We have a cold pool — Page 608, Lesson 585

I want to walk you through a specific type of weather phenomenon that has a very distinct characteristic: it disappears at night. This is a fascinating and operationally important feature for us as pilots. Let's start with the first item. We have a cold pool. A cold pool is a mass of cold air that becomes isolated from its parent air mass. In this context, it is most evident behind polar frontal weather in mid-temperate areas. So, picture a typical polar front depression moving through the mid-latitudes. Behind the cold front, you often get a pool of cold air that breaks away or becomes cut off. The key point here is that this cold pool is often most evident in that location, but here's the catch for us as professional pilots: it may show little or no sign on significant weather charts. That means the standard weather charts we use for flight planning might not clearly depict this cold pool. It's a feature we need to be aware of conceptually, even if it isn't always drawn out for us on the chart. Now, the defining characteristic of this entire section is that this is an area of weather which disappears at night. So, a cold pool that is present during the day will dissipate or vanish after sunset. This is critical because if you see indications of it during your daytime pre-flight briefing, you need to know that its influence will be gone by the time you're flying at night. The second item is: Air trapped on the leeward side of mountain ranges. This describes a different scenario. When wind flows over a mountain range, the air on the downwind side—the leeward side—can become trapped. It's essentially stagnant or recirculating air that doesn't mix with the surrounding flow. This trapped air can form its own local weather, like low clouds or fog, and it also shares that same key property: it is an area of weather which disappears at night. So, to tie it all together, we have two distinct phenomena—a cold pool behind polar frontal weather and trapped air on the leeward side of mountains—that both fall under the category of weather that is present during the day but vanishes after dark. This is a very practical piece of knowledge for your flight planning: if you're operating at night, you can generally expect these features to be absent, but during the day, you need to be alert for them, especially since the cold pool might not even show up on your significant weather chart.