52 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



in the shade, provided that this can be done without sacrificing the 

 essential requirement of a good windward exposure. The observer 

 should also remember that the instrument is quite sensitive to slight 

 variations in temperature, such as those resulting from the proximity 

 of the observer's body or of some object which has been strongly 

 heated by sunshine. The observer should, therefore, face the wind, 

 holding or whirling the psychrometer in front of his body, and 

 should also stand to windward of any overheated objects whose 

 presence might cause the thermometers to indicate false values. 



CLOUDS 



Amount of clouds. — The proportion of the sky covered by clouds, 

 irrespective of type, should be recorded on Form 1210A in the space 

 for total amount according to the scale to 10. In this scale rep- 

 resents a sky that is cloudless at the time of observation, 1 a sky 

 that is one-tenth covered, and so on to 10. which represents a sky 

 completely overcast. The amount of clouds should agTee in general 

 Avith the state of weather; that is, a sky with less than one-tenth of 

 cloud covering is clear, 1 to 5 tenths is partly cloudy, 6 to 9 tenths 

 is cloudy, 10 tenths is overcast. 



In the thin types of "mackerel" clouds there are almost always 

 gaps or spaces through which clear sky can be seen. When these 

 conditions prevail, therefore, the amount of cloud should never be 

 recorded as greater than 9, even though such clouds are spread 

 all over the sky. 



In case the sky is comi)letely obscured by dense fog, it should 

 be described as overcast, and ten-tenths of dense fog, without di- 

 rection of movement, should be entered in the space for cloud 

 amount. However, should the ship be enveloped in fog (or haze) 

 which allows blue sky, sun, moon, or stars to appear through and 

 there is no trace of cloud above the fog (or haze), the cloud amount 

 should be recorded as 0. On the other hand, if clouds are visible 

 above the fog, an attempt should be made to estimate the total 

 amount of cloud, and this should be recorded as though no fog 

 were present. 



Kind of clouds. — In recording the kind of clouds, observers should 

 conform strictly to the international system of classification, a full 

 description of which will be found, commencing on page 63, accom- 

 panied by illustrations to aid observers in identifying the various 

 cloud forms. 



In determining the kind of cloud, attention should be directed 

 mainly to the neighborhood of the zenith. Xear the horizon the 

 appearance of the clouds is much affected by perspective. 



Names of clouds should be entered on the observation form by 

 abbreviations, for example, Ci. for Cirrus, Cu. for Cumulus. The 

 proper abbreviations are shown in the condensed instructions on the 

 inside front cover of Form 1210A. 



Direction of cloud moveinent. — The direction of cloud movement 

 is always the direction from, which the clouds are moving. This is 

 frequently very difficult to determine on shipboard. Observers 

 should be guided by the remarks contained in the following para- 

 graphs : 



