In other words, the pressure drop per foot is less in warm air — Page 26, Lesson 26

I want to walk you through a key concept in pressure altimetry: the difference between QNH and QFF, and how temperature affects their relationship. Let's start with the core idea. The QNH is calculated using the ISA temperature, while the QFF is calculated using the actual temperature. That single difference drives everything that follows. Now, imagine we're at an aerodrome that is 270 feet above mean sea level, and the actual temperature is warmer than ISA. Since the actual temperature is warmer, the air is less dense, so the pressure change over that 270 feet is smaller in the actual conditions than it would be in the standard ISA atmosphere. In other words, the pressure drop per foot is less in warm air. Because we are above mean sea level, and the QNH uses the ISA lapse rate, it will reduce pressure more steeply as we go up. That means when we reduce the station pressure down to sea level using ISA, we get a higher sea-level pressure than if we used the actual, warmer temperature. So, when the aerodrome is above mean sea level and the temperature is warmer than ISA, QNH is greater than QFF. Let's look at Example 2 in the text. It asks: what is the relationship between QFF and QNH at an aerodrome 69 metres below mean sea level, if the QNH is 1005 hPa and the temperature is ISA minus 10 degrees? Here, the aerodrome is below mean sea level, so we are adding pressure to go down to sea level, not subtracting it. The temperature is colder than ISA, so the pressure change per foot is greater in the actual cold air than in the ISA. Since we are increasing pressure to go down, the QFF, which uses the actual colder temperature, will increase pressure more steeply than the QNH. That means QFF will be greater than QNH. But wait — the text says that in this case, QNH is once again greater than QFF. Let me read that carefully: "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." I need to reconcile that. The example says the aerodrome is 69 metres below MSL, and the temperature is ISA minus 10. The text states that QNH is greater than QFF. That matches the summary pattern we'll see in a moment. Let me give you the general rule from the summary. There are four combinations. If the aerodrome is above mean sea level and warmer than ISA — that's plus, plus — then QNH is greater than QFF. If it's above mean sea level and colder than ISA — plus, minus — then QFF is greater than QNH. If it's below mean sea level and warmer than ISA — minus, plus — then QFF is greater than QNH. If it's below mean sea level and colder than ISA — minus, minus — then QNH is greater than QFF. So the pattern is: same sign — both above MSL and warmer, or both below MSL and colder — gives QNH greater than QFF. Opposite signs give QFF greater than QNH. Now, what about stations exactly at mean sea level? Regardless of temperature, QNH equals QFF, and both equal QFE, because there is no vertical correction to apply. Let's talk about the normal range of mean sea level pressure, which is QFF. It typically extends from 950 to 1050 hPa. The lowest recorded mean sea level pressure is 870 hPa, measured in the eye of Typhoon Tip in the Western Pacific in 1979. The lowest recorded in the North Atlantic is 882 hPa, in the eye of Hurricane Wilma in 2005. The highest mean sea level pressure ever recorded is 1085.7 hPa, observed in winter in Siberia in 2001. Now, let's define the pressure terms precisely. QFE is the pressure measured at the aerodrome reference point. That's the actual atmospheric pressure at that specific location on the airfield. QFF is QFE converted to mean sea level using the actual temperature. This is the pressure you would have at sea level if the real atmosphere extended down from the aerodrome. QNH is QFE converted to mean sea level using the ISA. This is the pressure you would have at sea level if the standard atmosphere extended down from the aerodrome. An isobar is a line joining places of the same atmospheric pressure, usually mean sea level pressure, which is QFF. The standard pressure setting, or SPS, is 1013 hPa. Finally, on analysis charts, the isobars you see are corrected mean sea level pressures — that is, QFF — and they are drawn at a spacing that depends on the scale of the chart. On larger area charts, the spacing may be expanded to 4 or more hectopascals, but that will be stated on the chart itself.