Have you ever wondered how weather can change from clear and 20°C (68°F) one day to snow and freezing temperatures the next? Invariably, such dramatic changes in weather are linked to changes in the position of the polar-front jet stream. This high-speed band of wind travels up to 400 kilometers per hour (250 miles per hour) and encircles Earth in great curving arcs at altitudes of 10-15 kilometers (6-9 miles). Locations north of the jet stream are colder than locations south of it. The jet stream moves in sweeping loops thousands of kilometers long, hundreds of kilometers wide, and two to three kilometers (one to two miles) thick. As it moves, it steers the paths of storm systems around the globe.

Because of the Earth's tilt, some areas of Earth's surface receive more solar radiation than others and grow warmer. The amount of heat conducted back up into the atmosphere over a given region also varies, and both of these phenomena result in areas of warmer and cooler air. Warm air is less dense than cold air, and these density differences create air pressure differences from region to region.

Air moves horizontally from areas of higher pressure to areas of lower pressure. This movement creates wind. The greater the pressure difference, the stronger the wind blows. Moving upward into the atmosphere, the air thins, and the pressure difference between warm and cold air increases. Thus, the fastest winds occur at high altitudes where pressure differences are greatest, rather than at Earth's surface. These high altitudes are where the high-speed polar-front jet stream travels.

Air flowing towards areas of lower pressure is deflected toward the east by Earth's rotation. This deflection affects wind direction, which is represented by the red flags in the animation. The red flags point in the direction the wind is moving, generally west to east. At the end of the animation, along the East Coast, winds are generally moving southwest to northeast. The polar-front jet stream, represented by blue ribbons in the animation, transverses the U.S. from west to east.

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