CHAPTER 4 



TEMPERATURE, HUMIDITY, AND 

 PRECIPITATION 



Many parameters are important in the field 

 of meteorology; included among them are tem- 

 perature, humidity, and precipitation. In this 

 chapter the various terms associated with tem- 

 perature, humidity, and precipitation will be 

 defined, along with a description and use of 

 the various types of sensing equipment, both 

 manual and electronic, including a brief 

 resume of the oceanographic sensors. 



TEMPERATURE, HUMIDITY, AND 

 PRECIPITATION 



Before proceeding with a discussion of tem- 

 perature, humidity, and precipitation, one needs 

 to have a working knowledge of some of the 

 terms used to define them. 



DEFINITIONS 



1. Temperature. Temperature is defined as 

 the measure of molecular motion or the de- 

 gree of heat of a substance. It is measured 

 on an arbitrary scale from absolute zero where 

 the molecules theoretically stop moving. A 

 longer and more comprehensive definition is 

 given in chapter 12 of this training manual, 

 as are descriptions of the temperature scales 

 in use in meteorology today. 



Temperature, as used in surface observa- 

 tions, refers primarily to the free air or the 

 ambient temperature close to the surface of 

 the earth. Other temperatures are also ob- 

 served during the course of an observation, 

 such as the maximum, the minimum, sea water, 

 and others. 



2. Humidity. Humidity is the state of the 

 atmosphere with respect to water vapor con- 

 tent. However, since there are several ways 

 in which to express the water vapor content, 

 it Is better to specify which type of humidity 

 is meant. 



3. Dew point. The dew point is defined as 

 the temperature to which a sample of air must 

 be cooled, while the mixing ratio and barometric 

 pressure remain constant, in order to attain 

 saturation with respect to water. 



The dew point can never exceed the dry- 

 bulb temperature in any given observation. 

 When the air is saturated, the dew point and 

 the temperature are the same and the relative 

 humidity is 100 percent. 



4. Dry-bulb temperature. The dry-bulb tem- 

 perature is the natural temperature of the 

 ambient atmosphere at the point and time of 

 observation and is synonymous with the surface 

 temperature. 



5. Wet-bulb temperature. The wet-bulb tem- 

 perature is the lowest temperature to be se- 

 cured in the ambient atmosphere in its natural 

 state by evaporating water from the wick-cov- 

 ered bulb of a thermometer at a specified rate 

 of ventilation. It differs from the dry-bulb tem- 

 perature in an amount dependent on the tem- 

 perature and humidity of the air. This difference 

 is termed the wet-bulb depression. 



6. Relative humidity. Relative humidity, with 

 respect to water, is the ratio of the actual 

 vapor pressure in the air to the saturation 

 vapor pressure that would exist if the sample 

 of air were saturated at the same pressure and 

 temperature. 



7. Psychrometer. A psychrometer is an in- 

 strument used for measuring water vapor con- 

 tent of the atmosphere. It consists of two 

 thermometers, one of which (the dry bulb) is 

 an ordinary glass thermometer, while the other 

 (the wet bulb) has its bulb covered with a jacket 

 of clean muslin which is saturated with distilled 

 water prior to an observation. When the bulbs 

 are suitably ventilated, they indicate the ther- 

 modynamic wet- and dry-bulb temperature of 

 the atmosphere. 



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