WET-BULB  TEMPERATURE The  wet-bulb  temperature  is  the  lowest  temperature an  object  may  be  cooled  to  by  the  process  of evaporation. It is read directly from the wet-bulb thermometer  on  an  electric  psychrometer,  sling psychrometer,   or   rotor   psychrometer.   Water evaporating from the moistened wick on the wet-bulb thermometer bulb cools the thermometer bulb and lowers the temperature reading. The cooling effect of the evaporation from the bulb is inversely proportional to the amount of water vapor present in the air:  the more water vapor present, the less moisture will evaporate from the moistened wick, and the less cooling of the thermometer bulb will occur. From the dry- and wet- bulb readings, the dew-point temperature and humidity values may be calculated. The automatic weather observation  systems  do  not  provide  a  wet-bulb temperature, but automatically process equivalent measurements to compute dew-point temperature. T_F = 9 T_D 10 DEW-POINT  TEMPERATURE The  dew-point temperature is the temperature a parcel  of  air  must  be  cooled  to  in  order  to  reach saturation. Cooling  past  the  dew-point  temperature normally results in condensation or precipitation. Changes in temperature do not alter an air-parcel's dew- point temperature; therefore, dew-point temperature is termed a conservative property. The extraction or addition of moisture, however, from or to an air parcel will  respectively  decrease  or  increase  the  dew-point temperature. Dew-point temperature is calculated from the dry- bulb temperature and the wet-bulb depression by using the  CP-165/UM  psychrometric  computer.  The  wet- bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature. Dew-point temperature   is   automatically   calculated   by   the automatic weather systems. Many calculations that you will be using call for a dew-point  depression  as  a  value.  The  dew-point depression  is  the  difference  between  the  air  temperature and the dew-point temperature, expressed as a positive number. For example, if the air temperature is 78°F and the  dew-point  temperature  is  67.5°F,  the  dew-point depression is 10.5°F. FROST-POINT  TEMPERATURE The frost-point temperature is the temperature, below freezing, that a parcel of air must be cooled to in 1-41 order to reach saturation. Cooling past the frost-point temperature normally results in sublimation of ice crystals  from  the  air.  The  frost-point  temperature  is occasionally referred to as "the dew-point temperature with respect to ice." Calculations  using  the  "low" temperature side of the CP-165/UM psychrometric computer refer to use of the "Ti" scale when the wet- bulb thermometer wick is frozen, and to the "DP" scale if the wet-bulb wick is not frozen.  Both  scales  calculate a dew-point temperature with respect to liquid water, and not a frost-point temperature. The  frost-point temperature  may  be  approximated  by  the  following formula: where T_F is the frost-point temperature, and T_D is the dew-point  temperature. As an observer, you may be asked to calculate frost- point  temperatures,  especially  when  working  with  a Skew T, Log P diagram. Although the frost-point temperature is not usually computed for a surface aviation weather observation, we have introduced it at this point because it is so closely related to the dew- point  temperature.  Relative  humidity,  and  other humidity  computations  derived  from  temperature  and dew-point  temperature  readings  are  covered  later  in  this chapter. SEA SURFACE TEMPERATURE Another temperature reading in shipboard weather observations is the  sea surface temperature.  It  is supposed to reflect the temperature of the upper few inches of the sea surface. On some ships with OA divisions, installed sensors automatically measure this value. There are three other acceptable methods for obtaining  a  sea-surface  temperature  reading:  the bucket   temperature   method;   by   expendable bathythermograph;  and  by  use  of  the  seawater  injection temperature. The sea-surface temperature reading must be accurate since it is a major input into many undersea warfare (USW) acoustic products. Bucket Temperature The bucket temperature method is by far the most accurate, yet is also the most work intensive. In this method, a sample of seawater is obtained by casting a lightweight bucket or coffee can with a strong line attached over the side of the ship and retrieving a water sample.   This should be done as near to the bow of the