+// Invert the QNH calculation to get the field pressure from a metar
+// report.
+// field pressure _in pascals_
+// ... caller gets to convert to inHg or millibars
+// Field elevation in m
+// Altimeter setting (QNH) in pascals
+// Valid for fields within the troposphere only.
+double FGAtmo::fieldPressure(const double field_elev, const double qnh)
+{
+ using namespace atmodel;
+ static const double nn = ISA::lam0 * Rgas / g / mm;
+ const double pratio = pow(qnh / ISA::P0, nn);
+ return ISA::P0 * pow(pratio - field_elev * ISA::lam0 / ISA::T0, 1.0 / nn);
+}
+