Monday, 17 December 2012

What are these dangling inside Blisworth Tunnel?

Looking back through some photos I noticed that the southern entrance of Blisworth Tunnel seems to have several pencil-thin stalactites hanging from the roof.

I've not seen this effect before. I'm sure Shadow - crewed here by Fergus, Penny and David - didn't ping them off as it entered!

Are they jets of water? In which case, why don't they go all the way down? They certainly appear to be white things, dangling!

Any ideas, anyone? (Photo taken on 10th April 2012, shutter speed 1/110)

12 comments:

Anonymous said...

Perhaps it was raining? (And if you get this twice - sorry - these comments and I don't always get along) Lizzi

Nev Wells said...

Yep, rainfall I think...

Nev

Halfie said...

Lizzi, yes, it was raining - do you think it's raindrops, then?

Halfie said...

Oh - and Nev.

Val Poore said...

If it had been colder, I'd have said icicles, but not in April, I'm guessing.

Anonymous said...

I do indeed. Some of them look like they are hanging in mid air. But I had to enlarge the photo a lot to see it. I wasn't sure if it was a trick question. Lizzi.

Bill.S said...

I think they might be water drops, bigger than raindrops, from the trees. They only seem to be near the bows/tunnel mouth. The longer streaks would be from faster moving drops, so from higher branches. Was there any wind to shake the trees? Could the engine or exhaust shake the air enough to shiver the trees? Was there a great clang from the contact with the wharfage?
Regards.

Stephen said...

I agree, probably falling water drops. I have a similar effect in a photo taken in my back garden but the drops form streaks just a couple of inches long each and their directions are less parallel. Your exposure must have been much longer.

Halfie said...

Val, no it wasn't that cold!

Bill.S and Steve, yes, I think it must be water drops, and I like the suggestion that the longer streaks are from those which have picked up more speed from having had further to fall. They can't have reached terminal velocity, then, unlike normal rainfall. And they're visible because they're against the dark interior of the tunnel. It's an interesting effect, though, especially as the varying lengths of the streaks give an apparent perspective. They definitely look as though they're inside the tunnel, not in front of it.

Lizzi, no, it wasn't really a trick question!

Stephen said...

Terminal velocity of raindrops is about 10m/s and from my o'level physics I would estimate they acquire this in less than twenty metres of fall. If the trees nearby are not so tall then the drops won't have reached this speed. Even large drops (approaching instability) only reach about 13m/s and one might observe from the photographs that the longer streaks are from larger droplets. I would be interested to know if your exposure time bears this out?

Halfie said...

Stephen, it's difficult to estimate the lengths of the streaks as it's impossible to know how near the lens they are. They look like they're in the tunnel mouth, but that's only because that's where they're visible. So I'll work it out "backwards", knowing the shutter speed (1/110 s). Using your figure for the terminal velocity of raindrops of 10m/s this gives the distance travelled by a raindrop as [distance = velocity x time] therefore distance = 10 x 1/110 = 10/110 = 0.09m or 9cm.

I'm not sure what the next step is. The longest streaks are obviously much nearer the camera than the boat, but, without knowing how far away the boat is, I can't estimate how near the raindrops are. (I suspect measuring the front cabin side and comparing it with the rear cabin side might help.)

Stephen said...

Good point. Now we're getting into the trigonometry! Probably gone as far as we can.

Best wishes.