Twenty-three years later to the day, another "gale of November," following a similar path to the storm that sunk the Edmund Fitzgerald, generated hurricane-force winds across parts of the Great Lakes (see Figure 4.1). At night, a buoy on Lake Michigan measured giant waves over 6 m (20 ft) high. Meanwhile, barometers nose-dived to 28.43 inches of mercury in Minnesota, setting a new all-time state record for lowest air pressure.

If you have a barometer in your home, you probably are familiar with "inches of mercury" as a common unit of air pressure. If so, you may already know that a decreasing air pressure often heralds stormy weather, while rising air pressure is often the precursor of "fair weather" (as a side note, though you might think that "fair" is a word that meteorologists use to hedge their predictions, "fair" generically describes a weather pattern characterized by a lack of precipitation, a sky whose coverage by low clouds is less than four-tenths, and the absence of any other extreme conditions of cloudiness, visibility or wind).

With guidelines of "stormy" and "fair" on your home barometer in hand, you are well on your way to becoming an apprentice meteorologist because no principle is more fundamental to weather forecasting than the relationship between changes in air pressure over time and changes in the weather. In other words, it is crucial that forecasters know the rate at which air pressure increases or decreases (sometimes expressed as "barometers rising or falling," with "rapidly" or "slowly" tacked on for good measure).

Small variations of air pressure over horizontal distances are also crucial to weather forecasting, as we will soon learn. But first things first - we need a working model for pressure that will allow us to more easily understand the changes in air pressure, both in time and space, that play such a crucial role in weather.