Isotach

http://www.erh.noaa.gov/btv/events/28Oct2008/300mb_isotachs_28_18z.gif

Isotach lines are contour lines of equal wind speeds. They are important for locating jet streams because they are located in the upper atmosphere. It can also be helpful for captains of vessels, because it is very helpful to be warned when and where the high wind speeds are going to be.