A weather station can record temperature for many years. During that time, an instrument may be replaced, the station may move or the town around it may grow. Scientists write down these changes because they can affect how one part of the long record compares with another.
Researchers do not simply delete an unusual value. They compare nearby stations, read old notes and use tested methods to find changes that did not come from the climate. They keep the original record and explain each adjustment. This transparent process allows other teams to review the work. A good weather record preserves data and its history. Careful notes make a long record more useful, not less trustworthy.