INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 47 



It will be seen also that the code figures may represent two values 

 oi" barometric pressure, but this is true only with a very high or very 

 low barometer reading. For example, the code figures 48 may be 

 used for barometer readings of 28.00 and 30.95. In such cases the 

 i-ecipient of the radio message will be able to decide wdiich value is 

 intended. 



If the observer's barometer is graduated according to the milli- 

 meter scale it wdll be necessary for him to refer to the table, "(con- 

 version of millimeters to millibars", on page 115 of this manual. 



Observers who are not scheduled to transmit weather reports by 

 radio need not enter any figures in the "Barometer as coded" column. 



VISIBILITY 



Observations of visibility (V) should be made in accordance with 

 the international scale, which is given in the code table for V, below. 

 One code figure from this table should be entered in the "Visibility" 

 column on Form 1210A. When it is not possible to determine pre- 

 cisely what figure in the scale most accurately describes the con- 

 dition, the entry should be made according to the best judgment of 

 the observer. 



Code table for V (visibility) 



Code figures 



Visibility 



Dense fog. (Objects not visible at 50 yards.) 



Thick fog. (Objects not visible at 200 yards.) 



Fog. (Objects not visible at 500 yards.) 



Moderate fog. (Objects not visible at V2 nautical mile.) 



Thin fog. (Objects not visible at 1 nautical mile.) 



Poor visibility. (Objects not visible at 2 nautical miles.) 



Moderate visibility. (Objects not visible at 5 nautical miles.) 



Good visibility. (Objects not visible at 10 nautical miles.) 



Very good visibility. (Objects not visible at 30 nautical miles.) 



Excellent visibility. (Objects visible at more than 30 nautical miles.) 



Horizontal visibility is often very useful as an indicator of the 

 thermal structure of the lower atmosphere. As a general rule, the 

 visibility is good when the air temperature is lower than the sea 

 temperature and very poor wdien the reverse holds true. The reason 

 for this is that when the air temperature is low^er than the 

 sea temperature there is a transfer of heat from the sea to the 

 lowest layers of the atmosphere. This will tend to make the atmos- 

 phere thermally unstable and favor active vertical mixing, which in 

 turn tends to disperse haze or fog particles that may have accumu- 

 lated at low levels. An unstable atmosphere is characterized by cu- 

 muliform clouds and a showery type of weather. 



On the other hand, when the sea temperature is lower than the air 

 temperature it follows that there is a transfer of heat from the low- 

 est layers of the atmosphere to the sea. This will tend to make the 

 atmosphere thermally stable and prevent active vertical mixing, 

 which in turn favors the accumulation of haze 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. 



