A guide to this section of the website
This section of the website deals with the earth's atmosphere, and particulary with the science of meteorology. Meteorology has a theoretical aspect, a branch of classical physics dealing with the statics, dynamics and thermodynamics of the moist air that makes up the atmosphere. It also has a practical aspect, weather analysis and forecasting, which is of great practical utility to agriculture, navigation and aeronautics.
Additional information on the atmosphere can be found in the Geology section of the website, especially as a part of the earth as a whole. Atmospheric tides are also treated there, together with the familiar ocean tides.
The word meteorology is from the Greek metewrologia, which meant the discussion of high things, or astronomy. Indeed, an astronomer was a metewrologos. The adjective metewros meant high, lifted, raised, like things in the heavens, such as meteors.
The articles in this section offer a good course in meteorology, especially in the basic physics of the atmosphere. A few subjects are absent, such as the structure and movement of fronts, frontogenesis and frontolysis, and quite a few aspects of practical meteorology. However, the section will probably grow. Many internet links to sources of information are given in the individual articles, including links to satellite images and current weather maps. I have not yet found a good internet source of cloud pictures, but, after all, one has the outdoors!
Weather can be predicted to some degree by observing the state of the sky and the wind. Sailors and farmers had many empirical rules connecting clouds with weather. One curious property of weather that although it is of extreme variety and proverbial for changefulness, on the average it is remarkably constant. In a given district, the weather and its signs are a good guide to what can be expected on the average. In fact, tomorrow's weather will probably look much like today's. This statistical predictability is taken advantage of in preparing almanacs and other prognostications, but the accuracy is no more than statistical, and the predictions may prove wrong just when they are the most critical. In most cases, people are satisfied if only some of the predictions are correct. In many cases, the predictions can be stated in such a way that they can be interpreted to suit whatever happens.
The barometer and thermometer were perfected in the 17th century, and weather became quantitative. The barometer, in particular, had predictive power. A low barometer was associated with clouds, rain, storms and generally bad weather, while a high barometer implied clear skies and sunny, warm, dry weather. In fact, these explicit claims are usually displayed around the dial of an aneroid barometer. There is actually a closer relation between a falling barometer and bad weather, or a rising barometer and good weather, that with the absolute height of the barometer. It became a popular hobby to record the weather daily, which has not waned in popularity. Adding the wind, clouds, amount of precipitation and humidity, a rather good set of observations typical of a weather station can be assembled. The records assembled by 17th-century amateurs are still of value in studying long-term changes in the weather.
Synoptic weather analysis and forecasting, the assembly of data from a widely spread net of weather stations, could not really begin until the telegraph made instantaneous long-distance communications possible after 1850. When data was analyzed at a central station, the changes in the weather could be seen as they happened, and the weather at stations ahead of the weather could be advised of what to expect. For the first time, weather predictions could really be predictions, not just guesses. However, surprises continued to come over the oceans. In Britain, for example, the weather comes off the Atlantic at a much more rapid rate than a ship can sail, so weather surprises were the usual thing. Sometimes an approaching storm could be detected in the clouds, a typical sequence indicating the approach of a storm. Cable telegraphs were some help, as from Ireland to England, but mostly the weather at sea was unknowable.
The invention of radio allowed the extension of the weather observation net over the oceans, and this was one of its first important uses. Every ship became a weather station, radioing reports at regular intervals to a central office. This great advance happened by 1915, and weather prediction was of great service in the Great War. Significant theoretical advances were made during the middle of the 20th century, especially in the properties of the upper atmosphere, elucidated by such indirect methods as sound propagation and meteor trails. The importance of fronts and air masses was recognized. Aircraft made observations at altitude, but the greatest advance was the radiosonde, where the pressure, temperature and humidity of an air column from the surface to high altitudes could be determined in a few minutes. Originally clumsily called radiometeorographs when experimental devices were tried in the 1920's and 1930's, they were renamed radiosondes in 1930. Radiosondes were first used in service in 1937 at Boston, and there were 6 radiosonde stations by 1938. Radiosondes replaced all aircraft observations in 1940. This gave additional information that made weather forecasting a true science. Weather maps gained their maturity in this period. One article in this section takes a thorough look at weather maps and how to interpret them.
When aircraft began flying at altitudes of 35,000 ft and over, the unexpected jet streams were discovered, and the importance of these high levels to surface weather was made clear. Westbound bombers in World War II had unexpectedly encountered headwinds not much less than their airspeed, so they flew without getting anywhere. Sounding rockets finally illuminated the properties of the upper atmosphere by direct measurement. Radar then made remote sensing of weather possible. Military radar frequencies were chosen so that rain and similar phenomena would not affect the systems, but now frequencies are chosen that specifically report on such things, and weather radar is an important adjunct. Weather satellites send back images of the earth in visible, infrared, and other wavelength ranges, making clouds visible. Satellite data cannot supply the pressure, temperature and humidity figures that are the raw material for meteorology (at least not as readily as radiosondes), but can give useful data that is truly synoptic.
The hydrodynamic equations of motion, of continuity and of state that govern the atmosphere have been known for a very long time, and nothing important has been added to them for a century. However, solutions have been very, very difficult to obtain and for many years only approximate results were available. These results were very useful for theoretical purposes, but could not be used to predict the weather just from the equations. Computers have allowed an attack on such problems, but unfortunately computations yield many figures but little insight. Nevertheless, short-term predictions have been made much more accurate through computer analysis. The dream of long-term prediction by dynamic methods is still unrealized, perhaps because of the exponentially-growing effects of small variations, the problem of chaos. Computers can work much more rapidly and cheaply than the old forecasters with their charts and contours, but unfortunately cannot think.
The United States Weather Bureau began as a branch of the U.S. Army Signal Corps in 1870, on the founding of the Corps, showing the importance of the telegraph to weather. Also, the Signal Corps was the only part of the Army with the slightest technical knowledge, so it seemed a good home for the weather. In 1891 the weather service was made part of the Department of Agriculture, where it became famous as the USWB. In 1940 it was made part of the Department of Commerce, probably to be closer to aviation functions, for which weather was now essential. In 1965, the USWB was put into the Environmental Services Administration, still in the Department of Commerce. In the reorganization of 1970, weather became the responsibility of NOAA, and the USWB was renamed the National Weather Service (NWS). The NWS issues 5-day forecasts that are taken off the wire and repeated by TV presenters, as well as 30-day outlooks. The 5-day forecasts are actually fairly reliable, though longer-term forecasts seem to be wrong as often as right. Most TV stations give 7-day forecasts now, probably because they are provided by the NWS.
The properties of air and water vapor, aerostatics, the dry and wet adiabatic lapse rates, the lifting condensation level, the pseudoadiabatic and tephigram charts, the meaning of the wet bulb temperature, and the local and nonlocal stability of air columns are treated in Theoretical Meteorology.
Atmospheric dynamics and winds are discussed in Wind. The hydrodynamic equations governing atmospheric motions are reviewed, and then the geostrophic, gradient, thermal, isoallobaric and local winds are explained. There is a discussion of large-scale and seasonal winds, such as the monsoon, and finally the Beaufort wind scale is presented.
Cloudwatching discusses the nature and physics of clouds, and the forms and classification of clouds and what they mean. Unusual clouds, and a report on a day's clouds in Denver, are also included.
Atmospheric Radiation brings energy from the sun, and through the greenhouse effect keeps the surface of the earth at a temperature suitable for life. The energy budget of the earth is also described.
Atmospheric Turbulence is responsible for the lapse rate in the troposphere, and transports matter, momentum and energy invisibly and reliably.
Weather maps and what they show are presented in Weather Maps, with many examples taken from actual weather maps. All the symbols used on weather maps that I have seen are shown and defined, including the symbols for clouds.
The impressive convective storm or cumulonimbus is described in Thunder and Lightning. The life history of a thunderstorm is told, and the nature of lightning and thunder are explained.
A discussion of the current controversy over global warming and carbon dioxide appears in Global Warming, which has a Time article of 2001 as a basis.
Composed by J. B. Calvert
Created 11 July 2003
Last revised 7 August 2003