Types of Pressure


The atmospheric pressure measured at the aerodrome reference point. With QFE set on the altimeter, the altimeter will read zero feet when the aircraft is on the aerodrome. 


This is the barometric pressure at the airfield (QFE), converted to mean sea level (MSL) using the ISA temperature at the airfield and the ISA pressure lapse rate. This will provide a pressure which does not account for any temperature deviation away from ISA. The correction to be made to the surface pressure will depend solely upon the height of the airfield AMSL. QNH is always a whole number without any decimal places and is always rounded down. When on the aerodrome with QNH set the altimeter will read aerodrome elevation. 


Because temperature affects the change of pressure over height QNH is not a true mean sea level pressure (unless ISA conditions exist). The forecaster needs to know the true mean sea level pressure in order to construct accurate analysis charts and to help with the forecasting of future changes. 

The meteorological offices, therefore, convert QFE to MSL using the actual temperature and assuming isothermal conditions between the aerodrome and MSL. This pressure is known as QFF and, because of the differential rate of change of pressure over height at different temperatures, may differ from QNH. 

We can determine, from in formula in the previous lesson, that at temperatures below ISA we have a relatively small height change per 1 hPa change in pressure and a relatively large change at temperatures above ISA.

Example 1:

What is the relationship between QFF and QNH at Oxford (270 ft AMSL) if the QNH is 1020 hPa and the temperature ISA +10°? 

The QNH is calculated using the ISA temperature and the QFF using the actual temperature. Since the actual temperature is warmer than ISA the change in pressure over 270 ft will be greater in the ISA than in the actual conditions. As we are above MSL this means that the QNH will be greater than the QFF. 

Example 2:

What is the relationship between QFF and QNH at an aerodrome 69 m below MSL if the QNH is 1005 hPa and the temperature is ISA-10°? 

This time the change in pressure is greater for the calculation of QFF than for the QNH. As we are reducing pressure this time it means the QNH will once again be greater than the QFF. 

We can use similar arguments to show that at an aerodrome AMSL with a temperature colder than ISA or at an aerodrome below MSL with a temperature greater than ISA the QFF will be greater than the QNH. This is summarized next:

error: Content is protected !!