Emma White, of Balclutha Primary School, asks :-

Why does a tornado come down but suck things up?

Rebekah LaBar, a meteorologist with Metservice, responded.

A tornado is a rotating column of air extending from a cloud down to the ground. For a tornado to form, we usually need at least four ingredients: instability, a lifting mechanism, moisture, and wind shear.

If the atmosphere is unstable, it means the temperature is cooling with height and air will start to rise if it gets a little nudge from something such as a mountain or a weather front. The colder it is aloft (and warmer it is at low levels), the greater the instability and the more the air can rise. Warm air rises until it cools to the same temperature as the surrounding air. If there is enough moisture, a cloud can form.

Wind shear occurs when the wind is blowing in different directions and/or speeds at different levels. If you place a pencil between your hands and move your top hand but keep your bottom hand still, the pencil will rotate. The bigger the difference in speed and direction, the more turbulent air can become.

The rising air underneath the cloud can lift and tilt the tube of rotating air into the vertical, and if certain conditions are right, the now-rotating storm can produce a tornado starting from the base of the cloud.

Air pressure is much lower inside a tornado than outside. This causes air to move towards the tornado in order to balance out the pressure difference. However, if air moves towards low pressure it eventually has to go somewhere; at ground level, it can’t go down so it has to go up. The strong winds near the base of a tornado can pull things in, and if the objects are light enough, they can even be lofted above the ground in the rising, rotating air. Most objects, however, are too heavy for the winds to hold them up, so if they do get pulled off the ground they will fall back down shortly afterward.

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