INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS JQ? 



Surge. — A general change in barometric pressure apparently superposed upon 

 cyclonic and normal diurnal changes. 



Synoptic chart. — A chart, such as the ordinary weather map, which shows 

 the distribution of meteorological conditions over an area at a given moment. 



Synoptic meteorology. — The branch of meteorology that deals with the 

 analysis of meteorological observations made simultaneously at a number of 

 points in the atmosphere (at the ground or aloft) over the whole or a part of 

 the earth, and the application of the analysis to weather forecasting and other 

 problems. 



Tangent arc. — Any halo that occurs as an arc tangent to one of the heliocen- 

 tric halos. 



Thermogratn. — The continuous record of temperature made by a thermo- 

 graph. 



Thermograph. — A self- registering thermometer. 



Thermometer. — An instrument for measuring temperature; in meteorology, 

 generally the temperature of the air. Maximum and minimum thermometers 

 indicate, respectively, the highest and lowest temperatures occurring between 

 the times of setting the instrument. A wet-bulb thermometer is used in meas- 

 uring humidity. (See Psychrometer.) 



Thermometer screen. — A construction designed to screen a thermometer 

 from the direct rays of the sun and from other conditions that would interfere 

 with the registration of the true air temperature ; usually a wooden cage with 

 louvered sides. In the United States commonly called the instrument shelter. 



Thunder. — The sound produced by lightning discharge. 



Thunderstorm. — A storm attended by thunder and lightning. Thunder- 

 storms are local disturbances, often occurring as episodes of cyclones, and, in 

 common with squalls, are marked by abrupt variations in pressure, tem- 

 perature, and wind. 



Tornado. — 1. A violent vortex in the atmosphere, attended by a pendulous, 

 more or less funnel-shaped cloud. 2. In West Africa, a violent thunder- 

 squall. 



Trade-winds. — Two belts of winds, one on either side of the equatorial 

 doldrums in which the winds blow almost constantly from easterly quadrants. 



Trajectory. — The path traced out by a small volume of air in its movement 

 over the earth's surface. 



Transition zone. — The relatively narrow region occupied by a front wherein 

 the meteorological properties exhibit lai'ge variations over a short distance and 

 possess values intermediate between those characteristic of the air masses on 

 either side of the zone. 



Tropical disturbance. — The name used by the Weather Bureau for a cyclonic 

 wind system of the tropics that is not known to have sufficient force to justify 

 the use of the words "storm" or "hurricane." 



Tropical maritim,e air. — The term used to describe any air mass that orig- 

 inates over an ocean area in the tropics. Tropical maritime air is character- 

 ized by high surface temperatures and high specific humidity. 



Tropopause. — The point in the atmosphere at which the fall of temperature 

 with increasing height abruptly ceases. This point marks the base of the 

 stratosphere. Over most of the earth it is located, on the average, at eleva- 

 tions of between 10 and 15 kilometers (6 and 9 miles) above sea level. Its 

 normal level over the polar regions is somewhat below 10 kilometers and over 

 the equator somewhat above 15 kilometers. 



Troposphere. — The lower region of the atmosphere from the ground to the 

 tropopause, in which the average condition is typified by a more or less regu- 

 lar decrease of temperature with increasing altitude. 



Trough. — An elongated area of low barometric pressure. 



Turbulence. — Irregular motion of the atmosphere produced when air flows 

 over a comparatively uneven surface, such as the sui'face of the earth, or when 

 two currents of air flow past or over each other in different directions or at 

 different speeds. The existence of turbulence in the atmosphere is made appa- 

 rent by the character of the trail of smoke from a ship's funnel and by gusts 

 and lulls in the wind. 



Tivilight.— Astronomical twilight is the interval between sunrise or sunset 

 and the total darkness of night. Civil twilight is the period of time before 

 sunrise and after sunset during which there is enough daylight for ordinary 

 outdoor occupations. 



Typhoon. — The name applied in the Far East to a tropical cyclone. 



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