Chris - i'm not sure to what temperature that reading really refers to.
Consulting the
weatherzone 500hPa chart, it suggests an air temp of -22 to -24C over gippsland at an altitude of ~5680m. As a very rough rule of thumb, air temp drops about 0.7-0.8C per 100m of elevation. That would mean that you'd have to get to an altitude of nearly 7400m at least to see an air temp of -38 C at your long/lat.
I suspect it means that the sort of measuring device you're using can't be used to measure air temp in the manner you're describing. Point at a nearby solid object - for sure. straight up in the air - i have my doubts...
