Читать книгу Wind Power Basics - Dan Chiras - Страница 26

Like coastal winds, mountain-valley breezes arise from the differential heating of the Earth’s surface. To understand how these winds are formed, let’s begin in the morning.

As the Sun rises on clear days, sunrays strike the valley floor and begin heating the ground, valley walls and mountains. As the ground and valley walls begin to warm, the air above them warms. It then expands and begins to flow upward. This process is known as convection. (Convection is the transfer of heat in a fluid or a gas that is caused by the movement of the heated air or fluid itself.) While some of this warm air rises vertically, mountain valleys also tend to channel the solar-heated air through the valley toward the mountains (Figure 2.2). As the warmed air moves up a valley, cooler air from surrounding areas flows in to replace it. This wind is known as a valley breeze.

Throughout the morning and well into the afternoon, breezes flow up-valley — from the valley floor into the mountains. These breezes tend to reach a crescendo in the afternoon. When the Sun sets, however, the winds reverse direction, flowing down valley.

Winds flow in reverse at night because the mountains cool more quickly than the valley floor. Cool, dense air (high-pressure air) from the mountains sinks and flows down through the valleys like the water in a mountain stream, creating steady and often predictable down-valley or mountain breezes.

Fig. 2.2a and 2.2b: Mountain-Valley Breezes. Mountain-valley winds can provide a reliable source of wind power if conditions are just right. (a) Up-valley winds. (b) Down-valley winds.

Together, valley and mountain winds are known as mountain-valley breezes. As a rule, mountain breezes (down-flowing winds) tend to be stronger than daytime valley breezes.

Mountain-valley breezes typically occur in the summer, a time when solar radiation is greatest. They also typically occur on calm days when the prevailing winds (larger regional winds, which will be discussed shortly) are weak or nonexistent.

Mountain-valley winds also form in the presence of prevailing winds — for example, when a storm moves through an area. In such instances, mountain or valley winds may “piggy back” on the prevailing winds, creating even more powerful (and hence higher energy) winds. When consistently flowing in the same direction, such winds can provide a great deal of power that can be tapped to produce an abundance of electricity.