Worldwide, there are over 800 upper-air observation stations and through international agreements data are exchanged between countries. Most upper air stations are located in the Northern Hemisphere and all observations are usually taken at the same time each day (up to an hour before 00:00 and/or 12:00 UTC), 365 days a year. When severe weather is expected additional soundings may be taken at a select number of stations. NWS takes observations at 92 stations; 69 in the conterminous United States, 13 in Alaska, 9 in the Pacific, and 1 in Puerto Rico. NWS also supports the operation of 10 other stations in the Caribbean.