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.
In the image below, this surface dew point temperature information has been incorporated into a more complex surface map generated for the same time.
Reported dew points show that a dome of very moist air has pushed its way into Iowa, Illinois, and southern Michigan where dew point temperatures are above 70. Such high dew points usually mean humid and sticky weather conditions.
On the other hand, pleasant weather is found across North Dakota and Minnesota, where dew points are in the 50's and the air is much drier.