Historically, variations in pressure have long been associated with changes… — Page 16, Lesson 21

I want to walk you through the beginning of Chapter 2, which is all about pressure. This is a foundational topic in meteorology, and the chapter opens with a very important statement from the Handbook of Aviation Meteorology: "The study of atmospheric pressure may be said to form the foundations of the science of meteorology." So right away, we know this is central to understanding weather. Historically, variations in pressure have long been associated with changes in the weather. The phrase "falling glass" refers to a falling barometer reading, and it usually indicates the approach of bad weather. That's the practical, real-world connection. Let's define atmospheric pressure precisely. Atmospheric pressure is the force per unit area exerted by the atmosphere on any surface in contact with it. Another way to think about it is as the weight of a column of air of unit cross-sectional area above that surface. So if you imagine a vertical column of air with a one-square-metre base, the weight of all the air in that column pressing down is the pressure at the base. From that mental picture, you can see that the pressure at the top of that column will be less than the pressure at the bottom, because there is less air above it. Therefore, atmospheric pressure decreases with an increase in height. That's a fundamental relationship. Now, let's talk about the units we use to measure this. The standard unit of force is the newton, abbreviated N. An average value for atmospheric pressure at sea level is 101,325 newtons per square metre. That unit, the newton per square metre, is also called the pascal. For simplicity in aviation meteorology, we express this as 1013.25 hectopascals, which is written as hPa. The reason for this is that the earlier system of measurement used millibars, abbreviated mb, and one hectopascal equals one millibar. So 1013.25 hPa is exactly the same as 1013.25 mb. In some countries, millibars are still used. There are other units still in use, related to the height of a column of mercury in a barometer. These are expressed in inches or millimetres. For reference, the mean sea level pressure in the International Standard Atmosphere, or ISA, is 29.92 inches of mercury or 760 millimetres of mercury. So you'll encounter all of these units in practice. Let's bring that together. The key points are: pressure is force per unit area, it decreases with height, and we measure it in hectopascals or millibars, with the standard sea-level value being 1013.25 hPa.