Meteorology is the study of all atmospheric phenomena, but it tends to direct most of its attention to the weather. The term "weather" refers mainly to atmospheric phenomena that change over a short time scale, including temperature, precipitation, wind, barometric pressure, humidity, cloud classification and so on. Climatology on the other hand is the study of the long-term effect that weather has on a particular place or area.
The Earth's atmosphere is a very active environment. Solar radiation, cosmic rays, the Earth's rotation, temperature, elevation--these are all part of a long list of factors that bring about change in the atmosphere. Almost all weather activity takes place in the troposphere, the 9 miles (15 km) of air closest to the Earth's surface. What goes on in this region of the atmosphere most directly affects the Earth's inhabitants. Until relatively recent times, weather was interpreted as a manifestation of the wrath or reward of the gods. Aristotle, in the fourth century b.c., was one of few scholars of the ancient world to come forward with an objective explanation of weather. In his work Meteorologica, he deductively explained the atmospheric forces, as he did with other parts of the cosmos. His writings had little scientific foundation, yet were accepted as fact well into the eighteenth century.
During the 1600s and 1700s, weather was increasingly viewed as something to be observed and measured. Many inventions began to make that possible. The barometer was invented by Evangelista Torricelli in 1644. Also in 1644, the anemometer, which measures wind velocity, was invented by Robert Hooke. Various types of hygrometers, used to measure humidity, were invented by Guillaume Amontons (1687), Nicolas-Théodore de Saussure (1775) and John Frederic Daniell (1819). The wet-and-dry bulb psychrometer was invented by John Leslie in 1790.
What emerged during the 1800s and 1900s was a greater ability to collect and analyze meteorological information and an understanding of atmospheric circulation. In 1835, Gustave de Coriolis (1792-1843) proposed the Coriolis force--that air movements would appear to be deflected as they traveled above a rotating Earth. In 1858, William Ferrel quantified this deflection and established the law that bears his name. The advent of the telegraph in the 1800s made it possible to collect and transmit weather data. Manned, and later unmanned, balloon ascents took meteorological research directly to its source. From about 1900, radiosondes, the data collecting packages carried into the atmosphere by unmanned balloons, produced new information that rewrote the book on atmospheric processes.
As a result of evolving technical abilities and needs, it became possible for an individual like Cleveland Abbe (1838-1916) to advocate, then follow through on, the establishment of a national weather reporting service for the United States during the 1860s and 1870s. Abbe began using maps to plot weather information and to make forecasts. Atmospheric soundings with balloons coincided with the development of manned flight. A major outcome of this was the Bergen school of meteorologists. Under the leadership and inspiration of Vilhelm Bjerknes, who discovered the existenceof air masses and polar fronts and developed mathematical models for predicting weather, the Scandinavian group revealed the true complex and dynamic nature of the atmosphere. Their efforts began in 1918 and continued into the 1920s. Carl-Gustaf Rossby got his start at the Bergen office. From 1928 on, Rossby's research, based on Bjerknes's cyclone model, led to the discovery of jet streams, which determine the tracks of low pressure cells and have a daily impact on weather patterns. Much of what is seen on a modern weather map, including the cold and warm fronts, high and low pressure cells, the jet stream path, pressure ridges and troughs, can be attributed to the Bergen researchers.
Since World War II, meteorology has been enhanced by rocket and satellite technology. In the 1940s and 50s, rockets and high-altitude planes were used to go beyond the limit of balloon research. The first weather satellite, TIROS I, was launched in 1960. For 78 days it beamed back television pictures of clouds that changed our view of Earth's weather forever. Satellites, especially those with fixed (or geostationary) orbits, are actually weather observatories perched permanently over the equator at a height of 22,000 miles. Because they revolve around the Earth in exactly one day, geostationary satellites seem to maintain their position over one spot on the Earth. Beginning in the mid 1960s, a network of weather satellites were placed in geostationary orbit which now provide an unbroken global view of atmosphere over the Equator. Several polar orbiting weather satellites are also in place, providing views of the Earth's north and south poles. These satellites collect images of the atmosphere in a range of wavelengths, from visible light to the infrared, and also collect data on vertical profiles of temperature and humidity. The images satellites provide are indispensable to modern weather forecasting, especially over the oceans where there exist no surface weather stations. Images made in visible light provide a continuous picture of cloud motion (and hence the winds that blow the clouds), and are important for forecasting the movement of tropical cyclones (hurricanes) and extratropical storms (those outside of the tropics). Infrared images are used to track the movement of water vapor in the atmosphere, which can give important clues for forecasting the probability and severity of storms. In the 1980's, researchers began to study the possibility of using satellite mounted instruments called lidars to measure the winds at any level in the atmosphere in the absence of clouds. Lidar, which functions like radar, uses wavelengths close to those of visible light. The waves are bounced off of suspended dust and particulates, called aerosols; the returning light signal is then analyzed to yield the wind speed and direction.
The use of satellites has increased in step with advances in computer technology. Modern meteorology is heavily dependent on computers to analyze satellite data and to solve the equations that predict changes in the winds and the development of storms. The first weather forecast completed with the aid of a computer (called a numerical forecast) was made in 1952 by meteorologist Jule Charney and computer pioneer John von Neumann. By mid 1955 computer weather models began to be used routinely. These early models were quite limited in the accuracy and scope of their forecasts; with each new generation of larger, faster computers have come advances in the numerical weather forecasting. Today, the best numerical forecast can reach 97% accuracy.
The advancement of computer modeling has had a great impact on two areas of active meteorological research, namely the El Niño Southern Oscillation (ENSO) and global climate change. Atmospheric models have been instrumental in understanding ENSO, an alternating warm and cold oscillation in the waters of the eastern and western Pacific Ocean. These changes in sea surface temperature over two-year periods have global consequences, such as droughts in northern Africa and heavy rains in California. The possibility that humans are altering the Earth's climate, through the emission of carbon dioxide and other gases, can be studied with the aid of computer models that can recreate the Earth's long term climate, with its coupled atmosphere-ocean-land-ice cap sub systems.
The ultimate goal of meteorology is to use past and present data to project future weather events. Anyone who has awakened to a covering of snow that was not predicted will know that the science is far from perfect. On the other hand, people whose lives have been saved by early warnings of a deadly tornado can attest to how much our understanding of the atmosphere has increased in only a few decades.
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