This one is, I admit, rather obscure. The U.S. National Weather Service has, since the 1960s, operated a program called SKYWARN, through which thousands of trained volunteers report severe weather conditions to the NWS, mostly via HAM radio. This information fills in gaps in the data acquired by other means (such as Doppler radar and satellite), enabling officials to warn the public or take other appropriate action in the event of dangerous weather conditions. SKYWARN Recognition Day, observed every year since 1999, calls attention to this important volunteer work.

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