Comment by trailbits

Even non-physical numbers are problematic to signal 'invalid'. I had a customer use -999 as a placeholder for 'invalid' data. Years later somebody made a higher level data product that averaged and combined that data with other products, without knowing to first remove those 'invalid' values. The resulting values were all now within physical limits, but very very wrong. The best solution is to use IEEE NaN https://en.wikipedia.org/wiki/NaN so that your code blows up if you don't explicitly check for it.