Wind is an ever-present force. From a gentle breeze to a cold arctic blast, it is constantly shaping the landscape and the weather. But where does wind come from?
Simply put, wind is the motion of the air around us, generated by differences in pressure in the Earth's atmosphere. Air is a fluid, and just like water, it obeys the laws of fluid dynamics. It will seek to flow from a region of higher pressure to one of lower pressure, says Chris Maier, a meteorologist with the National Oceanic and Atmospheric Administration’s National Weather Service.
Earth's air-filled atmosphere is constantly but unevenly pressurized, with highs and lows at various places caused by the uneven heating of Earth’s surface by the sun. The air at the North or South Pole is colder and denser, while the air at the Equator is warmer and rises more easily. The colder, more highly pressurized polar air is constantly trying to move down to the Equator to replace the warm, rising air.
That creates Earth’s overall global circulation, says Maier. There are wind belts that circle the planet along latitudinal lines, each having particular characteristics and creating specific weather patterns.
One of those bands is the Intertropical Convergence Zone near the Equator, where the trade winds meet. Sailors named the trade winds, navigating by them because of their fairly dependable behavior.
In the Northern Hemisphere, the trade winds are created as warm air moves away from the Equator and is bent slightly right due to Earth’s rotation. This is known as the Coriolis effect. The warm air is pushed from the northeast to the southwest back toward the Equator thanks to down-rushing polar air. The same thing happens in the Southern Hemisphere, with the trade winds being pushed from the southeast toward the northwest.
As the northern and southern trade winds converge near the Equator, they create a zone with little to no wind and a tendency for short-burst intense storms full of rain. Sailors have called this zone the doldrums for eons.
In the contiguous U.S., a band known as the Westerlies is the primary force behind wind and weather, with most storms following along a west-to-east track, says Maier.
Jet streams also help direct those wind and climate patterns. The jet streams—there are at least several—range from 25,000 to 50,000 feet above Earth and “are basically fast-flowing rivers of air,” says Maier.
They help form boundaries—a counterforce to the polar air making its way toward the Equator and to the warm air moving away from the Equator, he says. The jet streams can change course, speed or altitude from day to day. The streams are often used by airlines and the military to increase speed without using more fuel.
In the U.S., the current El Niño weather pattern—in which warmer Pacific Ocean water off Peru and Ecuador creates a bubble of warmer, less-pressurized air that then moves north—is pushing North America’s usual jet stream northward and extending another jet stream across the southern part of America. That brings more snow and rain to California, more wet weather across the Gulf Coast, and milder temperatures to the East.
Wind is also noticeable on a more local level. In the winter, the air is colder, denser and more pressurized. Colder air will rush in upon the opening of a door from a heated, less-pressurized house. People who live near the mountains will notice that during the day, as the valley heats up, warm air will rise up the slopes, while in the evening, colder, denser air will push back down into the valley.
At the beach, denser air sits above cool water, while low-pressure air sits above warmer land. The colder air seeks to move inland to equalize the pressure, causing a breeze. That pattern is reversed at night, as the land cools rapidly and pushes its more pressurized air back to sea.
Storm systems, however, can upset that equilibrium. A storm undergoes many changes in pressure as it moves and often contains warmer, rising air within its confines, with cooler air pushing from behind. The stronger the difference in pressure between those systems, the more forceful the wind, as is seen with a hurricane or a tropical cyclone.
It's your turn to Ask Smithsonian.