The Atmosphere — Page 10, Lesson 13

I want to walk you through the significance of the tropopause height, the temperature patterns in the troposphere and lower stratosphere, the atmospheric hazards that become important at higher altitudes, and then the International Standard Atmosphere — the common datum we use in aviation. Let's start with the tropopause. The tropopause is the boundary between the troposphere below and the stratosphere above. Its height is significant because it usually marks several important things for us as pilots. First, it marks the maximum height of significant cloud — most of the weather-producing cloud lives below the tropopause. Second, it marks the presence of jet streams — those narrow, high-speed wind currents. Third, it marks the presence of Clear Air Turbulence, which is now referred to in aviation as TURB. Fourth, it marks the maximum wind speed you'll generally encounter. And fifth, it marks the upper limit of most of the weather — the bulk of meteorological activity happens in the troposphere, below the tropopause. Now, temperatures. In the troposphere, temperature increases from the poles toward the Equator — so the Equator is warmer, the poles are colder. In the lower stratosphere, the pattern is different. In summer, temperature increases from the Equator toward the poles. But in winter, the maximum temperature in the lower stratosphere is reached in mid-latitudes, not at the poles. Let's talk about atmospheric hazards that become important as we operate at higher altitudes. Two main ones are ozone and cosmic radiation. Above 50,000 feet, normal concentrations of ozone exceed tolerable limits for humans. That means the air needs to be filtered before it enters the cabin. The heat from the compressor system in the aircraft's air conditioning helps break down the ozone to an acceptable level — so the compressor heat actually assists in that process. Cosmic radiation is not normally hazardous, but at times of solar flare activity, a lower flight level may be necessary to reduce exposure. Advances in meteorological forecasting and communications should result in pilots receiving prompt and accurate information about these high-altitude hazards. But it's important that you as a pilot are aware of these hazards and prepared to take the necessary re-planning action. Now, the International Standard Atmosphere — ISA. Because temperature and pressure vary with time and position, both horizontally and vertically, we need a standard set of conditions in aviation. This gives us a common datum for two main purposes: the calibration of aircraft pressure instruments, and the design and testing of aircraft. The standard atmosphere used in aviation is the ICAO International Standard Atmosphere — ICAO stands for the International Civil Aviation Organization. ISA defines an 'average' atmosphere from -5 kilometres, which is -16,400 feet, up to 80 kilometres, which is 262,464 feet. For practical purposes, we just need to look at the ISA between mean sea level and 20 kilometres. Here are the specific values that define the ICAO International Standard Atmosphere. At mean sea level, the temperature is +15 degrees Celsius. The pressure is 1013.25 hectopascals — hPa. The density is 1225 grams per cubic metre. The lapse rate — that's the rate at which temperature decreases with height — is 0.65 degrees Celsius per 100 metres, which is 1.98 degrees Celsius per 1000 feet, and this applies up to 11 kilometres, which is 36,090 feet. Above that, from 11 kilometres up to 20 kilometres, which is 65,617 feet, the temperature is constant at -56.5 degrees Celsius. Then from 20 kilometres up to 32 kilometres, which is 104,987 feet, temperature increases at a rate of 0.1 degrees Celsius per 100 metres, which is 0.3 degrees Celsius per 1000 feet. One practical note: for calculations up to the tropopause, we commonly use a lapse rate of 2 degrees per 1000 feet — that's a rounded figure that makes mental arithmetic easier.