Page:Handbook of Meteorology.djvu/203

 High drum thermographs are used very generally in meteorological work. Low drum strips have all the temperature range necessary for greenhouses, refrigerating establishments and freight cars containing perishable goods. They are used in many Weather Bureau stations where the yearly range does not materially exceed 100 degrees. For most stations, and for general military use, a high drum thermograph is advisable.

Thermographs are not so accurate as standard thermometers. In very damp weather the expansion and swelling of the paper on which the record is made affects the accuracy. Inasmuch as the paper rests on the lower collar of the drum, the upward expansion of the paper may render the record of the maximum 2 or 3 degrees too low. In any case, the maxima and minima should be compared with those of the registering thermometers, and the corrections, plus or minus, noted on the record sheet.

If the thermograph record does not coincide with the thermometer readings an adjustment screw will bring the pen to the proper position. It is a good plan to adjust the pen so that the minimum coincides with that of the minimum thermometer. The time of the minimum may always be determined from the thermograph sheet, and this is one of its important uses. As a rule, the minimum temperature occurs a short interval before sunrise. During the progress of a cold wave there may be a steady fall of temperature covering a period of two days. Frequently the fall of temperature continues from 12:01 to 11:59  The “lowest this morning” is therefore not the minimum of the day; and though this fact may escape the notice of the observer, it will not escape the record of the thermograph.

The recording pen of the thermograph may lag anywhere from five minutes to forty-five minutes behind the actual temperature. In very damp weather the lag is usually the greatest, and, in fixing the time at which a given temperature occurred, this fact must be taken into the calculation. An observer who studies the vagaries of his instruments—and they are many—will learn how to master them.