Zenith wet delay The water vapor delay second term in refractivity )is not so easily integrated because of distribution of water vapor with height Surface measurements of water vapor pressure (deduced from relative humidity) are not very effective because it can be dry at surface and moist above and vIsa versa Only effective method is to sense the whole column of water vapor. Can be done with water vapor radiometer (WVR) which infers water vapor delay from thermal emission from water vapor molecules and some laser profiling methods(LIDAR). Both methods are very expensive(200Ks per site) 0409/03 12540Lec15 Zenith wet delay In meteorology, the term" Precipitable water (PW)is used. This is the integral of water vapor density with height and equals the depth of water if all the water vapor precipitated as rain(amount measured on rain gauge) If the mean temperature of atmosphere is known Pw can be related to zenith wet Delay(zWD)(See next page) 04903 12540Lec15Zenith wet delay • The water vapor delay (second term in refractivity) is not so easily integrated because of distribution of water vapor with height. • Surface measurements of water vapor pressure (deduced from relative humidity) are not very effective because it can be dry at surface and moist above and visa versa. • Only effective method is to sense the whole column of water vapor. Can be done with water vapor radiometer (WVR) which infers water vapor delay from thermal emission from water vapor molecules and some laser profiling methods (LIDAR). Both methods are very expensive (200K$ per site) 04/09/03 12.540 Lec 15 11 Zenith wet delay • In meteorology, the term “Precipitable water” (PW) is used. This is the integral of water vapor density with height and equals the depth of water if all the water vapor precipitated as rain (amount measured on rain gauge). • If the mean temperature of atmosphere is known, PW can be related to Zenith Wet Delay (ZWD) (See next page) 04/09/03 12.540 Lec 15 12 6