INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 53 



1. The direction of cloud motion should be determined by using 

 some heavenly body as a reference point. If the sky is completely 

 covered with thick clouds, it is best not to attempt to ascertain the 

 direction of cloud motion. 



2. When there are two or more cloud sheets with openings so 

 that cloud movement at more than one level is visible, the absolute 

 movement of each layer should be ascertained, if possible. This 

 can be done only when the cloud layers are so thin or broken that 

 the sun (or moon) is plainly visible from time to time. The mis- 

 take should not be made of determining the direction of motion of 

 one cloud layer with reference to another. 



3. In estimating the true (not the magnetic) direction from which 

 the clouds proceed, the course and speed of the vessel should be 

 taken into account, precisely as in the case of the wind. This ap- 

 plies especially to the determination of the motions of low cloucls, 

 because their rate of movement is usually less than that of high 

 clouds. The following example shows how wide, in the case of 

 slowly moving clouds, the variation between apparent and true cloud 

 direction can be : If the clouds are actually moving from NE. at a 

 speed of 12 knots and the ship is steaming due W. at 20 knots, the 

 clouds wall aj)pear to be moving from about NW. by W. The slower 

 the actual rate of cloud movement the more the apparent direction 

 wdll differ from the true direction, except when the true direction 

 is from dead ahead. 



Since the observer ordinarily has no way of knowing the exact 

 velocity of the clouds, it is quite impossible to make a really accu- 

 rate allowance for the movement of the ship. However, it should 

 be remembered that the lower clouds usually are floating within the 

 same air current as prevails at the sea surface. Therefore, in most 

 cases the true speed and direction of the lower cloucls will be 

 roughly the same as those of the wind at sea level. In the case of 

 upper clouds, this is seldom true. However, if the upper clouds 

 display a clearly noticeable motion, their actual speed must be large 

 compared with that of the ship. It is then unnecessary to make 

 much allowance for the ship's movement. On the other hand, if the 

 upper clouds appear to be almost motionless, it is advisable not to 

 attempt to ascertain the true direction of their movement. 



4. Finally, and in general, when there is any doubt about the 

 direction of cloud movement, it is better to enter a question mark 

 or a dash, since no observation at all is preferable to an incorrect 

 one. 



Cloud notes in Daily Journal. — In addition to the records of 

 clouds as a part of regular observations, notes in the Daily Jour- 

 nal should briefly describe important changes in the amount and 

 kind of clouds between observations. 



SEA AND SWELL 



When a light wind blows over a water surface it produces at 

 first a series of ripples, moving with the wind. As the ripples move 

 forward with the Avind they increase in size. A strong wind pro- 

 duces larger waves and, if it continues, a heavy sea results. 



After a wave moves beyond the influence of the winds which 

 caused it there is a change in its form. The most rapid change at 



