A
weather map displays various meteorological features across a particular area at a particular point in time and has various symbols which all have specific meanings. Such maps have been in use since the mid-19th century and are used for research and
weather forecasting purposes. Maps using
isotherms show temperature gradients, which can help locate
weather fronts. Isotach maps, analyzing lines of equal
wind speed, on a constant pressure surface of 300 mb or 250 mb show where the
jet stream is located. Use of constant pressure charts at the 700 and 500 hPa level can indicate
tropical cyclone motion. Two-dimensional
streamlines based on wind speeds at various levels show areas of convergence and
divergence in the wind field, which are helpful in determining the location of features within the wind pattern. A popular type of surface weather map is the
surface weather analysis, which plots
isobars to depict areas of
high pressure and
low pressure. Special weather maps in
aviation show areas of icing and turbulence for the other.