100 I^'«TRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



Drizzle. — Precipitation coii.si.stiny of iiniiicrons tiiiy (1r(>i)lcts. Drizzle origi- 

 nates from .stratns clonds. (See also part V.) 



Drought. — A proti-acted period of dry weather. 



Dry adiahatic htpac rate. — A rate of decrease of teniperatnre witli heiglit 

 approximately equal to 1° C. per 100 meters (1.8° F. per 328 feet.) This is 

 close to tlie rate at wliich an ascending body of imsaturated air will cool due 

 to adiabatic expansion. 



Dry hiilh. — A name given to an ordinary thermometer used to determine the 

 temperature of the air, in order to distinguish it from the wet bulb. 



Dry foy. — A haze due to the presence of dust or smoke in the air. 



Dii.'^t counter. — An instrument for determining approximately the number of 

 dust particles or condensation nuclei per unit volume in a sample of air. 



Dyiiaiiiie iiictcoroloyy. — The branch of meteorology that ti-eats of the motions 

 of the atmosphere and their relations to other meteorological phenomena. 



Eddy. — A more or less fully developed vortex in the atmosphere, constituting 

 a local irregularity in a wind. All winds near the eartli's surface contain 

 eddies, which at any given place produce '"gusts" and "lulls." Air containing 

 numerous eddies is said to be "turbulent." 



Equivalent potential teniperatnre. — The temperature that a given sample of 

 air would have if it were brouglit adiabatically to the top of the atmosphere 

 (i. e., to zero pressure) so that along its route all the water vapor present were 

 condensed and precipitated, the latent heat of condensation being given to the 

 .sample, and then the remaining dry air compressed adiabatically to a pressure 

 of 1,000 millibars. The equivalent potential temperature at any point is there- 

 fore determined by the values of absolute temperature, pressure and humidity. 

 It is one of the most conservative of air mass properties. 



Evaporiineter. — An instrument f<»r measuring the rate of evaporation of water 

 into the atmosphere. 



Expoaure. — In meteorology the method of presentation of an instrument to 

 that element whicii it is destined to measure or record, or the situation of the 

 station with regard to the phenomenon or phenomena there to be observed. 



Ejie of the storm. — A calm region at the center of a tropical cyclone or a 

 break in the clouds marking its location. 



Fahrenheit. — A thermometric scale on which 32° denotes the temperature 

 of melting ice, and 212° the temperature of boiling water, both under standard 

 atmospheric pressure. 



Fall-icind. — A wind blowing down a mountainside; or any wind having a 

 strong downward component. Fall-winds include the foehn, mistral, bora, etc. 



False cirrus. — Cirruslike clouds at the summit of a thunder cloud: more ap- 

 propriately called "thunderstorm cirrus." 



Fata Mori/ana. — A complex form of mirage, characterized by marked dis- 

 tortion of images. 



Festoon cloud. — Mammatocumulus. 



FiUiny. — The occurrence of increasing pressure in the center of a moving 

 pressure system. Filling is the opposite of deepening. 



Foehn. — A dry wnnd with strong downward component, warm for the season, 

 characteristic of many mountainous regions. The air is cooled dynamically 

 in ascending the mountains, but this leads to condensation, which checks the 

 fall in temperature through the liberation of latent heat. Tlie wind deposits 

 its moisture as rain or snow. In descending the opposite slope it is strongly 

 heated dynamically and arrives in the valleys beyond as a warm and very 

 dry wind. Some writers apply this term to any wind that is dynamically 

 heated by descent ; e. g., the sinking air of an anticyclone. 



Fog. — A cloud at the earth's surface. Fog consists of numerous droplets 

 of water, which are so small that they cannot readily be distinguished l>y the 

 naked eye. In ordinary speech the term "fog" generally implies an obscurity 

 of the atmosphere sufficiently great to interfere with marine and aerial 

 navigation. (See also part Y.) 



Foghoir. — A rainbow, colorless or nearly so, formed in a fog. 



Fog drip. — Moisture that is deposited on terrestrial objects by fog and drips 

 from them to the ground. 



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



Fracto-^tratus. — A form of cloud. (See part V.) 



Front. — A surface of discontinuity between two juxtaposed currents of air 

 possessing different densities, or, more simply, the boundary between two 

 different air masses. 



