Q45. Q46. Q47. Q48. Q49. Q50. REVIEW  QUESTIONS Any liquid that has a temperature below its normal freezing point but still exists in the liquid state is known by what term? What distinguishes drizzle from rain? What type of hydrometeor is composed of hard grains of ice formed from freezing rain or the refreezing  of  snowflakes? If rain is accumulating at a rate of 0.25 inches per hour, how should the intensity be classified? What does the abbreviation LTGCA mean? When is a thunderstorm considered to have begun? PRESSURE LEARNING  OBJECTIVES:  Recognize  the importance   of   an   accurate   pressure observation.  Describe  atmospheric  pressure, barometric  pressure,  and  station  pressure. Explain   sea-level   pressure   and   altimeter setting. Define pressure tendency. Pressure is an important weather analysis and forecasting  item  used  by  agriculturalists,  pilots,  and weather  forecasters.  Many  years  ago,  farmers discovered   that   falling   atmospheric   pressure   is associated with poor, unsettled weather and that fair weather is associated with rising atmospheric pressure. Today, most farmers rely on scientific forecasts to regulate their activity. Overland, at and below 18,000 feet,  pilots  fly  aircraft  at  their  assigned  flight  levels based  on  the  altimeter  setting  provided  by  local weather-observation  stations. In forecasting, pressure is used to analyze the isobar  patterns, or lines of equal pressure. From the isobar patterns, analysts can determine wind speeds, centers of high and low pressure, and other critical information. By tracking the movement of high- and low-pressure centers, forecasters may anticipate future movements of the centers, and their associated weather patterns. For pressure values to be meaningful to pilots, analysts, and forecasters, the reported readings must be accurate. An error in a reported sea-level pressure may cause an analysis to be in error, especially over data- sparse areas, such as the oceans. But an error in an altimeter setting can be disastrous for a pilot. The responsibility for observing, calculating, and reporting pressures accurately rests solely on you, the weather observer. In this section, we cover the different types of pressure  that  must  be  observed,  associated  pressure terms, and the pressure values that must be calculated for  an  observation. The standard units used to measure and report pressure values are inches of mercury and hectopascals. The  term  hectopascals  (hPa)  replaced  the  term millibars (mb) several years ago. A hectopascal is exactly equal to one millibar. See Appendix II for conversions  between  inches  of  mercury  and hectopascals. ATMOSPHERIC  PRESSURE Atmospheric  pressure  refers  to  the  pressure  exerted by the column of air on any point on the earth’s surface. The term is not specific as to where the point in question is located. The vagueness of the term causes some confusion in military weather because the observer can never be sure if the person asking for atmospheric pressure wants station pressure, sea-level pressure, or even an altimeter setting. BAROMETRIC  PRESSURE Barometric pressure is the pressure read directly from a precision aneroid barometer or a tactical aneroid barometer. On the ML-448/UM precision aneroid barometer, this value may be read in inches or in millibars. Readings in millibars can be converted directly to hectopascals; for example, 978.7 millibars equals 978.7 hPa. STATION  PRESSURE Station Pressure is the pressure value read on the barometer  (barometric  pressure  in  inches  or hectopascals) corrected for the difference between the height of the barometer and the station elevation. The correction that is added to the barometric pressure may be an instrument correction, a removal correction, and a temperature  correction. The  station elevation  is the height of the highest point on the runway above mean sea level (MSL). This is the height that is found published in the Flight Information Publications. Aboard naval ships, the station elevation is considered to be the height of the barometer above the water line, not the height of the flight deck. 1-38