We are starting Chapter 6: Humidity — Page 72, Lesson 66

We are starting Chapter 6: Humidity. This is a fresh topic, so let me walk you through what this chapter covers. First, I want to define a key concept you'll need throughout: latent heat. Latent heat is the heat energy absorbed or released when water changes state — from solid to liquid, liquid to gas, or the reverse — without changing its temperature. This is fundamental to understanding how moisture behaves in the atmosphere. We then move to evaporation. Evaporation is the process by which liquid water turns into water vapour. It requires heat energy — that's latent heat being absorbed from the surroundings, which is why evaporation has a cooling effect. Next comes saturation. Saturation is the state where the air holds the maximum possible amount of water vapour at a given temperature. Once the air is saturated, it cannot take on any more vapour unless the temperature rises or more vapour is removed. Condensation is the opposite of evaporation. It's when water vapour turns back into liquid water. This process releases latent heat into the surroundings, which warms the air. Condensation is what forms clouds and fog. Freezing is the change from liquid water to solid ice. When water freezes, it also releases latent heat. Melting is the reverse — ice turning back into liquid water — and it absorbs latent heat, which has a cooling effect. Sublimation is a direct change from solid ice to water vapour, skipping the liquid phase. This also absorbs latent heat. The reverse process, where vapour goes directly to ice, is called deposition, though the chapter lists sublimation as the key term here. After these definitions, the chapter covers humidity measurement. This includes instruments and methods for quantifying the amount of water vapour in the air. Then we have the Bergeron Theory. This is a key concept in cloud physics — it explains how ice crystals and supercooled water droplets interact to produce precipitation, especially in colder clouds. Next is the measurement of humidity in more detail, starting with the dry-bulb and wet-bulb hygrometer, also called a psychrometer. This instrument uses two thermometers: one measures the air temperature (dry-bulb), and the other has its bulb kept moist (wet-bulb). The difference between the two readings tells you how much moisture is in the air. We also cover dew point temperature. The dew point is the temperature to which air must be cooled, at constant pressure, to become saturated. When the air temperature drops to the dew point, condensation begins — that's when dew or fog forms. Finally, the chapter discusses the diurnal variation of humidity — how humidity changes over the course of a day, typically with higher values at night and lower values in the afternoon when temperatures are highest. The chapter ends with questions and answers for practice. So, in summary: we start with latent heat and the six phase changes of water — evaporation, saturation, condensation, freezing, melting, and sublimation. Then we move into how we measure humidity, the Bergeron theory for precipitation, the psychrometer, dew point, and daily humidity patterns. That's the roadmap for Chapter 6.