Humidity — Page 82, Lesson 69

I want to walk you through the Bergeron Theory, which is a key concept in understanding how precipitation forms in cold clouds. This theory is more accurately called the Wegener-Bergeron-Findeissen theory, named after the three scientists who discovered the relationship. Let me start with the core idea. We already know that as temperature decreases, the maximum amount of water vapour the air can hold decreases, and therefore the partial water vapour pressure at saturation also decreases. But here's where it gets interesting — there's a difference between saturation over a surface of liquid water and saturation over a surface of ice. Look at Figure 6.3, which shows the partial pressure of water vapour at saturation for temperatures from -30°C to +40°C. The small sub-diagram within it reveals something crucial: at temperatures below 0°C, the partial pressure at saturation for the formation of water is greater than the partial pressure for the formation of ice. Let me say that again in simpler terms — at the same sub-zero temperature, air can hold more water vapour before it becomes saturated over a liquid water surface than it can before it becomes saturated over an ice surface. This means that the air becomes saturated for the formation of ice before it becomes saturated for the formation of water. In other words, at temperatures below zero, water vapour will go directly to the solid state — that's ice — without first going through the liquid state. This process is called sublimation. The converse also applies: ice can go directly to vapour without melting first. The technical way to state this is: "the saturation vapour pressure over water is greater than over ice." Now, the table on this page shows the same effect expressed in terms of relative humidity for water and for ice. Let me walk through these numbers. At 0°C, the relative humidity for water is 100% and for ice it's also 100% — they're equal at freezing point. But as we go colder, they diverge. At -5°C, when the air is saturated for the formation of ice — meaning the relative humidity for ice is 100% — the relative humidity for water is only 95%. At -10°C, it's 91%. At -15°C, it's 87%. And at -20°C, it's 83%. So what does this actually mean in the real atmosphere? Here's the practical effect. When supercooled water droplets exist — and remember, supercooled means liquid water at temperatures below 0°C — the air around them is not saturated for water at that temperature. Those supercooled water droplets will evaporate, adding water vapour to the air. That water vapour then saturates the air for the formation of ice, and the water vapour now sublimes out as ice crystals. So the liquid droplets evaporate, and the vapour deposits directly as ice. This effect is critically important in two main areas: the formation of precipitation in clouds when the temperature is below 0°C, and the formation of fog. In cold clouds, this Bergeron process is the primary mechanism that allows ice crystals to grow at the expense of surrounding supercooled water droplets, eventually becoming large enough to fall as precipitation.