A common answer to the crossword clue “line on a weather map” is isobar. This term denotes a line drawn on a weather map connecting points of equal atmospheric pressure. For instance, if several locations on a map all register an air pressure of 1012 millibars, an isobar would be drawn connecting those points.
Isobars are crucial for interpreting weather patterns. The spacing between isobars indicates the pressure gradient; closely spaced isobars signify a steep pressure gradient and thus strong winds, while widely spaced isobars indicate a weak pressure gradient and light winds. Historically, these lines have been essential tools for meteorologists in forecasting weather conditions, helping to predict wind speed and direction as well as potential storm systems.