Re: Can we precisely measure humidity etc. at the Earth surface from orbit?

The instrumentation on the latest series of GOES satellites has 
greatly improved the capabilities for remote sensing of the atmosphere.  I 
have used the derived satellite sounding products over my forecast area 
during the convective season for the last two or three years and have found 
that they compare very favorably with balloon soundings.  The derived 
satellite soundings use the ETA forecast model as a first guess, and apply 
the information retrieved from the satellite sounder as a "correction" to 
that guess.  Therefore they should be considered a derived product rather 
than an observation.  Their greatest value to date has been as input into 
numerical forecast models although the use of the satellite soundings in 
real-time forecasting is growing as you can see from some case studies on 
the NESDIS 
Temperature and Moisture Soundings home page.

The sounding instrument has multiple channels, each of which samples a 
different layer of the atmosphere.  The latest generation of instruments 
allows this sampling to extend to the surface of the earth.  In the absence 
of clouds, the atmosphere is well sampled from the ground to around 100 mb 
(something around 16-17 kilometers).  The instrument does not sample 
specific points at every height, rather it sounds layers of the 
atmosphere, deriving a temperature and moisture value for the layer based 
on the absorption of radiation in each channel on the sounder.  The 
soundings are taken once every hour and are usually available on the web 
around an hour and a half after sounding time.  Most of this time lag is 
processing the data and loading onto a web server.  The actual sounding 
time is only a few minutes.  

My knowledge of the details of remote soundings is rather limited, so I 
refer you to a more detailed 
explanation of satellite sounding capabilities, as well as information 
on the quality and accuracy of remote soundings from the National Weather 
Service's Western Region Headquarters.