What causes turbulence?

[ahlam1399]

http://img-s-msn-com.akamaized.net/tenant/amp/entityid/BBGaFJf.img?h=100&w=100&m=6&q=60&o=f&l=fIt's a lot less scary than it feels.

أكثر... (http://www.msn.com/en-ca/travel/news/what-causes-turbulence/ar-BBGudCM?srcref=rss)