Humidity — Page 82, Lesson 67

I want to walk you through the topic of humidity, starting with a fundamental concept that underpins all the state changes we’ll discuss: latent heat. The latent heat of a substance is the heat that is absorbed or released without any change in temperature when that substance changes state. The key point is that the temperature stays the same during the change, but energy is either taken in or given out. Latent heat differs depending on what state the substance is moving between. When ice changes to water, or ice changes directly to water vapour, or water changes to water vapour, latent heat is absorbed — the substance takes heat in from its surroundings. Conversely, when water vapour changes to water, or water vapour changes to ice, or water changes to ice, latent heat is released — it gives heat out to the surroundings. Let’s look at each change of state in turn, starting with evaporation. Evaporation is the change of state from liquid to vapour. During evaporation, latent heat is absorbed. A critical point here is that evaporation can occur at any temperature — it doesn’t require boiling. For any given temperature, there is a particular amount of water per unit volume that the air can hold. When that maximum is reached, evaporation will cease. This brings us to saturation. Air becomes saturated by adding more water vapour to it. Alternatively, because warm air can hold more water vapour than cold air, saturation can also be achieved by cooling the air. Air is defined as saturated if it contains the maximum amount of water vapour that it can hold at that temperature. If saturated air is cooled further, condensation will occur. Condensation is the change of state from vapour to liquid. Here, latent heat is released. Condensation is what causes cloud and fog to form. For condensation to happen, it requires minute impurities or particles called hygroscopic or condensation nuclei. These are usually present in abundance in the troposphere, so condensation generally has plenty of particles to form on. Next, freezing. If a water droplet is cooled below zero degrees Celsius, it may change state again to ice. That process is called freezing. Freezing requires the presence of freezing nuclei, which are less common in the troposphere than condensation nuclei. Because of this scarcity, it is possible to have water droplets in the atmosphere with temperatures below 0°C. These are known as supercooled water droplets, and they give us the icing hazard that is discussed in Chapter 16. The opposite change of state, from solid to liquid, is called melting. There is no "superfrozen" state analogous to supercooled. Then we have sublimation. Sublimation is the change of state directly from water vapour to ice without water droplets being formed. Latent heat is released during this process. This process is also known as deposition. The change of state from ice directly to water vapour is also called sublimation — so the term covers both directions: vapour to ice and ice to vapour. Now let’s move into how we measure humidity. There are several key terms. Absolute Humidity is the weight of water vapour in a unit volume of air. It is usually expressed in grams per cubic metre, or g/m³. Humidity Mixing Ratio, abbreviated HMR, is the weight of water vapour contained in a unit mass of dry air. It is usually expressed in grams per kilogram, or g/kg. An important behaviour: in unsaturated air, the HMR remains constant during ascent, even while temperature and pressure decrease. Saturation Mixing Ratio, or SMR, is the maximum amount of water vapour a unit mass of dry air can hold at a specified temperature. Finally, Relative Humidity, abbreviated RH. Relative Humidity is defined as the ratio of HMR to SMR, multiplied by 100%. More simply, it is the amount of water vapour present in a volume of air divided by the maximum amount of water vapour that volume could hold at that temperature, expressed as a percentage. When RH equals 100%, the air is saturated.