WEATHER MAP

any map or chart that shows the meteorological elements at a given time over an extended area. The earliest weather charts were made well before 1835 by collecting synchronous weather reports by mail. The first telegraphic collection of synoptic meteorological reports and their mapping for forecasting was accomplished by Urbain-J.-J. Le Verrier during the mid-1800s. The most common type of weather map, normally issued by a central weather office, shows the distribution of surface isobars (lines of constant pressure) and the location of fronts and severe weather areas such as hurricanes and other storms. At many locations on the map, a standard plotting code indicates wind direction and speed, air temperature and dew point, barometric pressure and its change during the preceding hours, the amount and types of clouds, the weather type, including restrictions to visibility, and the amount and type of precipitation. In the United States, weather maps are issued every three hours by the National Weather Service. When used in conjunction with charts showing the upper-air flow pattern at 6,000 to 12,000 m (20,000 to 40,000 feet) and with satellite photographs of the distribution of clouds, these maps are a valuable forecasting tool.