Isotach

http://www.erh.noaa.gov/btv/events/28Oct2008/ua2.php

An isotach is very similar to an isobar except for they measure two different things but in the same way. An idobar is a line that connects points where equal pressure has been recorded. An isotach is a line that connects points where winds of equal speeds have been recorded. This isotach shows northwesterly winds of 75-100 kt across the North and South Dakota as well as Montana with broadly diffluent flow downstream across the mid and upper Mississippi River Valley.