The Atmosphere — Page 32, Lesson 34

Let’s start with the big picture. The atmosphere is the air we fly through, and for an ATPL pilot, three of its properties matter more than anything else: static pressure, temperature, and air density. These three drive how the aircraft performs, how the instruments read, and how the engine breathes. So I want to walk you through each one, then finish with the International Standard Atmosphere, which is the yardstick we use to compare everything. First, static pressure. Static pressure is the pressure of the air at rest, the ambient pressure around you, not the dynamic pressure from the aircraft’s motion. At a particular altitude, static pressure varies from day to day. At sea level, it’s about 1000 hPa. Now, hPa stands for hectopascal, and that’s the unit we use in Europe. But some countries measure static pressure in inches of mercury, abbreviated inHg. At sea level, that’s about 30 inHg. So remember the two sea-level figures: roughly 1000 hPa, or roughly 30 inHg. They’re just two different rulers for the same thing. Next, temperature. The unit for temperature is °C, or K. °C is degrees Celsius, sometimes called centigrade, and it’s measured relative to the freezing point of water. K is Kelvin, measured relative to absolute zero. And the conversion you need to hold onto: 0°C is equivalent to 273 K. So absolute zero, 0 K, is minus 273 degrees Celsius. Now here’s the key behaviour for flying: temperature decreases with increasing altitude up to about 36,000 feet, and then it remains constant. So from the surface up to roughly 36,000 feet, it gets colder as you climb. Above that, the temperature stops falling and just stays put. That’s a critical fact for performance, because temperature affects density, which we’re about to look at. Now air density. The unit for density is kg/m³, kilograms per cubic metre, and the symbol is the Greek letter rho, ρ. Density is defined as mass per unit volume. Think of it as the number of air particles packed into a given space. The more particles in that space, the denser the air. Density varies with three things: static pressure, temperature, and humidity. And the relationships are very specific. Density decreases if static pressure decreases. Density decreases if temperature increases. And density decreases if humidity increases. So higher pressure packs more particles in, higher temperature spreads them out, and more humidity means lighter water vapour molecules replace heavier air molecules, so the air gets less dense. There’s a neat way to summarise all this. Air density is proportional to pressure and inversely proportional to temperature. That’s the ideal gas law. The formula is ρ divided by P over T equals a constant. More usefully, we write ρ ∝ P over T. Let me read that carefully: rho is proportional to P divided by T, where p is pressure, T is temperature, and ρ is density. So if pressure goes up, density goes up. If temperature goes up, density goes down. That’s the whole relationship in one line. Now, how does density behave as you climb? Density decreases with increasing altitude because of decreasing static pressure. But here’s the subtlety: with increasing altitude, temperature also decreases, and lower temperature would tend to increase density. So you have two competing effects. The decreasing pressure pulls density down, the decreasing temperature pulls it up. And the winner is pressure. The effect of decreasing static pressure is dominant. So net result, density decreases with altitude. That’s why the air gets thinner the higher you go. Finally, the International Standard Atmosphere, ISA. The values of temperature, pressure, and density are never constant in any given layer of the atmosphere. They change day to day, hour to hour. So to enable accurate comparison of aircraft performance, and to calibrate pressure instruments, a standard atmosphere has been adopted. The standard atmosphere represents the mean, or average, properties of the atmosphere. It’s a fixed reference so that when we say an aircraft performs a certain way, we all mean the same conditions. Europe uses the standard atmosphere defined by the International Civil Aviation Organization, ICAO. And the ICAO standard atmosphere assumes these mean sea level values. Temperature: 15°C. Pressure: 1013.25 hPa. Density: 1.225 kg/m³. Hold onto those three numbers, because they’re the baseline for everything in performance and instrument calibration. 15 degrees Celsius, 1013.25 hectopascals, and 1.225 kilograms per cubic metre. So to tie it together: static pressure, temperature, and density are the three pillars. Pressure and temperature combine through the ideal gas law to give density, and density is what really drives lift and engine performance. And ISA gives us the fixed baseline to compare it all against.