Weather Forecasting

Weather forecasting is the science of making predictions about general and specific weather phenomena for a given area based on observations of such weather related factors as atmospheric pressure, wind speed and direction, precipitation, cloud cover, temperature, humidity, frontal movements, etc. Meteorologists use several tools to help them forecast the weather for an area. These fall under two categories: tools for collecting data and tools for coordinating and interpreting data.

• Atmospheric Pressure - Atmospheric pressure is the pressure above any area in the Earth's atmosphere caused by the weight of air. Standard atmospheric pressure (atm) is discussed in the next section. Air masses are affected by the general atmospheric pressure within the mass, creating areas of high and low pressure.
• Weather Front - In meteorology, a weather front is a boundary between two air masses with differing characteristics (e.g., air temperature or humidity). When a weather front passes over an area, it is marked by changes in temperature, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern. Weather fronts are often closely associated with atmospheric pressure systems. They are generally guided by the jet stream and travel from west to east. This movement is due to the coriolis effect, caused by the Earth spinning about its axis. Weather fronts can also be affected by geographic features like mountains and large bodies of water, especially at the lower levels of the atmosphere.
• Jet Stream - Jet streams are fast flowing, confined air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south (the meridional temperature gradient). Because of the effect of the Earth's rotation the streams flow west to east, propagating in a serpentine or wave-like manner at lower speeds than that of the actual wind within the flow.
• Wind Chill - Wind chill is the apparent temperature felt on the exposed human (or animal) body due to the combination of air temperature and wind speed. Except at higher temperatures, where wind chill is reckoned not to exist, the wind chill temperature — or "wind chill factor" as it is more commonly called — is always lower than the air temperature, because the wind increases the rate at which moisture evaporates from the skin and carries heat away from the body (some meteorologists, however, disagree with this, on the grounds that the average wind speed is not calm at any locality on Earth, and propose the establishment of a benchmark wind-speed figure — typically in the range of 5 to 7½ miles per hour — so that any wind speed slower than that would result in the "wind chill factor" indeed being higher than the air temperature thus adjusted). The phase change of water (in sweat) from liquid to vapor requires that the molecules reach a higher energy state. That energy is acquired by absorbing heat from surrounding tissue by conduction.

• Tools for collecting data include instruments such as thermometers, barometers, hygrometers, rain gauges, anemometers, wind socks and vanes, Doppler radar and satellite imagery (such as the GOES weather satellite).
• Tools for coordinating and interpreting data include weather maps and computer models in the form of Numerical Weather Predictions.

In a typical weather-forecasting system, recently collected data are fed into a computer model in a process called assimilation. This ensures that the computer model holds the current weather conditions as accurately as possible before using it to predict how the weather may change over the next few days. Weather forecasting involves processing a lot of data, but interpretation can be difficult because of the chaotic nature of the factors that affect the weather. These factors can follow generally recognized trends, but meteorologists understand that many things can affect these trends. With the advent of computer models and satellite imagery, weather forecasting has improved greatly. Since lives and livelihoods depend on accurate weather forecasting, these improvements have helped not only the understanding of weather, but how it affects living and nonliving things on Earth. The chaotic nature of the atmosphere imposes a limit on the predictability of the weather. The predictability limit is estimated to be about two weeks. Predictions beyond this limit are necessarily statistical rather than deterministic. Current operational weather prediction has not yet reached this predictability limit. Below is a sample Hurricane Warning issued by the Cape Cod Hurricane Center.

History

Historically, the two men most credited with the birth of forecasting as a science were Francis Beaufort (remembered chiefly for the Beaufort scale) and his protegé Robert Fitzroy (developer of the Fitzroy Barometer). Both were influential men in British Naval and Governmental circles, and though ridiculed in the press at the time, their work gained scientific credence, was accepted by the British Navy and formed the basis for all of today's weather forecasting knowledge. Television weather reporters have sometimes used gimmicks to attract viewers. One trend that started in the 1970s was "backyard" weather where the forecaster would stand in an outdoor setup while making predictions. WNEP-TV in Scranton, Pennsylvania has been doing this since 1978.