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.

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.

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.

Sunny paradoxes – part 2

A hazy sun

Since the solar system's beginning, the sun has increased its energy output by about 25 percent. What has that meant for earth? From ancient rocks, we can tell that a younger earth had surface liquid which can be taken to mean that earth has remained roughly the same temperature as we still have liquid water. If the sun was cooler back then, why wasn't the earth cooler? This is known as the 'faint young sun paradox'. A number of mechanisms may have been responsible for keeping a relatively constant surface temperature. Probably a combination of things are involved, but no one knows for sure. Here are some possibilities:

The Earth was warmer despite less incoming solar energy because of a larger greenhouse effect. For this to work, the greenhouse effect responsible would have tapered off as the sun grew brighter. The greenhouse effect is caused by an atmosphere rich in 'stuff' that prevents radiation from escaping. Two of the better culprits are water vapour and carbon dioxide (methane is a good at this too, as is nitrous oxide i.e., laughing gas). Assuming carbon dioxide played a big role, where would extra carbon dioxide come from? Way back, a more geologically active earth spewed more carbon dioxide from volcanoes. This excess carbon dioxide eventually was sunk into places like our oceans thereby reducing this greenhouse gas over time (only in the last 200 years have people begun spewing out our own contribution of this gas). So, when the sun was cooler the extra carbon dioxide created a greenhouse effect that has decreased at a similar rate as the sun heating up. Other processes probably put more of the other greenhouse gases into our atmosphere long ago and removed them slowly over time as well.

A recent thought based on big assumptions is that the early atmosphere also had more nitrogen. Nitrogen all on its own isn't a greenhouse gas on Earth (on Saturn's moon Titan, some funky stuff happens to the nitrogen, so there it acts a greenhouse gas), however extra nitrogen bounces around and hits the greenhouse gases which gives the greenhouse gases extra energy and makes them unstable or wobbly. This molecular wobble spreads out absorption lines (the range where a particular molecule absorbs energy) resulting in a wider band to absorb the radiation – thus more radiation is absorbed. On the flip side, extra nitrogen could also increase Rayleigh scattering, thus reflecting more of the incoming radiation away. So knowing which process dominated would be important.

As for why early earth wasn't colder with a cooler sun, we have good some ideas, but we really don't know for sure.

Refs:

Walker, C.G., P.B. Hays and J.F. Kasting, 1981: A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. Journal of Geophysical Research, 86, 9776-9782

Goldblatt, C., M.W. Claire, T.M. Lenton, A.J. Matthews, A.J. Watson and K.J. Zahnle, 2009: Nitrogen-enhanced greenhouse warming on early Earth. Nature Geoscience, 2, 891-896.

Sunny paradoxes – part 1

A sunset over Cumberland Sound

The sun continuously shines on Earth, but how much sunlight reaches us varies through the year. Since the Earth's orbit is elliptical, we are closest to the sun around 3 January and farthest around 4 July. Every illustration of the elliptical orbit of Earth that I've seen shows a hugely skewed orbit, which could lead one to think Earth would have its hottest global temperature in January.

However, the elipticalness of our orbit doesn't have much of an impact. From Wikipedia: Earth's farthest point is 152,098,232 km and the closest 147,098,290 km – a difference of 4,999,942 km, which is a small fraction of the orbit's radius (yeah, it's still a huge number, but everything in space is huge). Another way to look at this is to consider the eccentricity of Earth's orbit. Eccentricity is a measure of how circular an orbit is, zero is a perfect circle and one isn't a closed orbit (like a slingshot). Earth's eccentricity is about 0.02 – really close to a circle.

For those of us in the northern hemisphere, we are closest to the sun in the middle of winter – not the hottest time of year. Instead, it's the Earth's tilt that creates the seasons – we are tipped 23.5 degrees from the plane of Earth's orbit. When the pole closest to us is tipped away from the sun, we get winter. At the pole itself there is complete darkness (good for vampires).

The tilt of the Earth's rotation plays a greater role in our temperatures than the elipticalness of Earth's orbit. Either way, we still get a free trip of 150 million kilometres each year.