Phy 541

• About 1–2% of the energy coming from the sun is converted into wind energy
• Wind


  A natural movement of air of any velocity. Wind is caused by the uneven heating of the Earth by the sun and the Earth's own rotation. Winds range from light breezes to natural hazards such as hurricanes and tornadoes.
• Differences in atmospheric pressure generate winds
• How winds are generated

  1. Warm equatorial air rises higher into the atmosphere and migrates toward the poles (low-pressure system)

  2. Cooler, denser air moves over Earth's surface toward the Equator to replace the heated air (high-pressure system)

  3. Winds generally blow from high-pressure areas to low-pressure areas
• Front


  The boundary between high-pressure and low-pressure areas
• Prevailing winds


  Winds that blow from a single direction over a specific area of the Earth
• Convergence zones


  Areas where prevailing winds meet
• Coriolis effect


  The effect that makes wind systems twist counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere
• Geostrophic winds


  Winds that travel along the edges of the high-pressure and low-pressure systems
• When you stand with your back to the wind in the Northern Hemisphere, low pressure is always to your left. In the Southern Hemisphere, low-pressure systems will be on your right.
• Major wind zones


  • Polar easterlies
  • Westerlies
  • Horse latitudes
  • Trade winds
  • Doldrums
• Polar easterlies


  • Dry, cold prevailing winds that blow from the east
  • They emanate from the polar highs, areas of high pressure around the North and South Poles
  • They flow to low-pressure areas in sub-polar regions
• Westerlies


  • Prevailing winds that blow from the west at midlatitudes
  • They are fed by polar easterlies and winds from the high-pressure horse latitudes
  • They are strongest in the winter, when pressure over the pole is low, and weakest in summer, when the polar high creates stronger polar easterlies
  • The strongest westerlies blow through the "Roaring Forties" in the Southern Hemisphere
• The westerlies of the Roaring Forties were very important to sailors during the Age of Exploration
• The westerlies drive the powerful Antarctic Circumpolar Current, the largest ocean current in the world
• Horse latitudes


  • A narrow zone of warm, dry climates between westerlies and the trade winds
  • They are about 30 and 35 degrees north and south
  • Many deserts are part of the horse latitudes
  • The prevailing winds are usually light and short in duration
• Trade winds


  • Powerful prevailing winds that blow from the east across the tropics
  • They have been instrumental in the history of exploration, communication, and trade
  • Continental trade winds are warmer and drier than maritime trade winds
  • Most tropical storms develop as trade winds
• Doldrums


  • The area around the intertropical convergence zone (ITCZ), where the trade winds of the two hemispheres meet
  • Prevailing winds are very weak, and the weather is unusually calm
• Wind shear


  • A difference in wind speed and direction over a set distance in the atmosphere
  • It is measured both horizontally and vertically
• Beaufort scale


  A scale for measuring the amount of force that wind is generating, with 17 levels from 0 (calm) to 12 (hurricane)
• Anemometer


  A device for measuring wind speed
• Wind carries heat, moisture, pollutants, and pollen to new areas
• Wind interacts with a mountain range

  Rain shadows are created as wind brings moisture which condenses as precipitation before coming over the crest of the mountain, leaving dry "downslope winds" on the other side
• The Föhn wind is a warm, dry downslope wind that develops in central Europe
• Coriolis force


  The apparent bending force that causes any movement on the Northern hemisphere to be diverted to the right, and in the Southern hemisphere to the left
• In the Northern hemisphere, the wind tends to rotate counterclockwise as it approaches a low pressure area. In the Southern hemisphere, the wind rotates clockwise around low pressure areas.
• Prevailing global wind directions


  • Important when siting wind turbines, to place them in areas with the least obstacles from the prevailing wind directions
• Effects influencing wind flow near the Earth's surface


  • Sea breezes (land masses heated more quickly than sea in daytime)
  • Mountain breezes (air ascending slopes during the day, descending at night)
  • Canyon winds (air moving up or down sloped valley floors)
• Wind rose


  A diagram that shows the distribution of wind speeds and the frequency of varying wind directions based on meteorological observations
• Wind energy is one of the oldest sources of energy used by mankind, comparable only to the use of animal force and biomass
• Ancient cultures, dating back several thousand years, took advantage of wind energy to propel their sailing vessels
• These early wind energy converters were essential for pumping water and grinding cereals
• In Europe, wind wheels were introduced around 1200 BC probably as an after-effect of the crusade to the orient
• These European windmills were mainly used for grinding, except in the Netherlands where wind wheels supplied the power to pump river water to the land located below sea level
• Between 1700 and 1800 AD the art of windmill construction reached its peak
• Later on, theories were developed, e.g. those by Euler, providing the tools to introduce new designs and, thus, to substantially improve the efficiency of energy conversion
• Around the beginning of the twentieth century, windmills were further improved and the design of a multi-blade farm windmill originated in the USA
• By the middle of the century, more than 6 million windmills were in operation in the USA
• Worldwide, many of these wind wheels were used to produce mechanical power or as decentralized electricity suppliers on large farms
• Use of wind energy worldwide

  • Table showing use of wind energy worldwide