The most common type of surface 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 850, 700, 500, 300, and 200 millibars of atmospheric pressure and with satellite photographs of the distribution of clouds, these maps are a valuable forecasting (weather forecasting) tool.