The daily amount of data varies considerably over the period of record.
See "Period of Record" in <a href="http://dss.ucar.edu/datasets/ds353.4/docs/more.summary.html#POR" >this</a>
*  definitions of raob, pibal and other upper air station observations are provided <a href="http://dss.ucar.edu/datasets/common/definitions/upa_obs.html" >here.</a></ul>