Weather map

A weather map, or synoptic weather chart, shows meteorological conditions over a specific region at a given moment, using symbols with defined meanings. These maps have been employed since the mid-1800s for both research and forecasting. Maps with isotherms reveal temperature gradients, helping to locate weather fronts. Isotach maps, which chart lines of equal wind speed on constant pressure surfaces at 300 or 250 hPa, pinpoint the jet stream. Constant pressure charts at 700 and 500 hPa can track tropical cyclone movement. Two-dimensional streamlines based on wind speeds at various levels highlight areas of convergence and divergence in the wind field, aiding in identifying wind pattern features. A common surface weather map is the surface weather analysis, which uses isobars to show high and low pressure areas. Cloud codes are converted into symbols and plotted on these maps, along with other meteorological data from synoptic reports submitted by trained observers.

The modern use of weather charts began in the mid-19th century to develop storm system theories. During the Crimean War, a storm destroyed the French fleet at Balaklava; French scientist Urbain Le Verrier demonstrated that a chronological map of the storm could have predicted its path, allowing the fleet to avoid it. In England, Francis Galton learned of this work and of Robert FitzRoy’s early weather forecasts. Collecting data from weather stations across the country for October 1861, Galton plotted it on a map using his own symbols, creating the world’s first weather map. He used it to prove that air circulates clockwise around high-pressure areas, coining the term "anticyclone." He also helped publish the first weather map in a newspaper, modifying a pantograph to transfer the map onto printing blocks. *The Times* began printing weather maps using these methods with data from the Meteorological Office.

Country-wide weather maps required national telegraph networks to gather real-time data. The *Manchester Examiner* newspaper first used the telegraph for this purpose in 1847. Standardized time across time zones was also necessary for accuracy; Greenwich Mean Time was first used to coordinate the British railway network in 1847. In the United States, the Smithsonian Institution built a network of observers across the central and eastern U.S. from the 1840s to the 1860s under Joseph Henry. The U.S. Army Signal Corps took over this network between 1870 and 1874 by an act of Congress, expanding it to the west coast. Initially, not all map data was used due to a lack of time standardization; the U.S. fully adopted time zones in 1905 when Detroit established standard time.

The use of frontal zones on weather maps began in Norway in the 1910s. Polar front theory, attributed to Jacob Bjerknes, came from a coastal observation network in Norway during World War I. The theory proposed that a cyclone’s main inflow was along two convergence lines: one ahead of the low (the steering line or warm front) and one trailing behind (the squall line or cold front). Clouds and rainfall focused along these zones. The concept of frontal zones led to the idea of air masses. The three-dimensional structure of cyclones awaited the development of the upper air network in the 1940s. Because the leading edge of air mass changes resembled military fronts of World War I, the term "front" was adopted. The United States began formally analyzing fronts on surface analyses in late 1942, when the WBAN Analysis Center opened in Washington, D.C.

Weather agencies also began generating constant pressure charts. In 1948, the U.S. started the Daily Weather Map series, initially analyzing the 700 hPa level (about 3,000 meters above sea level). By May 14, 1954, the 500 hPa surface (about 5,520 meters above sea level) was being analyzed. Automation of map plotting began in the U.S. in 1969 and was completed in the 1970s; India’s Meteorological Department started a similar initiative in 1969. Hong Kong finished automated surface plotting by 1987. By 1999, computer systems and software allowed workstations to overlay satellite imagery, radar imagery, and model-derived fields (such as atmospheric thickness and frontogenesis) with surface observations for the best possible surface analysis. In the U.S., this was achieved when Intergraph workstations were replaced by n-AWIPS workstations.