The saying goes “When flying from high to low, watch out below” and then there is a similar one for “When flying from hot to cold, watch out below”. The first one is pretty intuitive, but I was having trouble grasping the second one. The reason is simple, pressure altitude is defined as indicated altitude corrected for atmospheric pressure. Nothing about temperature, right? So why would temperature change pressure altitude? Density altitude is a different story. Yes, temperature is a fundamental to calculating density altitude, ok, but pressure altitude? This has circled in my mind for a bit. Thanks for Emilio for giving me the hint to find a solution to this “problem” (at least it works for me :)). Here it goes…
First things first! Temperature does not change pressure altitude directly, it does indirectly though. That explains everything, right? Well, to understand that we have to think about how the altitude instrument functions. It measures air pressure and translates it into altitude. Air pressure can be visualized as weight of the air. At sea level, the air column above you has a specific weight. As you climb you have less air above you, hence less weight and less pressure. The altitude instrument uses these changes to show differences in altitude. Now atmospheric pressure changes with high and low pressure systems also. You can adjust for atmospheric pressure changes in your altitude indicator. Let’s go a step back and think about what is shown in the altitude instrument. It shows altitude, right? And it measures pressure. So how does it calculate the altitude from that? Well, we know that the air pressure decreases with altitude in a certain scale. This scale is based on the International Standard Atmosphere. This is the key! Altitude is not calculated directly, but is translated from pressure using the International Standard Atmosphere. Flying from hot to cold is an experiment where you are changing temperature while leaving all other variables unchanged. The standard atmosphere is a theoretical combination of temperature, pressure, humidity, etc. a single value. If you are lucky, the hot to cold experiment goes through the standard at a certain point. More likely this is not going to happen. See it? The scale is based on a standard day, and the standard day doesn’t really exist. It’s only close! So changes in temperature by definition cannot be standard. Maybe at the beginning its standard, in the end or in the middle, but it can’t be standard all the time. Being much higher or lower than “standard” will make these deviations even more pronounced.
Get it? Temperature changes pressure altitude, because the scale that translates altitude from pressure is based on a standard that doesn’t exist in reality (or rarely). Say the conditions actually reflect exactly the standard atmosphere. You are flying from hot (15 degrees or less) to colder. You changed you standard atmosphere right there. Here in Guatemala, I would not wait for a standard date. 15 degrees Celcius in San Jose? Right!
I can now sleep again :). Please leave a comment!