Wind

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" colspan=2 height=125 BGCOLOR="#996699" TEXT="#080000" bgproperties="fixed" background="145467d1erlcll.gif"> <div> <FONT SIZE="5" COLOR="#FFFFFF" FACE="Arial" ><B>Paragliding</B></FONT><B>     </B><B>|     </B><A HREF="index.htm" TARGET="_top" TITLE="Paragliding"><U><B>home</B></U></A></div> <div> <A HREF="clouds.htm" TARGET="_top" TITLE="Clouds"><U>Clouds</U></A>   |   <B>Wind</B></div> <div> </div> </td> </tr><tr> <td valign="top" width=190 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> <B>Back to </B><A HREF="dynamic.htm" TARGET="_top" TITLE="Dynamics"><U><B>Dynamics</B></U></A></div> </td> <td valign="top" height=355 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> Wind</div> <div> Find a lot of information regarding all sort of aspects in weather, even weather calculations in this <A HREF="usatoday.htm" TARGET="_top" TITLE="USA Today"><U>address</U></A>.</div> <bR> <div> Understanding air pressure</div> <bR> <div> The air's pressure is caused by the weight of the air pressing down on the Earth, the ocean and on the air below. Earth's gravity, of course, causes the downward force that we know as "weight." Since the pressure depends on the amount of air above the point where you're measuring the pressure, the pressure falls as you go higher. </div> <div> The air's pressure also changes with the weather. Air pressure, in fact, is one of the important factors affecting the weather. For more on this see:</div> <div> <IMG BORDER="0" SRC="Symb075c00b700f000000000.gif" HEIGHT="16" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0"> <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/tg/whighlow/whighlow.htm','Graphic: How high and low air pressure affect the weather','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Graphic: How high and low air pressure affect the weather</A>  </div> <div> <IMG BORDER="0" SRC="Symb075c00b700f000000000.gif" HEIGHT="16" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0"> <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/wfbarrow.htm','Using barometer readings, wind as forecasting tools','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Using barometer readings, wind as forecasting tools</A>  </div> <div> <IMG BORDER="0" SRC="Symb075c00b700f000000000.gif" HEIGHT="16" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0"> <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/wstorm0.htm','Storms and fronts index','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Storms and fronts index</A>  with links to more on air pressure and weather. </div> <div> The information below tells you more about how air pressure is measured and its relation to air density. Below you'll also find the mathematical formulas used to describe air's decreasing pressure with altitude. </div> <div> Units of pressure </div> <div> In the U.S., air pressure at the surface is reported in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals. </div> <div> In the rest of the world, measurements are usually given in hectopascals although you will sometimes see them in centimeters of mercury, especially on older barometers. </div> <div> The term "hectopascals" is replacing the term "millibars." The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a conveinent measure of Earth's air pressure. Inches of mercury measure how high the pressure pushes the mercury in a barometer.</div> <div> To convert between inches of mercury and millibars, one millibar is equal to 0.02953 inches of mercury. The El Paso, Texas, National Weather Service Office has a  <A HREF="javascript:void(0)"ONCLICK="open('http://www.srh.noaa.gov/elp/wxcalc/wxcalc.html','weather calculator','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">weather calculator</A>  posted on the web that can be used to make the conversion. </div> <div> Centimeters of mercury can be translated into inches of mercury by dividing the number of centimeters by 2.54.</div> <div> The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather. </div> <div> A sidelight: In the International System (SI) of measurements, the unit of pressure is the Pascal, named after Blaise Pascal, the 17th century scientist who made important discoveries about air pressure. The standard atmospheric pressure at the Earth's surface of 1013.25 millibars is equal to 101,325 Pascals. To avoid large numbers, air pressure is reported in hectoPascals, which are the same as millibars. In many nations, you are now likely to hear reports such as, "air pressure, 1020.0 hectoPascals." This is the same as 1020.0 millibars. </div> <bR> <div> Pressure corrections </div> <div> When you read a barometer the reading directly from it is the "<B>station pressure</B>." </div> <div> Two things affect the barometer's reading, the  <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/tg/whighlow/whighlow.htm','high or low air pressure','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">high or low air pressure</A>  caused by weather systems, and the air pressure caused by the station's elevation, or how high it is above sea level. No matter what weather systems are doing, the air's pressure decreases with height. If you're trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station's elevation. That is, you want to see what the pressure would be at the station if it were at sea level. </div> <div> The air's pressure is related to its density. A USA TODAY online file has more on  <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/wdensity.htm','understanding air density','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">understanding air density</A>  </div> <div> You need to calculate, <B>sea-level pressure</B>, which is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station." To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature. </div> <div> Another kind of barometric reading is the <B>altimeter setting</B>, which aircraft use. It's defined as: "The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined." For it, all you need is the station pressure and the elevation, you can ignore the temperature. </div> <div> Information like this is found in the  <A HREF="javascript:void(0)"ONCLICK="open('http://www.nws.noaa.gov/oso/oso1/oso12/fmh1.htm','Federal Meteorological Handbook No. 1','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Federal Meteorological Handbook No. 1</A>  , which is the official guide to taking weather observations. The definitions above are from  <A HREF="javascript:void(0)"ONCLICK="open('http://www.nws.noaa.gov/oso/oso1/oso12/fmh1/fmh1ch11.htm#ch11link','Chapter 11','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Chapter 11</A>  , which covers pressure measurements.</div> <bR> <div> How pressure decreases with altitude</div> <div> As you go higher in the air, the atmospheric pressure decreases. The  <A HREF="javascript:void(0)"ONCLICK="open('http://covis.atmos.uiuc.edu/covis/visualizer/help/upperair/sfc.shtml','Constant pressure surfaces','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">Constant pressure surfaces</A>  page on a University of Illinois web site explains how air temperature affects upper air pressures. USA TODAY online graphics on  <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/tg/wridge/wridge.htm','upper-air ridges','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">upper-air ridges</A>  and  <A HREF="javascript:void(0)"ONCLICK="open('http://www.usatoday.com/weather/tg/wtrough/wtrough.htm','upper-air troughs','toolbar=1,location=1,resizable=1,scrollbars=1,width=600,height=400')">upper-air troughs</A>  explain more about pressure systems above the surface. </div> <div> The exact pressure at a particular altitude depends of weather conditions, but a couple of rules of thumb (approximations) and a formula give you a general idea of how pressure decreases with altitude. </div> <div> A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you're using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules of thumb work pretty well for elevations or altitudes of less than a two or three thousand feet. </div> <div> Here's the formula if you want a more precise answer: </div> <div> Pressure decreases with height in the first 100 kilometers above the earth's surface according to the formula P(z)=P(sea level)*exp(-z/H). P(z)= pressure at height z, P(sea level)= sea level pressure(~1013 millibars), z= height in meters, H= scale height( to keep the formula simple, we're using 7 kilometers for the scale height) </div> <div> If you want to use the formula to determine station pressure when you have an altimeter setting or a sea level pressure report from a weather station, use the altimeter setting or sea level pressure value for P, instead of using 1013 millibars. Since the formula is in metric units, you'd have to convert an altimeter setting, which is always given in inches of mercury in the U.S., to millibars, using the conversion: one millibar is equal to 0.02953 inches of mercury </div> <div> In all the formula, / means to divide, * means to multiply, "exp(-z/H) means to divide minus z by H and then take the inverse of the natural log of the answer. The standard rules of algebra apply.</div> <bR> <div> Infornation source: <A HREF="usatoday.htm" TARGET="_top" TITLE="USA Today"><U>USA Today Weather</U></A></div> <bR> </td> </tr></table></body>