Pilot Weather Wisdom

Airline schedules will be redirecting their flights southward as the days get shorter. You may be planning an adventuresome Caribbean cross-country flight, flying a biz jet or airliner to a sunny destination, or heading there on vacay.

Flying within the Horse latitudes

At around 30° north and south of the equator semi-permanent high pressure systems form due to subsidence —sinking air. Diverging air at the surface causes very light winds. That’s why pleasant weather predominates in the Caribbean and latitudes like Hawaii. These “highs” are named after the areas in which they form. The Bermuda High (western Atlantic), the Azores High (eastern Atlantic) and the Pacific High are three such semi-permanent high pressure features. This high pressure belt (anticyclone) hosts most of the world’s deserts ­— Mojave (California), Sonoran (southwest U.S and northwest Mexico), Sahara (North Africa), Arabian (Arabian peninsula — Middle East), Gobi (Mongolia), etc. When the Bermuda High moves inland, its arrival is frequently associated with heat waves in the eastern United States. The Pacific High has the same effect on the West Coast. Generally, these high-pressure areas are known as subtropical high pressure systems. (“Subtropical” means bordering on the tropics, or more specifically, the area between the tropical zone and the temperate zone — about 25° to 40° north and south latitude).

Further south of these Highs, the subsiding air will return to the equator along the surface. The surface winds returning to the equator do not flow straight back, but are forced to the right due to the Coriolis effect north of the equator. These winds, which flow predominantly from the northeast, are known as the northeast trade winds or simply the trade winds (or even simpler, the “trades”). The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, but you may see them grouped in some reference materials as the tropical easterlies.

Because of these northeast trade winds, most if not all of your landings and take-offs will be in an easterly direction. Pilots in northern latitudes know a northeast wind is a “weather” wind but in the Caribbean, it is a sunny warm wind great for cervezas on the beach.

Why “Horse Latitudes?” The term “horse latitudes” came about during the age of sailing. When sailing ships got caught in calm winds within this zone, the ships couldn’t move, with these conditions lasting for days. The supply of water and food dwindled. Many ships carried horses, and what cargo do you think was sent overboard (or eaten) to lessen the demand for water and to lighten the load?