A tornado requires patterns of both cold and warm air in the same region in order to form. The most likely time for this to occur is during warm or hot weather when a thunderstorm is approaching. The tornado is created when warm, moist air rises from the ground and comes into contact with a mass of colder air at the bottom of a thundercloud. The rising air pushes against the colder air, and the rotation of the earth causes the air to spin, (18) _______.

As mentioned, tornadoes generally, but not always, develop in conjunction with a thunderstorm. Thunderstorms that result in tornadoes are referred to as supercells. (19) ______, especially when other conditions, such as a cold front or low-level converging winds, are also present. (20) ______, these clouds get caught up in the tornado’s swirling wind, which creates the menacing funnel that most people associate with danger and destruction of property.
(21) ______. The first stage occurs just before the onset of a thunderstorm, when the wind changes direction and increases its speed. As the wind moves faster and higher in altitude, it begins an invisible horizontal spin. Once the thunderstorm starts, rising air inside the storm causes the spinning wind to become vertical, or to shift from side to side to an up-and-down position. The thunderstorm then surrounds this area of rotating air, which can vary in length. Clouds that have formed in the lower part of the thunderstorm (22) ______. Shortly after the completion of the previous step, the tornado develops.
(Adapted from Longman Preparation Course for the TOEFL iBT® Test)