http://www.wired.com/autopia/2013/12/airplanes-weather-sensors/

With the help of Aeronautical Radio Incorporated (ARINC), the
National Oceanic and Atmospheric Administration (NOAA), and SpectraSensors, 87
Boeing 737s have been equipped with Water Vapor Sensing Systems (WVSS-II) that
keep tabs on humidity when the planes take off and land.

The WVSS-II systems aren’t new — they’re used by both meteorologists and
aviation forecasters to determine everything from fog formation to cloud
ceilings — but by mounting these sensors onto planes already buzzing around the
sky, the amount of data will increase dramatically.

“The WVSS-II observations add a critical new piece of weather data to the
forecasting puzzle,” says Jeannine Hendricks, ARINC’s manager for the program.
“For the first time in aircraft operations, we are collecting water vapor data
that measures the humidity in the air. This has the potential to revolutionize
weather forecasting — especially when predicting thunderstorms — a significant
weather occurrence for aviation.”

The ability to sample the moisture in the atmosphere — something that’s
surprisingly under-sampled — and how those levels change between destinations
will provide a new way of forecasting weather patterns, increasing accuracy and
allowing NOAA and other agencies to get severe weather warnings out even
earlier. And maybe even predict flight delays before you leave home for the
airport.