Let me start with the chapter structure — Page 399, Lesson 371

I want to welcome you to Chapter 21 of your meteorology syllabus — Local Winds and Weather. This chapter is a practical tour of specific, named winds that affect aviation around the world, and the weather patterns that ride along with them. The last chapter gave you the general theory of climatology; now we apply that theory to real, named winds you will encounter as a professional pilot. Let me start with the chapter structure. The winds are organised into five sections: Föhn-type winds, Valley winds, some Mediterranean winds, Storm squalls, and a West African wind. That gives you the roadmap. We begin with Föhn winds. Now, Föhn winds were actually introduced back in Chapter 10, and the diagram explaining the temperature increase on the lee side of a mountain range is shown here as Figure 21.1. The key point is that although the name "Föhn" originally refers to winds that blow in the Alps, the term is used generically — that is, as a general category — to describe winds that blow with a similar effect in other parts of the world. The first specific example of a Föhn-type wind is the Chinook. The Chinook blows on the eastern side of the Rocky Mountains of North America. If you look at Figure 21.2, it shows you the location of the Chinook. Now, let me tell you about its behaviour, because this is important for flight planning. The Chinook usually blows during the winter months. Its defining characteristic is that it produces a rapid and considerable rise in temperature. To give you a sense of scale: a rise of 20° in 15 minutes is not unusual. That is a dramatic, sudden warming. The wind may blow for several days, and when it does, snow on the eastern side of the Rockies may clear completely. The area covered by the Chinook runs from southern Colorado all the way up to the Mackenzie Basin — that is a very large geographical footprint. So, to summarise what I want you to take away: a Föhn wind is a generic category of warm, dry wind on the lee side of a mountain range. The Chinook is a specific winter example on the eastern Rockies, famous for its rapid temperature rise of up to 20° in just 15 minutes, and its ability to clear snow over a vast area from Colorado to the Mackenzie Basin. That is your foundation for the rest of the chapter.