This practice is no longer followed. Additionally, sea- level   pressure   should   not   be   converted   from hectopascals into inches for use as an altimeter setting, since differences in such calculations could yield false altimeter  settings. Aircraft flying above 18,000 feet overland and on over  water  flights  more  than  100  miles  offshore routinely use the standard pressure, 29.92 inches, as an altimeter setting. During low-level tactical flights and landings  aboard  aircraft  carriers,  however,  accurate altimeter settings are required. Now let’s consider pressure  tendency. PRESSURE TENDENCY The pressure tendency is the net change in the barometric pressure during a period of time and the trend or characteristic of the change. Normally the pressure tendency is observed for 3-hour periods ending at the intermediate synoptic times 0000Z, 0300Z, 0600Z, etc. Pressure tendencies for 12- and 24-hour periods may also be observed, and routinely replace the 3-hour pressure tendencies in observations taken in the tropics. The  net  change  is  determined  by  taking  the difference in the station pressure between the current observation and the station pressure 3, 12, or 24 hours ago. The trend or characteristic is determined from the barograph  trace,  or  the  actual  recorded  station  pressures during  the  period. The general trends of pressure "higher," the "same," or "lower" than at the beginning of the  period  are  further  described  in  both  NAVMET- OCCOMINST 3 141.2 and NAVMETOCCOMINST 3144.1 for reporting purposes. We will discuss some related pressure calculations on pressure altitude and density altitude later in this chapter in a section on aircraft performance indicators, but first we must cover temperature and moisture observations,  which  are  necessary  for  those calculations. REVIEW  QUESTIONS Q51.  Ten  millibars  is  equal  to  how  many hectopascals? Q52.  What is meant by the term "removal correction"? Q53. What would be the "r" factor for a shipboard barometer located 45 feet above the water line? Q54. What is an altimeter setting used for? Q55. How is the overall trend or characteristic of pressure tendency determined? TEMPERATURE LEARNING   OBJECTIVES: Define temperature.  Define  and  describe  how  to obtain  dry-bulb  temperature  and  wet-bulb temperature readings. Define and describe how  to  calculate  dew-point  temperature  and frost-point temperature. Define sea-surface temperature and describe three methods used to obtain  this  reading. Temperature is defined as the amount of sensible heat in a substance or as the measurement of molecular motion  in  a  substance.  Molecules  in  motion  cause  heat. As energy is added to a substance in the form of light or as infrared radiation (heat energy), the molecules absorb the energy, which increases molecular motion. This increase in molecular motion is measured as an increase  in  temperature.  Higher  temperature substances will also give off energy by radiation. Higher temperature substances can also transfer energy from faster moving molecules (warmer) to slower moving (cooler) molecules as the molecules collide. This process is known as conduction. In  surface  aviation  weather  observations,  observers take dry-bulb temperature and wet-bulb temperature readings by using sling psychrometers or electric psychrometers,  or  they  obtain  the  readings  from automatic weather station equipment. From these readings,  the  observer  may  calculate  the  dew-point temperature by using the CP-165/UM psychrometric computer, although automatic systems will calculate this  important  value. Related  to  the  dew-point temperature is the frost-point temperature, which may need  to  be  calculated.  Another  temperature  required  for shipboard surface aviation weather observations is the sea-surface  temperature. Also, once each day, the observer must obtain a maximum and a minimum temperature  reading. DRY-BULB  TEMPERATURE The dry-bulb temperature (also called the ambient air temperature, or simply the air temperature) reflects the amount of heat present in the air. It is read directly from  a  ventilated  thermometer  on  an  electric psychrometer, sling psychrometer, rotor psychrometer, or   from   automatic   measuring   equipment. The temperature must be obtained to the nearest 1/10 degree and may be read in either Fahrenheit or Celsius degrees. 1-40