when the air temperature is lower than the sea 

 temperature and very poor when higher. The 

 reason for tliis is that in the former condition, 

 the lowest layers of the atmosphere are being 

 heated by the sea. This tends to make the 

 atmosphere thermally unstable and favors 

 active vertical mixing, which in turn tends to 

 disperse haze or fog particles that may have 

 accumulated at low levels. An unstable atmos- 

 phere is characterized by cumuliform clouds and 

 a showery type of weather 



On the other hand, when the sea temperature 

 is lower than the air temperature, it follows that 

 the sea cools the lowest layers of the atmosphere. 

 This tends to make the atmosphere thermall.y 

 stable and prevents active vertical mixing, 

 which in turn favors the production of haze and 

 at low levels. The cooling of the surface air 

 also favors the production of fog. A stable 

 atmosphere is, therefore, characterized by poor 

 visibility and, if it is sufficiently moist, by fog, 

 low stratus clouds and drizzle. 



11-6 Wind Speed and Direction. — When a 

 ship is equipped with an anemometer the true 

 wind speed is recorded in knots and the direc- 

 tion in degrees True rather than magnetic. The 

 data ma_y be recorded in sucli manner on the 

 A-sheet, but speed must be converted to meters 

 per second and direction coded when tran- 

 scribed to the E-sheet. These values are found 

 in tables lOA and 8, respectively. When 

 observing aboard a ship without an anemometer, 

 the Beaufort scale given in table 9 is used and 

 the force estimated. 



The appearance of the sea surface serves as 

 the best means of estimating the true wind 

 speed, just as it affords the best means of ascer- 

 taining wind direction. The direction can be 

 determined by making use of the fact that the 

 crest lines of the smallest ripples on the sea 

 surface are perpendicular to the direction of the 

 wind. These ripples are very sensitive to 

 sudden changes in character of the wind. 

 Accentuation of them by a localized increase in 

 wind velocity produces an apparent darkening 

 of the sea surface, which serves to show the rate 

 of travel of individual gusts or puffs. With wind 

 forces of 6 (Beaufort) or more, the wind direc- 

 tion also may be estimated correctly from the 

 direction of the streaks of foam which are 

 formed. 



11-7 Temperature of the Air. — The air 

 temperature should be read from a reliable 

 portable thermometer or the dry bulb of a 

 sling psj'chrometer and recorded on the A- and 

 E-sheets to the nearest 0.1° C. and on the B- 

 shect to the nearest 0.1° F. Conversion tables 

 are given in table 13. Wet-bulb reading of the 



sling psychrometer are read and recorded to the 

 same degree of accurac3^ It is important that 

 the temperature of the air and also the wet-bulb 

 temperature be measured by a sling psychrome- 

 ter since this method is more accurate than that 

 of stationary thermometers. Furthermore, the 

 formula employed for determining the relative 

 humidity from the readings of a psychrometer 

 is not applicable to the readings of a stationary 

 wet-bulb thermometer. 



The mercury bulb of the wet-bulb thermome- 

 ter in the sling psychrometer is covered with a 

 muslin wick. The wick must be wet with fresh 

 water before each observation. The wick must 

 be kept clean and free of salt. 



11-8 Relative Humidity. — Relative hu- 

 midity is determined from table 14. The table 

 is entered with the Fahrenheit readings to the 

 nearest whole degree, of the sling psychrometer 

 dry and wet bulbs and the percent of relative 

 humidity is read directly. 



11-9 Barometric Pressure. — Barometric 

 pressure may be recorded on oceanographic log 

 sheets — A and B in inches of mercurj' after 

 correcting for any barometer error; however, the 

 readings must be converted to millibars when 

 transferring the data to the E-sheet. Conver- 

 sion tables for barometers reading in inches of 

 mercury and millimeters to millibars are given 

 hi tables llA and llB respectively. 



When the barometer is pumping, the observer 

 should take 2 or 3 pairs of readings. Each pair 

 should contain one of the highest and one of 

 the lowest readings obtainable. The reading 

 to be recorded is that which is obtained from 

 averaging the whole set. 



11-10 Wind Waves (Sea) and Swell.— 

 Sea and swell have been discussed in chapter 10. 

 Complete instructions for taking and recording 

 these observations are given on H. O. Pub. No. 

 606-e, Sea and Swell Observations. Tables 

 for recording the data are in the above publica- 

 tion and in tables 5 and 6. 



11 11 INSTRUCTIONS FOR OPERAT- 

 ING THE RECORDING PYRHELIOME- 

 TER. — The pyrheliometer assembly is designed 

 to obtain data to determine the ratio between 

 total incoming solar and sky radiation and 

 that reflected from the sea surface. The 

 pyrheliometer is a thermopile enclosed in a 

 glass bulb and measures radiation incident on 

 a horizontal surface. The receiving surface 

 consists of two flat concentric disks, a black disk 

 forming an absorbing surface and a white disk 

 forming a reflecting surface. The resulting 

 temperature difference between the two disks 

 acts on the thermopile and produces an electro- 

 motive force (EMF) which is proportional to the 



H. O. 607 



111 



