Complex sentence: In addition to the surface weather observations of temperature, precipitation and wind, Radiosonde observations began September 1,1939 with two flights a day which still continue at 12z and 00z through the present day.
Split into simpler sentences: In addition to the surface weather observations of temperature, precipitation and wind, Radiosonde observations began September 1,1939. At least 2 Radiosonde flights were done a day which still continues at 12z and 00z through the present day.