Isotherms are lines that connect places at the same temperature.
You will also be introduced to what is called the simple surface-station model. We'll discuss some of the other numbers and symbols at a later time. It is unlikely that many stations will report exactly 50, so you will need to do some estimating. Looking at the chart below, we find the upper station is at 48 and the lower one at 55. Mark the location where you estimate it to be 50 degrees with an x .