In meteorology, station models are symbolic illustrations showing the weather occurring at a given reporting station. Meteorologists created the station model to fit a number of weather elements into a small space on weather maps. This allows map users to analyze patterns in atmospheric pressure, temperature, wind speed and direction, cloud cover, precipitation, and other parameters. The most common station plots depict surface weather observations although upper air plots at various mandatory levels are also frequently depicted. Station model plots use an internationally accepted coding convention that has changed little since August 1, 1941. Elements in the plot show the key weather elements, including temperature, dew point, wind, cloud cover, air pressure, pressure tendency, and precipitation. Measurement location and units Weather maps primarily use the station model to show surface weather conditions, but the model can also show the weather aloft as reported by a weather balloon's radiosonde or a pilot's report. Plotted winds The station model uses a wind barb to show both wind direction and speed. The wind barb shows the speed using "flags" on the end. Each half of a flag depicts Each full flag depicts Each pennant (filled triangle) depicts Winds are depicted as blowing from the direction the flags are facing. Therefore, a northeast wind will be depicted with a line extending from the cloud circle to the northeast, with flags indicating wind speed on the northeast end of this line. Once plotted on a map, an analysis of isotachs (lines of equal wind speeds) can be accomplished. Isotachs are particularly useful in diagnosing the location of the jet stream on upper level constant pressure charts, usually at or above the 300 hPa level. The flags and pennants point to the low pressure, so it is possible to determine at which hemisphere the station is standing. The barbs in the figure at the right are located at the Northern Hemisphere, because the wind is circling counter clock-wise around a low-pressure area at the Northern Hemisphere (the wind is blowing in the opposite direction at the Southern Hemisphere, see also Buys Ballot's law). More than a century ago, winds were initially plotted as arrows facing downwind, with feathers on both sides of the staff to indicate wind speed. In the United States, the change to the modern convention of flags shown on one side of the staff to indicate wind speed took effect on August 1,