WV Location MET
Half the mass of the atmosphere is below 18000' (the 500hPa level, which is about half of 1013) but most of the the vapour is towards the bottom of this. From the NOAA: "Nearly half the total water in the air resides below an altitude of about 1.5 km. Less than 5% is in the upper troposphere (above 5 km), and less than 1% is in the stratosphere, nominally above 12 km. " Ensure you read the question - it could be about the mass, or the water vapour.