INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 99 



Conservative property. — Any air mass property the nature or value of which 

 is affected comparatively little by the various modifying influences to which 

 a moving body of air is exposed. 



Continental elimate. — The type of climate characteristic of the interior of a 

 continent. As compared with a marine climate, a continental climate has a 

 large annual and daily range of temperature. 



Convection. — The upward or downward movement, mechanically or thermally 

 produced, of a limited portion of the atmosphere. Convection is essential to the 

 formation of many clouds, especially of the cumulus type. 



Converfjence. — The condition that exists when the distribution of winds within 

 a given area is such that there is a net horizontal inflow of air into the area. 

 The removal of the resulting excess is accomplished by an upward movement 

 of air ; consequently areas of convergent winds are regions favorable to the 

 occurrence of precipitation. 



Corona. — (See part VI.) 



Corposant. — (See St. Elmo's fire.) 



Countertrades. — ( See Antitrades. ) 



Counter sun. — (See Anthelion. ) 



Crepuscular rays. — (See part VI; see also "Sun drawing water.") 



Cumuliform. — A general term applied to all clouds having dome-shaped upper 

 surfaces which exhibit protuberances, the bases of such clouds being generally 

 horizontal. Cumuliform clouds are characteristically distinct and separated 

 from one another by clear spaces. 



C'unmloninihus. — A form of cloud. (See part V.) 



Cumulus. — A form of cloud. (See part V.) 



Cyclogcnesis. — The term applied to the process which creates or develops a 

 new cyclone. The word is applied also to the process which produces an inten- 

 sification of a pre-existing cyclone. 



Cyclone. — An area of low barometric pressure wnth its attendant system of 

 winds. The cyclones occurring within the Tropics (tropical cyclones) are 

 smaller, on an average, than those of higher latitudes and in many cases are 

 the most violent of all storms, except tornadoes. Those occurring in higher 

 latitudes (extra-tropical cyclones) whether originating there or in the Tropics, 

 usually bring about marked changes of weather and temperature during their 

 passage ; their winds may be high or otherwise. Tropical cyclones are also 

 called hurricanes (when violent), typhoons, or bagnios. Extratropical cyclones 

 are commonly known as lows or barometric depressions. 



Dark segment. — The shadow of the earth which, in clear weather, rises from 

 the eastern horizon at sunset and sinks below the western horizon at sunrise. 



Debacle. — Breaking up of the ice in the spring in rivers and seas. 



Deepening. — The occurrence of decreasing pressure in the center of a moving 

 pressure system. 



Depression. — A cyclonic area, or low. 



Deviation of the ivind. — The angle between the direction of the wind and 

 the direction of the pressure gradient. (Cf. Inclination of the wind.) 



Dew. — Atmosplieric moisture condensed. In liquid form, upon objects cooler 

 than the air, especially at night. 



Dewpoint. — The temperature at which, under ordinary conditions, condensa- 

 tion begins in a cooling mass of air. It varies with the specific humidity. 

 The dewpoint is a conservative air mass property. 



Discontinuity. — The term applied in a special sense by meteorologists to a 

 zone within which there is a comparatively rapid transition of the meteorologi- 

 cal elements. 



Disturbance. — A local departure from the normal or average wind conditions 

 of any part of the world, or, in other words, a feature of what is sometimes 

 called the "secondary" circulation of the atmosphere, as distinguished from the 

 general circulation. In everyday usage disturhance has come to be synonymous 

 with cyclone and depression. 



Divergence. — The condition that exists when the distribution of winds within 

 a given area is such that there is a net horizontal flow of air outward from the 

 region. The resulting deficit is compensated by a downward movement of air 

 from aloft ; consequently areas of divergent winds are regions unfavorable to 

 the occurrence of precipitation. 



Doldrums. — The equatorial belt of calms or light variable winds, lying 

 between the two trade-wind belts. 



