Let me walk you through each option so you understand the correct concept — Page 90, Lesson 75

I want to walk you through the start of Chapter 7, which is titled "Adiabatics and Stability." This is a core chapter in your meteorology syllabus, and it builds directly on what we've been discussing about temperature and moisture. First, let's clarify the definition that appears at the very top of the page, because it sets the scene for the measurements we'll be using. The "temperature reached by the surface of the earth" is defined as the temperature measured by a thermometer placed 1.2 metres above the ground. That's the standard height for a surface air temperature measurement in meteorology — not at the ground itself, but at that specific height. Now, before we dive into the chapter proper, there's a practice question here about atmospheric humidity. Let me walk you through each option so you understand the correct concept. Option b states: "The absolute humidity is the mass of water vapour contained in unit volume of air." That is the correct definition. Absolute humidity is a measure of the actual water vapour content, expressed as a mass per unit volume — typically grams per cubic metre. It tells you how much water vapour is physically present in a given sample of air. Option a says: "If the air temperature falls then the absolute humidity must increase." That is incorrect. Absolute humidity depends on the actual water vapour content, not directly on temperature. If temperature falls but no water vapour is added or removed, the absolute humidity stays the same. Option c says: "The diurnal variation of dew point temperature is greatest when skies are clear at night." This is actually a statement about dew point behaviour, but it's not the correct answer here. The dew point temperature does vary, but the greatest diurnal variation of dew point typically occurs under clear skies during the day, not specifically at night. Option d says: "The dew point temperature is the temperature indicated by the wet bulb thermometer." That is incorrect. The wet bulb thermometer indicates the wet bulb temperature, which is different from the dew point. The dew point is the temperature to which air must be cooled at constant pressure for it to become saturated, and it is measured by a different method. So, the correct statement is b: absolute humidity is the mass of water vapour per unit volume of air. Now, let's move into the chapter itself. The title is "Adiabatics and Stability." The chapter outline shows we will cover several key topics in order. First, Adiabatic Temperature Changes — that's the idea that a parcel of air can change temperature without exchanging heat with its surroundings, purely due to expansion or compression as it rises or sinks. Then we have The Dry Adiabatic Lapse Rate (DALR) , which is the rate at which an unsaturated parcel of air cools as it rises. Next is The Saturated Adiabatic Lapse Rate (SALR) , which applies once the air becomes saturated and condensation releases latent heat. There's a subsection on the Variation of the SALR with Temperature, because the amount of latent heat released changes with temperature. Then we have The Environmental Lapse Rate (ELR) , which is the actual temperature profile of the surrounding atmosphere at a given time and place. After that, we get into Stability — the concept of whether a parcel of air, if displaced vertically, will tend to return to its original position, continue moving away, or just stay where it is. The chapter breaks stability into four categories: Absolute Instability, Absolute Stability, Conditional Instability, and Neutral Stability. There's a Stability Summary section, followed by Examples, then Questions and Answers. This is the foundation for understanding cloud formation, turbulence, and vertical motion in the atmosphere — all critical for your professional pilot knowledge. Let's begin with the first topic: adiabatic temperature changes.