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Back To Weather School

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I know it’s summer and school is out for most people, but for us it is time to get back to weather school. Before I do that I just need to touch on one question that I am hearing more and more of so far this summer – “So where is this so-called global warming?”

To answer this I just have to emphasize one word – global! It’s not “Western Canada” warming, its global warming. Head up to Alaska were they have been breaking daily records for daytime highs. Even better, check out Pond Inlet up on the northern tip of Baffin Island. Just the other day they broke an all-time record high when they hit 22.3 C. That means that they have never, during the years of record-keeping, recorded a temperature that warm. In fact, this is only the sixth time they have recorded a temperature over 20 C since 1923 (note: they are missing weather data between 1960 and 1975).

OK, enough on that, it’s back to school time.

It has been a while since our last official weather school class. We took a break and looked at thunderstorms, and in between we complained about the lousy weather we have been having so far this year. So to jog your memories here is where we left off – exploring angular momentum.

The formula for calculating angular momentum is: A = mvr where: m equals mass, v equals velocity, and r = radial distance. The mass we are talking about is the mass of the air which we will take as basically remaining constant no matter where we are. Velocity is how fast the air will be moving, and radial distance is the distance between the parcel of air and Earth’s rotational axis. It is this final part of the equation that helps to explain why we have strong upper atmospheric winds.

A BIT OF MATH

If you remember our lesson on the general flow of our atmosphere you will remember that air rises at the equator and then flows northward in the upper atmosphere. As this air moves northward it needs to conserve its angular momentum. That is, our value for A in our equation wants to stay the same. The mass of the air will also remain the same but the radial distance is decreasing. So in order for A to remain the same something will have to happen with the velocity – it will increase. For example, if we say all the values in this formula are equal to one then it would look something like this: 1 = 1 x 1 x 1. Now if we cut the radial distance in half it would look like this: 0.5 = 1 x 1 x 0.5. Our angular momentum has not remained constant and we pointed out that the air needs to conserve its angular momentum as it moves northwards.

So, in order for the air to conserve its angular momentum (A to equal 1 in our example) we need to change one of the two remaining values in our formula (mass or velocity). Since the mass of the air does not change that only leaves velocity. So we have to change our velocity to two, or double it for our angular momentum to stay the same (1 = 1 x 2 x 0.5).

JET STREAMS

Now how’s that for jumping back into weather school – yikes! This whole idea of conservation of angular momentum helps to explain why we can have these really strong upper atmospheric winds but this alone does not result in a jet stream.

Jet streams form along the boundaries between warm and cold air. Along these boundaries we have a sharp contrast in temperatures, and this results in a rapid change in pressure. This rapid change in pressure sets up a steep pressure gradient, which allows the already-strong upper level winds to intensify into a jet stream.

Within the jet stream we can see wind speeds as high as 300-400 kph but typically they are between 100 and 200 kph. Two main jet streams usually form – the polar jet and the subtropical jet. Here in our part of the world we deal mostly with the polar jet. Since these jet streams are partly the result of strong temperature contrasts, the polar jet tends to be strongest in the winter and weakest in the summer. If the polar jet is to our north, then we are in warm air and if it dives to our south we are in cold air (which seems to be happening too much this year).

Well, I have to wrap it up here. Our next topic of discussion will be clouds and cloud classification.

About the author

AF Contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Manitoba.

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