Because the water cools more slowly, air above it is warmer. Warm air expands and rises. Cooler high pressure air flows from the land to the water at night (Figure 2.1b). The result is an offshore breeze: steady winds that flow from land to water.
Fig. 2.1a and 2.1b: Onshore and offshore breezes. Onshore (a) and offshore (b) breezes occur along the coastlines of major lakes and oceans.
Offshore and onshore breezes operate day in and day out on sunny days, providing a steady supply of wind energy. Because offshore and onshore winds are fairly reliable, coastal regions of the world are often ideal locations for small (and large) wind turbines.
Coastal winds are more consistent than winds over the interior of continents and also tend to be more powerful because of the relatively smooth and unobstructed surface of open waters. That is to say, wind moves rapidly over water because lakes and coastal waters provide very little resistance to its flow, unlike forests or cities and suburbs, which dramatically lower surface wind speeds.
Mountain-Valley Breezes
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.
Large-Scale Wind Currents
Local winds can be a valuable source of energy. The winds on which most people rely, however, are those produced by much larger air masses that result from regional and global air circulation. They create dominant wind-flow patterns, known as prevailing winds.
Prevailing winds, like local winds, are created by the differential heating of the Earth’s surface, but on a much larger scale. Here’s how they are formed: As shown in Figure 2.3, the Earth is divided into three climatic zones: the tropics, temperate zones and poles. Because the tropics are more directly aligned with the Sun throughout the year, they receive more sunlight and are, therefore, the warmest regions on Earth.
Fig. 2.3: Climate Zones. The Earth is divided into three climate zones in each hemisphere. In the Northern Hemisphere, warm air from the tropics flows northward by convection, creating the global circulation pattern.
The temperate zones lie outside the tropics, in both the Northern and Southern Hemispheres.They receive less sunlight than the tropics and so are cooler. The North and South poles receive the least amount of sunlight and are the coolest regions of our planet.
As shown in Figure 2.4a, global air circulation is created by hot air produced in the tropics. This air expands and rises (as in local air circulation patterns). Cool air from the northern regions — as far north as the poles — moves in to fill the void. The result is huge air currents that flow from the poles to the equator.
Although air generally flows from the North and South poles toward the equator, circulation patterns are a bit more complicated. In the Northern Hemisphere, some of the warm air moving northward cools and sinks back to the Earth’s surface, as shown in Figure 2.4b. It then flows back toward the equator creating the trade winds. Because the trade winds blow quite consistently, they are a potentially huge and reliable source of energy for residents and nations fortunate enough to lie in the winds’ path.
As shown in Figure 2.4b, a substantial amount of the northward-moving air continues on toward the North Pole, traveling over the temperate zone.
The figure also shows air masses flowing northward across the temperate zone split into two, creating higher- and lower-level winds. When the upper winds reach the North Pole, this cold air sinks and then flows southward back toward the equator.
Fig. 2.4: Global Air Circulation. (a) As shown here, warm tropical air rises and flows toward the poles. Cold polar air flows toward the equator. (b) Warm tropical air loses some of its heat and sinks toward the Earth’s surface, then flows back toward the equator, creating the trade winds. Air masses moving over the temperate zone split into upper and lower winds. (c) Wind patterns caused by the Coriolis effect, resulting from the rotation of the Earth on its axis.
If no other forces were at work, winds flowing back to the equator would flow from north to south. As shown in Figure 2.4c, they don’t. Other factors influence the movement of air masses across the surface of the planet. One of the most significant is the Earth’s rotation, which results in a phenomenon known as the Coriolis effect.
The Coriolis Effect
To understand why prevailing winds deviate from the expected patterns based solely on convection, let’s start with the trade winds. As shown in Figure 2.4c, the trade winds in the Northern Hemisphere flow