INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 31 



Disappearances with horizontal distances of less than that amount 

 are usually due to other causes, such as haze, smoke, fog, etc. Occa- 

 sionally the balloon will move directly across the sun's disk, making 

 observation extremely difficult. In such cases the amount of light 

 entering the object lens should be reduced. This may be accomplished 

 by means of a movable disk attached to the end of the sunshade or 

 by partially covering the end of the sunshade by any other means, 

 such as holding the fingers of one hand over it and alloAving the light 

 to enter between the fingers. Other instances will occur when the 

 balloon will be lost behind the anemometer, chimney, or other obstruc- 

 tion. At a properly selected station, possessing low-angle obstruc- 

 tions, this reason for disappearance will be of small frequency. The 



entry will be made, "obscured by " During periods of 



low, surface wind velocity the elevation angles for the first few 

 minutes will be relatively high. In fact, the balloon may be directly 

 overhead. The balloon may change its course and come back directly 

 over the observation station. The change of the angles, especially 

 the azimuth angle, will then be rapid. In case of disappearance due 

 to either of these causes an explanatory note should be entered after 

 the entry "Overhead." There will be a certain amount of loss due 

 to accident; which is caused by the kicking or knocking of theodolite 

 sufficiently to throw the balloon out of the field. This disappearance 

 is due to carelessness, and with due attention to the work at hand 

 will be eliminated altogether. Strong surface winds will sometimes 

 throw the theodolite into such a state of vibration that the balloon 

 cannot be accurately placed at the cross hairs, and this will finally 

 result in the loss of balloon altogether. An explanatory note must 

 also accompany the entry of accident. In some cases the balloon 

 will be abandoned to permit the early file of a coded message containing 

 the observed data. In order that the observations may provide as 

 much datum as possible for high levels and still not interfere with 

 other scheduled station duties, it is requested that the one daily 

 observation scheduled to begin at 5:00 p. m. (E. S. T.) be continued 

 to the greatest possible height. Observations made at other times 

 may be abandoned when absolutely necessary but must be continued 

 at least 22 minutes for the intermediate (11:00 a. m., E. S. T.) obser- 

 vations and at least 30 minutes for the 5:00 a. m. (E. S. T.) observa- 

 tions, unless cut short by adverse atmospheric conditions. Further- 

 more, during that part of the year when lanterns must be used for 

 the 5:00 p. m. observations by stations located in the 75th-meridian- 

 time zone, the 11:00 a. m. observations are to be continued to the 

 greatest possible height by those stations. 



93. Checking theodolite setting. — The setting or orientation of the 

 theodolite should be carefully checked immediately after the observa- 

 tion, before the theodolite has been disturbed. If there is no change 

 in the setting within a few tenths of a degree, the readings will be 

 entered in their proper columns on the second line under the last 

 entry of the observed angles, and the word "Check" written directly 

 below. Otherwise, corrections will be made on the observed data. 

 It should also be noted that the azimuth angles have been read from 

 the same vernier used in the original orientation or setting. 



