A line connecting points of equal Dew Point Temperature is called an isodrosotherm. That means, at every point along a given isodrosotherm, the values of dew point temperature are the same.
Isodrosotherms are represented by dashed green contours. An image of reported dew point temperatures and isodrosotherms has been given below.
The isodrosotherms were generated from the observed values of dew point temperature, and have been plotted at intervals of 5 degrees Fahrenheit. The small green numbers are contour labels, which identify the value of an isodrosotherm (55, 65 degrees Fahrenheit).
For example, high dew point values of greater than 70 have been reported in Iowa, Illinois and southern Michigan and are therefore located south of the 70 degree contour.