Fiat Uno in the stand turns the cooling indicator up

When driving, I noticed that the cooling indicator (city trip) was unusually high (at 90 degrees). While driving, it was noticeable that the indicator went back again and only slowly but steadily climbed up again in the standing position. So stopped – cooling water tested – was missing, tilted after and continued driving. But at the stationary gas the indicator climbed up again. What can that be? lg Phil