Cheryl Tobias, age 12, of Vancouver, B.C., for her question:
What causes the prevailing westerlies?
These prevailing winds stream eternally across the wide temperate belt of North America. Vancouver and its majestic island stand directly in their path. All year long, they blow and buffet from the Pacific toward the Atlantic, bearing along alternate pockets of balmy and stormy weather.
All the world's prevailing winds are performing a breezy hoedown on a global scale. In each hemisphere, they girdle the globe in three belts meanwhile swooping in and out of upstairs and downstairs wind systems. Our downstairs westerlies are merely one part of all this activity, emerging from one swerve and sweeping on to the next. Before we place them in the planetary picture, let's fix some directions on our fingertips. West winds blow from west to east, north winds blow south from the north. A wind is named from where it comes. When blowing south, a right turn veers it westward to become an east wind. When blowing north, a right turn changes it into a westerly wind.
Our dizzy planet spins eastward, independently of the breezes blowing above its surface. What's more, its rotation dwindles from about 1,000 miles per hour at the equator to almost nothing near the poles. This rotation moves the ground from under the winds blowing above it. It causes winds in our hemisphere to veer right and those south of the equator to veer left. Arctic winds start from the tropics and deflected to become the polar easterlies. As our tropical trade winds blow oward the equator, they are deflected to become northeasterlies. The prevailing westerlies are sandwiched between these two wide easterly winds belts. They are deflected to become west winds because they start blowing from the south.
The global hoedown is kept swirling mainly because of temperature zones, upstairs and downstairs in the weathery shell of the earth's atmosphere. Cool air contracts and becomes heavy. Warm air expands and becomes light. Dense, heavy air blows toward light, thin air. In the Arctic, chilled heavy sir sinks downstairs and starts toward the tropics. Near the equator, warm air expands and rises upstairs.. There it cools off and separates to blow north and south. Near Latitude 30 degrees, the cooled upstairs air is heavy enough to sink downstairs.
These descending currents pile up and start moving. Some rejoin the trade winds and blow back toward the equator. The rest turn toward the Artic and, like all winds in our hemisphere, they are deflected to the right. They start as south winds in our hemisphere, they are deflected to the right. They start as south winds from the south and the spinning earth deflects them away from the west. They become the prevailing westerlies that stream eternally across the wide mid section of North America, on and on in a never ending circle around the globe. Their hoedown partners south of the equator are north winds deflected to the left in a belt of northwest prevailing winds.
The westerly wind belt is about 4,000 miles wide, roughly between latitudes 30 and 60 degrees. Its southern boundary is a narrow zone of descending air with merely a few baffling breezes. It forms a calm belt called the horse latitudes. Along their northern boundary, the brisk westerlies brush and tangle with the brisker polar easterlies, on their way toward the tropics. This global zone is the polar from of weather that hatches many of our storms.