Jon does so well that I hesitate to suggest.  However, I will. If the plane were on autopilot  it might well travese a thin, flat cloud. That being said, I suggest the engine's exhausts, composed of heat and various hydrocarbons, reacted with the cloud's  fragile composition.  The speed of the plane will certainly allow for good mixing in the stream. Viola, the translucent  water vapor of the cloud is now transparent. Maybe the water molecules now have a new flim on them; or are smaller and not grouped.

I think an aircraft could easily cut a clear path through a thin cloud layer.

Please use the link to see this copyrighted photo by Steve Morris

http://www.airliners.net/photo/Virgin-Nigeria-Airways/Boeing-767-3Y0-ER/1229391/L/

I answered this question in detail last week and now I see that it's vanished. Let's hope this system works a bit better this time.

The photograph shows a distrail. This is caused by an aircraft flying through the cloud. For a detailed explanation, see <http://weatherfaqs.org.uk/node/23>.

Though nowhere near as common as contrails, I see them quite often. I've mainly seen them in high cloud but occasionally in low cloud. One I saw some years ago was in a thinning sheet of stratocumulus at a height of about 4000 ft. As well as the turbulent downdraft causing a clear track through the cloud, updraft also caused a thickening of the cloud on either side. The sheet of cloud continued to thin until only the two banks of cloud bordering the distrail were left, forming a sort of pseudo-contrail.

Dear Neil,

This is a surprisingly common phenomenon when you start to look out for it. The hole is caused by an aircraft penetrating the cloud layer - depending on whether the plane was flying horizontally or was climbing the hole produced can either be a long line (like this one) or a circular 'hole-punch'. They can occur for one of 3 reasons: 1) the hot aircraft exhaust is enough to evaporate the water droplets in the cloud; 2) the turbulence in the aircraft's wake mixes in dry air from above/below the cloud layer, again evaporating the droplets; or 3) the passage of the plane produces a contrail of growing ice crystals which cause the water droplets to evaporate at their expense: this happens because of the difference in vapour pressure between the surface of an ice crystal and a liquid water droplet. The droplets around where the aircraft penetrated evaporate away, the crystals fall out, and a clear hole is left behind.

Clouds affected by mechanism number 3 can be particularly striking, since the hole is often accompanied by bright, brush-like streaks of ice crystals falling below it:

http://www.met.reading.ac.uk/radar/publications/HolePunchPaper.pdf

http://www.google.co.uk/images?hl=en&client=firefox-a&hs=XfT&rls=org.mozilla:en-GB:official&q=hole+punch+cloud&um=1&ie=UTF-8&source=univ&ei=3w_YTIC3CYy2hAfD0u20BQ&sa=X&oi=image_result_group&ct=title&resnum=2&ved=0CCUQsAQwAQ&biw=1047&bih=633

Best wishes,

Chris Westbrook

While this appears to be a "hole" in the cloud, Occam's razor suggests that it is merely the shadow of a contrail at a higher altitude, cast on a lower layer of cloud.  Searching for "contrail shadow" produces many examples--one particularly good set of images can be found at http://www.weatherscapes.com/album.php?cat=clouds&subcat=contrail_shadows