George Hadley was an English lawyer and amateur meteorologist in the days before anybody thought there could be such a thing as a professional meteorologist. In 1735 he published a detailed description of why the trade winds blow, and whey they blow in the direction they do. He specifically was refuting a theory of Edmond Halley (of comet fame). Halley was wrong, and Hadley was right (mostly). Unfortunately, like many great ideas, Hadley’s was ignored during his lifetime. It was almost a hundred years later that his work was rediscovered and he gradually got the credit he deserved.
What Hadley described was a model for the way air moves around the globe, both South to North, and East to West. Directly under the Sun, the atmosphere heats the most. The hot damp air becomes light, and rises. Warm, damp air results in lower pressure at the surface. Cooler air from the North (in our half of the world!) flows in to replace the rising air which loses most of its water as rain while it rises and cools.
The rising hot air reaches an altitude of several kilometers, and begins to flow North. At this higher altitude, it cools further. Eventually, it becomes cool and dry enough that it begins to sink. Cool, dry air is heavy, and generates high pressure at the surface. This sinking occurs at a latitude about 30 degrees North of the initial rising air. As the Sun moves North and South with the seasons, this band of rising and falling air follows.
Many of the Earth’s large scale weather patterns are explained by this circulation. For example, most of the world’s major deserts are located in the latitudes where the very dry air sinks back to the surface. Since the moisture in the air is a key driving force for weather, and there is very little in these high pressure areas, there is very little wind, or weather. These areas of the ocean were known as the “horse latitudes” because ships becalmed in them had to kill any livestock they were carrying to save food and water. If you talk to anyone sailing between Hawaii and Northern California, one of the considerations for the course to follow is avoiding sailing into the center of the Pacific High where there is little or no wind for days and days.
Here along the coast of the Eastern Pacific, we see seasonal changes due to the movement of what is called the “Hadley Cell”. In the summer, the cool, dry high pressure system is located just north of San Francisco. Wet weather systems hit this immovable block of dense, dry air and are deflected northward, making Seattle’s famous wet weather. In the winter, the high pressure side of the Hadley Cell moves south, following the Sun. The winter storms now have a clear path into Northern California. In the words of the TV weatherman, “The storm door is open.”
Bill Kinney is a core instructor at OCSC Sailing. Bill started sailing on small boats 23 years ago. Upon moving to California in 1998 he bought a Northstar 40 foot center- cockpit ketch to sail and live on, and continues to live the perfect life, sailing regular in the bay and along the California coast.