Here is copy & paste from the NAT MNPS Operations Manual
6.5 METEOROLOGICAL REPORTS
6.5.1 Some aircraft flying in the NAT are required to report MET observations of wind speed and direction plus outside air temperature. Any turbulence encountered should be included in these reports. From among the aircraft intending to operate on the organised track system, OACs designate those which will be required to report routine meteorological observations at, and midway between, each prescribed reporting point. The designation is made by the OAC when issuing the Oceanic Clearance using the phrase “SEND MET REPORTS“, and is normally made so as to designate one aircraft per track at approximately hourly intervals. Pilots flying routes which are partly or wholly off the OTS should include routine MET observations with every prescribed report. The midpoint observation should be recorded then transmitted at the next designated reporting point. The format to be used for the reporting of such additional observations must be by reference to the latitude (degrees and minutes) and longitude (degrees only) for the intermediate mid-point. It should be recognised that the use of the term “MID” is insufficient for direct input into MET computers.