Science Questions

Where does air turbulence come from?

Sun, 1st Apr 2012

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Question

Pieter Horn asked:

Dear Chris,

 

When an aircraft flies into an area of air turbulence, is this turbulence cause by varying wind speeds and/or crosswinds or a change in air density?

 

Pieter Horn

 

 

 

Answer

Dave -   When you're flying through the air at about 500 miles an hour, if you're flying through beautifully smooth air; air which is not moving at all, or itís all moving at the same speed, you'll have a nice smooth journey. 

But you can be in complicated messy air; where wind is blowing off a rough surface or where the air is moving upwards, other areas where the air is going downwards, and yet more where itís going sideways.  As a plane flies through these, it will suddenly be flying too fast and have too much lift, making it go upwards. Then it will suddenly be flying too slowly, so it will go downwards.  So it will get shaken around because itís going through these little pockets of moving air very quickly and you feel that as being shaken around all over the place.

The issue with aircraft not being able to fly very close to each other is related.  When a plane flies through the air, itís pushing lots of air downwards over its wings.  The air at the side isn't getting pushed downwards, so it gets lifted upwards and you get these two swirls, two vortices, one at the end of each wing.  From a small plane, that's not very dangerous, but for something like a 747 or A380, those are incredibly powerful.  There's a huge amount of energy in these things and if a small aircraft gets caught in one of those behind a large aircraft, it can get flipped upside down, and could crash really viciously.

Chris -   So you don't want them too close for that reason which is also why midair collisions or near misses are so important as well I suppose.

Dave -   Yes, if you had a near miss flying into these vortices, wake vortices from another aircraft, you could cause problems but itís not as bad as near the ground.  If you're up high, you've got more time to deal with the fact that you are now upside down.  If you're at 100 feet, you don't have any time at all and itís very messy.

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