Originally Posted by
BECMG
My question is:
a)What do they mean by an isothermal lapse rate?
b)In altimetry we are taught to find the QNH from a given QFE by using 1 hpa=27ft then isnt the QNH reduced to sea level using the standard pressure lapse rate instead of 1.98/1000 ft ?
c)Why dont the meteorologists use the QNH?
a) "Iso-thermal" means "same-temperature" - so isothermal lapse rate means constant temperature.
b) Pressure reduces with height and, at the surface (ISA), it approximates to 27'/mb. However, reduction with height is not linear and. as height increases, so the difference in height corresponding to each millibar increases. 1mb approximates to 27' at the 1000mb level, to 50' at the 500mb level and to 100' at the 200mb level. A more accurate formula to show the relationship between height h (feet) and pressure P (mb) at mean temperature T (degrees Kelvin) is:
h2 - h1 = 221.1T (logP1 -logP2)
c) QNH is calculated using ISA because the altimeter is designed to assume ISA conditions and QNH is therefore likely to produce the most accurate indication of the altitude on the altimeter. QFF is obtained by taking the airfield pressure (QFE) and reducing it to sea level, assuming the actual temperature at the airfield is constant (isothermal) down to that level. This value is used by meteorologists on surface weather charts because it is more likely to accurately reflect actual conditions.
Hope this helps ...
JD
