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Tornadoes only form when a thunderstorm has a particular combination of winds. Air rising in a thunderstorm can begin to spin when it is affected by winds blowing it in different directions. It starts to rise and is pushed to the side by wind. It rises a bit more and is jostled again by wind moving in another direction. Winds moving in different speeds and directions at different altitudes cause the rising air to start spinning. Air that spins as it rises is typical in a supercell, the strongest type of thunderstorm, but not all spinning air creates a tornado. For a tornado to form, there also needs to be spinning air near the ground. This happens when air in the storm sinks to the ground and spreads out across it in gusts. Gusts of warmer air rise and gusts of cooler air sink as they blow across the land. If there are enough rising and sinking gusts, the air near the ground starts spinning. The spinning air near the ground speeds up as it is drawn inward toward its axis of rotation. This happens in the same way that figure skaters spin faster when their arms are drawn in rather than when their arms are outstretched. This is called conservation of angular momentum. The rotating air moves horizontally across the ground, and can be tilted vertically by the force of the rising, rotating air. This allows a tornado to form. Most tornadoes form during supercell thunderstorms, but not all supercell thunderstorms produce tornadoes. Usually, the rotating air near the ground doesn't rotate fast enough for a tornado to form. If the rotating air near the ground is very cold, it will spread away from the storm along the ground and slow down like a figure skater with extended arms, and a tornado will not form. 只有當雷暴具備特定的風向組合時，龍捲風才會形成。 雷暴中上升的空氣在受到不同方向風的影響時會開始旋轉。它開始上升，並被風推向一側。它繼續上升，然後再次受到另一方向風的擾動。不同高度不同速度和方向的風會導致上升的空氣開始旋轉。 上升過程中旋轉的空氣是超級單體雷暴（最強的雷暴類型）的典型特徵，但並非所有旋轉的空氣都會形成龍捲風。 龍捲風的形成也需要近地面的旋轉空氣。當風暴中的空氣下沉到地面並以陣風的形式擴散時，就會發生這種情況。暖空氣陣風上升，冷空氣陣風下沉，它們吹過地面。如果上升和下沉的陣風夠多，近地面的空氣就會開始旋轉。 近地面的旋轉空氣在被拉向其旋轉軸的過程中速度加快。這就像花式滑冰選手收臂時旋轉速度比伸臂時更快一樣。這稱為角動量守恆。 旋轉的空氣在地面上水平移動，並可能因上升旋轉空氣的力而垂直傾斜。這使得龍捲風得以形成。 大多數龍捲風形成於超級單體雷暴期間，但並非所有超級單體雷暴都會產生龍捲風。通常情況下，近地面的旋轉空氣旋轉速度不足以形成龍捲風。如果近地面的旋轉空氣非常冷，它會沿著地面遠離風暴擴散並減速，就像花式滑冰運動員伸臂時一樣，因此不會形成龍捲風。