Wednesday, October 26, 2011

No zombies here

Do zombies eat popcorn with their fingers? No, they prefer to eat fingers separately.

This post has nothing to do with zombies, I just heard this joke the other day and found it bizarrely funny. I guess there is no accounting for my taste in humor. Instead of zombies, I’m going to write about lee waves as they are cool (in my mind) and I gave a talk about them recently. Lee waves can be found everywhere if you know what you are looking for. They lurk in your sink, form over mountains and even beneath the ocean’s surface (I wouldn’t be surprised if they can be found out in space too).

Topography, like mountains or under-sea ridges, affects the flow that passes over it. Fluid (air or water) in the lower layers is pushed up the windward or upstream side where it squeezes in with the upper layers causing the flow to speed up. On the lee side, flow slows down again and a disturbance to the flow is formed. This 'disturbance' is often a wave that travels in the opposite direction of the flow. When the speed of the flow and this wave are the same, the wave is stationary and called a lee-wave. (ever notice a bump in the water right below a weir? It’s a non-linear form of a lee wave called a hydraulic jump).

Lee waves were discovered by glider pilots in the 1930s. If a glider catches a lee wave in the right place, the unpowered aircraft can gain significant altitude. All these early papers are in German, so I don’t know what these early pilots had to say about lee waves, my guess is that they found it pretty exciting. As a teenager, I regularly flew in gliders – which typically included a full day of pushing the glider around on the ground and about 7 minutes of actual flight time. I loved it. We flew out of a field on the prairies in Alberta, too far from the Rockies to gain altitude through a lee-wave.

A soon as lee waves were discovered, scientists started looking at why and how they form. The theory requires looking at the Navier-Stokes equations – a mighty difficult task which only recently became doable with computers. As an alternative, lab experiments were conducted. These experiments (and there was lots of them) offered a straightforward way to look at the factors influencing lee wave formation – combinations of the obstacle height and width, and the fluid velocity and density. Once this parameter space was full, it could be used to predict real world phenomenon.

Ocean lee waves are common and in shallow waters and beneath them a significant amount of turbulence is created. Oceanographers can look at them in detail using current meters (I don’t think there is an equivalent measurement instrument for the atmosphere yet). Because of the augmented flows and turbulence, the sea floor under a lee wave makes great habitat for critters – especially stationary filter feeders, as a buffet of tasty treats is whooshed by.

Filter feeders are often also builders, such as coral reefs, glass sponge reefs or even mussel beds. Sometimes the structures they build can intensify the turbulent flows they moved there to take advantage of. They can add to the roughness of the bottom (thus creating more drag) or even make the slope steeper. A steeper slope will result in a steeper lee wave, steeper lee waves may even break (remember the hydraulic jump?) creating even more turbulence. More turbulence means more food can be churned through the water giving the filter feeders more to eat.

As a tangent: this is post number 100.

Wednesday, October 19, 2011

Ageing maple leaves

a maple leaf in the sun
Yesterday, in the parking lot at work, a maple leaf rested on the pavement. The golden-hued morning light caught the leaf highlighting the red-tending-to-maroon tones. The leaf sharply contrasted the cold grey of the pavement, its vividness catching my eye. What if I picked up the leaf and saved it? Could archeologists in the far future figure out when the leaf fell from the tree?

Currently, we can estimate how old plant-based objects are using radiocarbon dating - often just called carbon dating. In 1949, Willard Libby and his team accurately estimated the age of the wood in an ancient Egyptian barge – a barge with a recorded age. This process works through knowing the ratio of carbon-12 (the ordinary stuff) to carbon-14 (a radioactive isotope) in the atmosphere.

Carbon-14 isn't particularly stable and decays quickly. It has a half-life of about 5,730 years - only a moment of time compared to the approximately 4.5 billion year half-life of uranium-238 (which is roughly the age of Earth). Continuously formed in the atmosphere by cosmic rays, carbon-14 reacts with oxygen becoming carbon dioxide. Plants take up some of this carbon dioxide along with carbon dioxide formed from the more abundant carbon-12. When the plant dies, no more carbon dioxide is taken in and the existing carbon-14 begins to decay.

If we assume the carbon-12 to carbon-14 ratio was the same when the plant died to now, using the decay rate of carbon-14 will give us the item's age (back to about 60,000 years). But, we know this ratio has fluctuated over time. To compensate, the age results are calibrated to something known like written records or tree rings. The biggest change to the carbon-12 to carbon-14 ratio has occurred in modern times through nuclear testing. Carbon-14 levels in the atmosphere were boosted around 1950 and peaked in the 1960's (at which time, testing bans were agreed to).

So, could a future archeologist figure out the are of my leaf using carbon dating? Probably not accurately because we've messed with the carbon-12 to carbon-14 ratio in our atmosphere. It would be more accurate for that archeologist to look at the date of this article.

As a tangent: At the end of the day when I returned to my car, the leaf was still there. Without the sunlight shining on it, the leaf looked brown and uninteresting.

Friday, October 14, 2011

Polar Bear Hair

Polar Bear photographed by Iva Peklova
Bears scare me, in fact, they scare me more than anything else. As a child, I would lay awake in my second story bedroom fearing that a bear would crash through my window at any moment. Even then, I was well aware the black bears in the area preferred to forage for berries and grubs over breaking into a child's bedroom but, I still feared them.

If I camp in the woods, any twig breaking or rustling sound will immediately start me thinking of bears. I've seen plenty of wild bears (black bears, grizzlies and polar bears) and I've never had a bad experience – mostly the bears acted terrified of me (perhaps as cubs they feared people would break into the dens). I'm forced to conclude that my life-long bear fear is irrational – at least I no longer fear bears will break into my urban second story bedroom.

In the temperate climate I live in, I don't see a bear every time I step in the forest. In fact, I rarely see them. However, every time I've been to the arctic, I've seen polar bears. I've seen more polar bears in the wild than any other type of bear. The arctic is huge and there are not a lot of polar bears, so I find it somewhat strange that I see them most often.

When I was shopping for my dad's birthday present (he ties flies for fishing), I was drawn to a swatch of polar bear hair. I wanted to touch it, so I bought the package and took it home. Polar bears aren't truly white, instead they are more of a cream colour. In a southern zoo setting they can even acquire a tint of green from algae growth.

If you look closely at a polar bear's hair, it is hollow and transparent. At some point an urban myth was promulgated that the hairs were acted like natural fibre optic cables, channelling the light, especially UV down to the bear's black skin. It doesn't quite work that way, instead light just passes through the hair to heat the skin. In this case, the simple answer is the right one.

As a tangent, the polar bear hair felt wiry rather than soft like I expected.