SNOTEL (SNOwpack TELemetry). Across the western half of the USA, remote weather stations in mountainous locations record the level of snowfall in their respective areas. This data can then used to calculate the potential amount of melt water in the catchment area of a particular river.
Radio bursts around 40 MHz are sent at the remote sites and these signals are then reflected off trails of ionised gas left by small meteors in the upper atmosphere. These trails decay quite rapidly so the signals tend to be of a short duration.
The frequencies I have seen listed for SNOTEL are 40.530 MHz, 40.670 MHz and 41.530 MHz.
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