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Doug wrote:
> The barometric pressure you are getting from the ATIS is not absolute > pressure, but "sea level" pressure (as if sea level existed at that point). > > As you go up, the pressure decreases, the barometric reading in the window > stays the same, and the pressure decrease is reflected in the altitude > reading. [This information is for US Airports only, as other places might not use Hg inches at all.] The ATIS does NOT report "sea level pressure." The ATIS reports the altimeter setting. Unless standard conditions exist, they are NOT the same. METARs might report SLP in addition to altimeter setting. When a METAR reports the SLP, it is encoded in hectopascals (millibars). For example, a sea level pressure of 998.2 hectopascals will be reported in the METAR as SLP982. (The first two digits of the hectopascal number are not included in the METAR. The third digit in the METAR is the first digit after the decimal point.) Notice that more often than not converting a METAR altimeter setting to hectopascals will not produce the same METARs SLP. |
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