vapor transport, the formation of clouds and precipitation; and for the absorption and reflection of solar radiation, and the absorption and emission of infrared (heat) radiation. Together, all of these equations run as a computer program are called a "numerical weather forecast model".

Finally, the Interpretation of Model Output: The Forecaster’s Role For many years, weather fcorecast models were not good enough to compute what the weather at the surface would be like…they could only compute the future weather at different levels up through the atmosphere. What happens at the surface (where we live) is very complicated because so much energy is exchanged there, and in a variety of complex ways. For instance, the presence of vegetation greatly cools the surface due to evapo-transpiration--a jungle is much cooler than a desert (but much more humid, too). Forecasters would examine the output from the computer models and would make an educated guess of what the model forecast meant in terms of surface weather. Also, the models were typically biased—under certain conditions, they might habitually make storms too intense, not produce enough rain, etc., -- so these biases had to be factored into the forecaster’s thinking as well.

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