The lack of atmospheric water vapor data is a well known issue currently being addressed by the TAMDAR in-situ aircraft atmospheric sensor. Since 2005, AirDat TAMDAR Sensors have been providing useful atmospheric data, which includes air temperature, winds, icing, GPS and pressure altitude, turbulence, and relative humidity (RH), from a large number of SAAB 340 turbo props. Relative humidity has been of particular interest because it is generally not included in the ACARS data stream. Also, radiosonde water vapor data is sparse and satellite data suffers from lack of resolution. AirDat has successfully modified and adapted industrial grade capacitive RH sensors to gather high quality RH data with the TAMDAR sensor at all aircraft altitudes and speeds. The success of the effort has most recently been demonstrated on regional jet aircraft equipped with TAMDAR sensors.

The TAMDAR sensor uses two capacitive RH sensors (for redundancy) with custom hydrophobic filters to prevent direct water contact. The sensing elements are easily field replaceable—a fifteen minute effort—although experience has shown them to be very robust. The uses of the hydrophobic filters along with individual sensor calibration are important developments that have made the technology successful.

Starting in March of 2008, TAMDAR Sensors on Republic Airlines ERJ regional jets began supplying data. These planes provide coverage primarily in the eastern and central CONUS. This provided a good opportunity to evaluate TAMDAR's RH performance at higher speeds and altitudes than what are presently covered by the SAABs. This paper describes the excellent RH results seen on the ERJ platforms.

Measuring atmospheric RH directly on an aircraft requires significant corrections be applied to the RH sensor element output based on temperature, pressure and Mach. At very cold temperatures and high Mach numbers, RH errors are magnified by this process and can become large unless care is taken in the algorithms and RH sensor element calibration. The TAMDAR RH sensor elements are calibrated based on the common conditions in which commercial jet aircraft fly; specifically, calibration points are included for cold and dry conditions which are what the sensor would see at high altitudes and where Mach corrections would cause the greatest errors. A 3D curve based on temperature and RH sensor element output; along with Mach, temperature and pressure corrections achieve the desired accuracy. Lengthy analysis of the SAAB fleets have already shown very good performance. Recent evaluation of the Republic ERJ145s has demonstrated excellent RH performance even at the high Mach speeds. This is based on our comparisons to the AirDat MM5 RT-FDDA model forecasts, the GSD RUC dev 2 model forecasts, and radiosonde data when available. To date, the results reported by NOAA's GSD are similar. These comparisons are presented and reviewed. Future winter data will also be included.