B-13 Wave Reflection.— At steep coasts, 

 where there is no beach and deep water is close 

 inshore, wind waves and swell will travel to the 

 coast without undergoing shallow water trans- 

 formation. Under these conditions, the waves 

 will be reflected from the shoreline and proceed 

 seaward, causing an interference pattern. If 

 this phenomenon is pronounced, one can see 

 pyramidal waves shooting upward where the 

 crest of a reflected wave meets the crest of an 

 oncoming wave. Beach gradients generally 

 must be steeper than 1 in 10 before reflection 

 occurs. 



B-14 Wave Forecasting. — Recent develop- 

 ments in wave forecasting theory indicate that 

 a much more thorough description of the sea 

 surface is needed than is possible by visual ob- 

 servations alone. Consequently, automatic wave 

 recorders of various types are being evaluated 

 as they are developed. This equipment eventu- 

 ally will enable the Oceanographic Forecasting 

 Central of the Oceanographic Office to improve 

 tlie prediction of sea conditions over large por- 

 tions of the ocean on a round-the-clock basis. 

 The immediate need, however, is for accurate 

 observational data, preferably instrument data, 

 to be used in checking and improving forecast- 

 ing procedures. 



B-15 Bottom Pressure Fluctuations. — In 



shallow water, pressure devices have been used 

 successfully to measure waves. For example, a 

 differential pressure gage that produces an 

 electrical signal proportional to the pressure 

 variation can be placed on the bottom and con- 

 nected to a recording system on an anchored 

 ship. Analog tapes of time versus amplitude 

 will yield bottom pressure fluctuations which 

 are caused by wave motions at the surface and 

 are related to wave periods and heights. 



B-16 Solar Radiation Measurements. — 



Solar radiation measurements are made using 

 a recording pyrheliometer system. The pyrhe- 

 liometer is an instrument designed primarily 

 for the measurement of direct solar radiation at 

 normal incidence. When mounted on a boom in 

 an inverted position, reflected radiation also 

 may be measured. The type pyrheliometer gen- 

 erally used by the Oceanographic Office is a 

 thermopile enclosed in a spherical glass bulb. 

 The receiving surface consists of two flat con- 

 centric disks, a black disk forming an absorbing 

 surface and a white disk forming a leflecting 

 surface. The resulting temperature difference 

 between the two disks acts on the thermopile 

 and produces an electromotive force (EMF) 

 which is proportional to the intensity of the 

 radiation. This signal then is transmitted to a 

 recording potentiometer. Each pyrheliometer is 

 given a sensitivity value to convert the EMF to 



gram-calories per square centimeter per minute. 

 This value is usually stamped on the pyrheliom- 

 eter identification tag. 



B-17 The Pyrheliometer Installation.— The 



pyrheliometer installation is dependent upon 

 the pyrheliometer used and the type radiation 

 to be measured; however, the following state- 

 ments are applicable in all instances and are 

 included as a guide. In order to obtain an ac- 

 curate measurement of the radiation, mount 

 the instrument in a location where shadows are 

 at a minimum. The pyrheliometer bulb and 

 sensor are extremely fragile and must be treated 

 carefully both in. use and in shipping. When- 

 ever possible, shock mounts and/or ruober gas- 

 keting should be used to reduce the vibration 

 and shock normally encountered on shipboard. 

 Two-conductor insulated cable should be used 

 to connect the pyrheliometer with the recorder, 

 ^ly type instrument that will record EMF 

 can be used for recording pyrheliometer data, 

 but the recorder manual should be consulted for 

 operating instructions for the particular in- 

 strument used. 



An installation for measuring total incoming 

 solar radiation and the radiation reflected from 

 the sea surface includes one topside pyrheliom- 

 eter and two inverted (180° shielded) outboard 

 pyrheliometers (fig. B-29). 



B-18 Taking Pyrheliometer Measure- 

 ments. — At the beginning of the operation and 

 at frequent intervals thereafter, the pyrheliom- 

 eter should be calibrated by placing the bulb 

 in total darkness and zeroing the recorder. The 

 recorder should be turned on approximately 1 

 hour before sunrise and turned off approxi- 

 mately 1 liour after sunset. Recorder speed 

 should be relatively slow ; 1 or 2 inches per hour 

 usually is sufficient. 



B-19 Marking the Recorder Chart.— The 



following information is entered on the recorder 

 chart : 



1. At the beginning of the roll, enter ship's 

 name, date (GMT) , pyrheliometer bulb number 

 and sensitivity value, recorder speed ( inches per 

 hour), and recorder range (in millivolts). 



2. Wlien the recorder is turned on enter time 

 and date (GMT) , position and heading. 



3. Time should be annotated every 2 to 4 

 hours during the measurements, or whenever 

 significant events occur; e.g., .station, recalibra- 

 tion, cleaning bulb, etc. 



4. W^hen recorder is turned off, enter time and 

 date (GMT), and position. 



B-20 Maintenance. — The pyrheliometer 

 bulbs may become encrusted with salt spray or 

 engine soot from the stack and must be wiped 



B-13 



