By Jonny Lupsha, Wondrium Staff Writer
Cargo shipowners are beginning to supplement fuel with wind energy. Modern wind sails and rotors look different than those of old, but they share similar functions. Sunlight and Earth’s rotation cause wind.
In the last several years, societal pressure has mounted for governments and privately owned companies to reduce their carbon emissions. The European Union is even drafting a carbon tax on imported goods. Meanwhile, the shipping industry causes considerable carbon emissions. Modernized wind power for shipping took 20 years to become light and affordable enough for practical use, but its time seems to be approaching. Many shipowners are beginning to go green by supplementing their fuel use with modern sails and rotors.
Wind power, whether harnessed in turbines or on ships, obviously comes from the wind blowing, but what really causes the wind to blow? In his video series The Science of Energy, Dr. Michael Wysession, Professor of Earth and Planetary Sciences at Washington University in St. Louis, said that differences in temperature caused by sunlight, as well as Earth’s rotation, lead to wind.
Under Pressure
“Wind, in general, blows from areas of high pressure to areas of low pressure,” Dr. Wysession said. “These high pressures and low pressures are driven largely by differences in temperature caused by sunlight. The actual patterns of surface winds are very complicated and very variable, and are affected by a wide variety of factors that include the distribution of landmasses, the topography of the land, and also Earth’s rotation.”
However, he said, winds are also always shifting due to changing weather patterns, which can vary by day, month, and year, which happens due to the relationship between weather and wind. According to Dr. Wysession, the way that weather and wind are related is that in places where the air is cold, it’s heavier and it sinks, which creates a region of high air pressure. Air moves horizontally away from these regions along the ground toward low-pressure areas, but the areas don’t last long.
“If skies are clear, sunlight will come and heat the ground, which heats the air above it, and then the air starts rising, and it cools; water vapor condenses to form clouds, and then it rains,” Dr. Wysession said. “As the air rises, the pressure drops, and air now flows along the ground toward the new low-pressure region.”
The Coriolis Effect
Dr. Wysession said that another major factor in wind patterns is Earth’s rotation, through the “Coriolis force.” It’s a mechanism that causes air to veer towards the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This Coriolis effect, he said, is also the reason there’s a large-scale global pattern of airflow being separated by latitude into six separate cells.
“What happens is warm air rises at the equator, heated by the Sun, and then generally falls about 30° north and 30° south—that’s why there are often deserts with clear, dry air about 30° north and 30° south. Air also rises about 45° to 60° north and south, and flows both up high to the poles and then back toward the equator, to those bands at 30° north and south.
“As far as wind is concerned, the key to all this is that to complete these cycles, air has to flow back along Earth’s surface—both north and south—away from those subtropical highs at 30° north and south.”
However, as the wind blows, it gets bent due to the Coriolis effect. Within the aforementioned 30° to 60° north and south bands, eastward winds called westerlies blow. Meanwhile, the more equatorial bands produce west-flowing winds called trade winds. Winter offers stronger winds than summer, so the Northern Hemisphere is generally much more windy in January than July.
Other factors help to cause and change the wind, such as the local topography, but shipowners looking to take advantage of wind power would be wise to start their studies with high- and low-pressure regions and the Coriolis effect.