Performance of the Meteolabor "Snow White" chilled-mirror hygrometer in the tropical troposphere: Comparisons with the Vaisala RS80 A/H-Humicap sensors

The "Snow White" hygrometer is a low-cost, chilled-mirror hygrometer for radiosonde applications provided by a Swiss company, Meteolabor AG. A total of 54 Snow White soundings were conducted at five tropical stations in different seasons in 2000-01. All soundings were made with Vaisala RS80 radiosondes equipped either with the A-Humicap (22 soundings) or H-Humicap (32) relative humidity (RH) sensor. Comparisons of the RH with respect to liquid water between the Snow White and the different RS80 Humicap sensors are made. The Snow White measurements show reasonable agreement with the H-Humicap measurements from the surface up to ∼12 km (above -50°C air temperature), the region where the H-Humicap sensor can be considered reliable. Above 12 km, the H-Humicap sensor tends to miss small vertical-scale structures in RH due to the time lag error, but on average both instruments show no significant difference up to 14 km (-65°C). The comparison between the Snow White and A-Humicap sensors shows the known A-Humicap dry bias error at low temperatures and second dry bias error in the wet lower troposphere. The latter error [(A-Humicap RH) ≃0.9 × (Snow White RH) above 50% RH] may be a common problem for the recent A-Humicap sensors. These intercomparisons confirm the validity of the Snow White measurements at least up to the tropical upper troposphere and above 3%-6% RH.