While driving uphill on my 79 300sd, I have been noticing the temp gage needle moving up. It has been hot lately here in Southern California.

Did not have this problem last year around the same time.

What could be causing the temp gauge needle to move to the line between 80 deg C and 120 deg C. I'm assuming that based on the temp gauge markings that the indicated line is the 100 deg C marking.

Please advise