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This is likely due to the source data being vended in whole number increments in degrees Celsius. Converting between Celsius and Fahrenheit using only whole numbers would not allow for 60 to show up (it is between 15 and 16 degrees C, which would convert to 59 and 61). There was a large story around this same issue in the Apple weather app, not showing 69 degrees F prior to iOS 15.

15C is 59F, 16C is 60.8F so I'd say using integer Celcius and converting to Fahrenheit

My guess is the data is stored in Celsius but displayed in Fahrenheit.

15C = 59F 16C = 60.8F