Weather

Weather comprises all the various phenomena that occur in the atmosphere of a planet. On Earth the regular events include wind, storms, rain, sleet, hail, and snow, which occur in the troposphere or the lower part of the atmosphere. Weather is driven by differences in energy from the sun. Due to the various angles that sunlight intersects the earth, different parts of it are heated to different extents. This causes temperature differences, which lead to global wind, as well as, indirectly, all other weather phenomena. Direct causes of weather are temperature, humidity, atmospheric pressure, cloud cover, wind speed, and elevation. All but elevation are influenced to some degree by variant heating.

Weather prediction

Weather stations all around the world measure these conditions on a continuous basis. The weather system is an inherently chaotic system. Practically this means that meteorology can only predict the weather a few days in the future. In fact it was through studying weather systems that chaos theory first emerged, pioneered with the aid of early computers by Edward Lorenz. Recently, though, it has been shown that the theoretical time ahead that it is possible to forecast is not as short as previously thought. Research suggests that problems with the formulas used in current models remains a limiting factor in the accuracy of predictions, rather than the inherently chaotic nature of the weather. This still only extends the forecast time by at most a week. However, scientists are continuously working to extend the length of the forecast period. In the last decade, advances in the science now allow forecasters to give a general idea of future conditions, such as whether the temperature or precipitation will average above or below normal well into the future. These types of forecasts have become increasingly more accurate.

Weather prediction is an area of much active research, due to the advantages that an accurate forecast of the weather would bring. Chaos theory says that small changes in the initial conditions increase exponentially over time, until the weather is very different from what it would have been under slightly different circumstances. Recognizing the fundamentally chaotic nature of the atmosphere, forecast centers use a technique called ensemble prediction in which forecasts are made by calculating a number of different scenarios. The collection of different scenarios (i.e., the ensemble) is constructed by making small changes to the initial conditions of each prediction, by using different models, or some combination of the two. The members of the ensemble are then combined in some way, for example by taking the mean of all the scenarios as the forecast.