In this video, we focus on the important characteristics of wind include wind speed, wind speed variability, wind shear, surface roughness, turbulence, wind direction, and air density. Let's look at each of these in some detail. Wind speeds are generally reported in meters per second. Intuitively, winds of 1-5 meters per second are mild breezes, 10-15 meters per second are stiff breezes, 20-25 meters per second are strong winds. In wind speeds above 30 meters per second, it would be difficult to stay on your feet. The wind speed ranges shown here in green are wind speeds useful for generating wind energy from wind turbines. This map shows average wind speeds around the globe. Red regions are areas with wind speeds greater than 10 meters per second. Like blue regions have mean wind speeds of around five meters per second. Lavender regions show regions with wind speeds of only about one meter per second. We see that the windiest region on Earth is the South Indian Ocean, where there are no landmasses and so of course there's no population. The least windy regions on Earth are in North, Central, South America, and South Central Africa. Areas with modern wind speeds are in North America, Europe, most of Asia, North Africa, Australia, and Southern South America. These moderate regions are attractive locations for extracting energy from the wind. When speed maps are also available for countries, regions, and locations. This first map is a wind map for India, showing high-speed winds along its Eastern coast. The second exhibit is a local wind map for the Quiberon Peninsula in France. Wind speeds, of course, are not steady but vary by the year, the month, the day, the hour, and even by the minute, as shown in the wind speed plots to the right. Even by the second, is shown in this second wind speed plot. Wind speeds are often summarized using a frequency distribution, as shown in this histogram, with wind speeds on the horizontal axis, and the frequency of occurrence on the vertical axis. For those of you interested in statistics, wind speed distributions that are often approximated using a Weibull distribution. Wind speed distributions are useful for analyzing and comparing wind farm sites, since wind speeds vary greatly from site to site. We'll come back to wind speed distributions when we discuss wind power. Another attribute of wind is that wind speed increases with altitude, or height above the ground. This is called wind shear, which is due to friction between flowing air and the static ground surface. As a rule of thumb or approximation, wind speed increases with height to about the 1/7 power. Starting with a base speed at some height, if we double the height we find about 10 percent more wind speed. If we quadruple the height, wind speeds they're 22 percent greater than the base. At eight times the base height, wind speeds are 35 percent greater. At 12 times the base height, wind speeds are about 43 percent greater than the base speed. As we go higher and higher, we see that increases in wind speed diminish. Above 100 meters, increases in wind speeds with height are small. This observation will be important later, when we discuss wind turbines. Related to wind shear is the concept of surface roughness. Surface roughness is variation in the height of surface obstacles, that impede wind and increase wind shear. Wind shear increases with surface roughness, due to the increased friction caused by rough surfaces. This diagram shows how wind shear is much greater in areas with rough surfaces such as cities and suburbs, than at flat regions such as open country or flat train such as deserts or oceans. The second table compares the roughness measures for various surfaces. For example, open seas have very low surface roughness. Agricultural land areas with high crops have moderate roughness. Forests, suburbs, and cities have the largest roughness measures. Surface roughness is an important consideration when evaluating wind farm sites. Another important characteristic of wind is turbulence. Turbulence is defined as a rapid fluctuations in wind velocity and direction. Turbulence is caused by two factors. Frictional forces such as surface roughness and topographic features such as hills, mountains, forests, and buildings. A second cause of turbulence are thermal gradients, which cause air to swirl and move upward and downwards very rapidly. If you've ever flown on an airplane, you probably have experienced turbulence, which can cause a very bumpy and uncomfortable ride. Turbulence is measured by turbulence intensity, which is the ratio of wind speed variability relative to average wind speed. For those of you interested in statistics, this is calculated as the standard deviation of wind speed, divided by mean wind speed. Another useful characterization of wind speed is a wind rose. A wind rose indicates both the frequency that wind blows from a particular direction, and the frequency of wind speeds from each direction. For example, this wind rose shows that winds blow predominantly from the Southwest, Northwest, and Northeast, and that the speediest winds or strongest winds are from the South, due West, Northwest, and Northeast. This can be very important when designing a wind farm. A final important characteristic of wind, is air density. Since when energy increases with heavier air. First, air density decreases with elevation. For example, Mexico City, at 2,300 meters above sea level, air density is about 25 percent less than at sea level. Air and air density decreases with temperature. For example, air at 30 degrees Celsius has about eight percent less density than at zero degrees. Finally, perhaps, counter-intuitively, humid air is less dense than dry air because water vapor is less dense than dry air. In summary, wind characteristics important for evaluating wind energy include, wind speed, how fast is the wind blowing, wind speed variability, the degree and frequency with which wind speeds vary over time. Wind shear, how quickly wind speeds increase with altitude. Surface roughness, the amount of surface friction that slows wind, turbulence, short-term variability in wind speed and direction, wind direction from whence do winds blow, and air density, which determines the heaviness of air. In the next video, we'll use many of these concepts when we investigate wind power. We'll see you there.
