METAR Temperature Groups MET
A relatively straightforward question with two traps.
The temperature and dew point are both rounded to the nearest degree. M before the number stands for minus. As you were taught in school, if the figure is .5 then you round up so 0.5 rounds up to 01. The first potential trap is with negative numbers - for example -1.5 rounds up to -1, not -2.
The second trap is that if you round to 0 degrees from below zero you keep the M so 0.4 would round to 00 and -0.4 would round to M00.