from an explosion on the ground or at sea level. In such a case, where the fire-ball touches the earth or sea, tons of dirt or water will be drawn up into the cloud. This will become coated with RA. material from the bomb and there will be a greater radioactive fallout. The various particles vary considerably in size, some are extremely small while others are as large as grains of sand or small water droplets. As the cloud rises, the smallest particles will be taken up with it but the largest ones will begin to "fallout" from the stem and the base of the cloud. A burst at ground or sea level will not be included in Operation "Grapple".
      The first thing the meteorologist must do is to provide the safety officers with the data which they need to calculate how high the cloud will rise. The elements used in this calculation are the pressure, temperature and humidity in the upper air and the estimated power of the bomb. Measurements of upper air data can be made by aircraft: Shackletons for low and medium level and Canberras for high level flight. But to reach and investigate the really great heights in the atmosphere to be of value in the trials of atomic weapons, a meteorological balloon is used. This is filled with hydrogen until it is about 7 feet in diameter. It rises at about 1,200 feet per minute, swelling as it does so due to the fall off in atmospheric pressure with altitude, and finally it bursts at about 100,000 feet, by which time it is rather larger than the average four-bedroomed house.
Attached to the balloon is a device known as radio-sonde and this ingenious instrument measures temperature, pressure and humidity. The measuring elements are connected to a small radio set with a battery which transmits the value of each in turn. The signals are picked up by an ordinary M/F receiver on the ground and the results plotted out on a special form known as Te-phi gram.
      When this form is completed the safety officers are then in a position to make their calculations for estimating the height to which the cloud will rise.
      In Operation "Grapple" the Met. organization is as follows. At Headquarters, Christmas Island there is the central Met. Office where radio-sonde ascents, wind finding to high altitudes, and surface observations are made. The principal task for which this office is responsible is the forecast of the special meteorological conditions for atomic trials, and also for general forecasts for the area and for flying. There is another shore station at Penrhyn Island which carries out radio-sonde ascents and wind finding and makes surface observations. All H.M. Ships taking part in the Operation will make observations as follows: the permanent Met. station for the target area once she is fully equipped for radio-sonde ascents, wind-finding and surface observations as are the shore stations. No other ship is equipped for radio-sonde as it is not necessary to have too many sonde stations close together. H.M.S.Warrior is equipped for high altitude wind-finding and surface observations and is also equipped to carry out the functions of a H.Q. office afloat, corresponding to H.Q. Christmas Island. In addition two R.N.Z.N. frigates H.M.N.Z.S. Rotoiti and Pukaki have been specially equipped for high altitude wind-finding and surface observations. The apparatus used is the standard apparatus for shore stations, but this is the