2 00:00:09,821 --> 00:00:14,621 All of these winds and currents on Earth that have to be simulated with 3 00:00:14,621 --> 00:00:17,981 our gridded climate model, are heavily impacted 4 00:00:17,981 --> 00:00:21,090 by the fact that the Earth is rotating. 5 00:00:21,090 --> 00:00:22,320 In fact, you can't even look at a 6 00:00:22,320 --> 00:00:24,990 weather map without seeing the effect of Earth's rotation. 7 00:00:26,160 --> 00:00:30,050 Here, we take into account rotation 8 00:00:30,050 --> 00:00:33,620 using an idea called the Coriolis Acceleration. 9 00:00:33,620 --> 00:00:35,150 Which is basically a fake, 10 00:00:35,150 --> 00:00:39,060 it's a fudged factor which is intended to 11 00:00:39,060 --> 00:00:42,620 account for the fact that the rotation is happening. 12 00:00:42,620 --> 00:00:46,040 To see what's going on imagine 13 00:00:46,040 --> 00:00:50,260 a merry go round with two people on it on opposite sides. 14 00:00:50,260 --> 00:00:54,550 And if we're looking down on the merry go round from a stationary frame. 15 00:00:54,550 --> 00:01:00,670 If we're not rotating but the thing is spinning you'll see say Bob here 16 00:01:00,670 --> 00:01:05,760 throw the ball toward Alice. But by the time the ball 17 00:01:05,760 --> 00:01:11,650 gets to Alice she has moved to there and, and Bob is now here. 18 00:01:11,650 --> 00:01:13,750 And that all makes perfect sense, the ball is going in 19 00:01:13,750 --> 00:01:17,880 a straight line just like Newton's Laws say it should do. 20 00:01:17,880 --> 00:01:21,070 But if we want to spin around with the 21 00:01:21,070 --> 00:01:26,180 merry go round and still make sense of things 22 00:01:26,180 --> 00:01:31,080 we actually see, we don't see Bob or Alice moving anymore. 23 00:01:31,080 --> 00:01:32,990 and what we see is the ball that Bob 24 00:01:32,990 --> 00:01:37,140 throws, it looks like it curves off to the side. 25 00:01:37,140 --> 00:01:42,990 We could sort of hack Newton's Laws of Motion to make 26 00:01:42,990 --> 00:01:48,700 the ball do that by adding this fake force, the Coriolis Acceleration. 27 00:01:48,700 --> 00:01:51,740 It always goes at 90 degrees 28 00:01:51,740 --> 00:01:55,880 to the way the thing is moving, because we want this ball to turn. 29 00:01:55,880 --> 00:01:58,360 If the fake force was pointing forward, 30 00:01:58,360 --> 00:02:00,922 it would make it go faster, and we don't want that. 31 00:02:00,922 --> 00:02:03,060 Or if it was pointing behind it, it would 32 00:02:03,060 --> 00:02:05,180 make it slow down, and we don't want that either. 33 00:02:05,180 --> 00:02:06,620 It has to be at 90 degrees. 34 00:02:08,490 --> 00:02:13,455 It's easy to understand the Coriolis Acceleration on a 35 00:02:13,455 --> 00:02:17,000 merry go round, because everything is spinning at the same rate on the merry go round. 36 00:02:17,000 --> 00:02:20,290 But we're on a planet which is a little more complicated. 37 00:02:22,380 --> 00:02:27,068 And the way to visualize the the rotation of the Earth. 38 00:02:27,068 --> 00:02:29,660 I mean one really cool way to visualize it, 39 00:02:29,660 --> 00:02:33,080 is with a device known as a Foucault's pendulum. 40 00:02:33,080 --> 00:02:35,720 They have these at science museums, usually. 41 00:02:35,720 --> 00:02:38,410 They have a big atrium, because it has to be very tall. 42 00:02:38,410 --> 00:02:42,080 They want this thing to get going in the morning and swing all day long. 43 00:02:42,080 --> 00:02:44,900 And what happens is as you're standing 44 00:02:44,900 --> 00:02:48,380 there, the Earth is actually rotating under the pendulum. 46 00:02:48,380 --> 00:02:52,100 A pendulum that's going in one axis at the beginning of the day 47 00:02:52,100 --> 00:02:56,360 will be going in a slightly different axis later on in the day. 48 00:02:56,360 --> 00:02:59,550 You can set up a little ring of little dominoes or something, and 49 00:02:59,550 --> 00:03:04,190 every ten minutes it'll knock one over as the Earth is rotating under it. 50 00:03:04,190 --> 00:03:08,180 Imagine a Foucault pendulum at the North Pole. 51 00:03:08,180 --> 00:03:10,770 That's totally like the merry go round. 52 00:03:10,770 --> 00:03:12,630 It's no different, 53 00:03:12,630 --> 00:03:17,090 the pendulum is going to want to stay in a fixed plane relative 54 00:03:17,090 --> 00:03:22,070 to the fixed stars, and the Earth will just spin underneath it. 55 00:03:22,070 --> 00:03:25,690 What you see is 360 degrees of rotation 56 00:03:25,690 --> 00:03:28,800 in 24 hours, the actual rotation rate of the Earth. 57 00:03:28,800 --> 00:03:32,310 Now, let's set up a Foucault pendulum at the equator. 58 00:03:32,310 --> 00:03:36,050 And we'll get it going north and south. 59 00:03:36,050 --> 00:03:38,120 That's the easiest way to visualize this, 60 00:03:38,120 --> 00:03:39,830 for me anyway. Hopefully it is for you, too. 61 00:03:41,000 --> 00:03:43,110 We'll get it going North and South, and it 62 00:03:43,110 --> 00:03:48,920 wants to stay in that orientation relative to the fixed stars. 63 00:03:48,920 --> 00:03:51,100 Stars up here, stars down there. 64 00:03:51,100 --> 00:03:53,970 It's going to keep going North South as it rotates around. 65 00:03:53,970 --> 00:03:56,590 Of course it'll always be pointing down 66 00:03:56,590 --> 00:03:58,950 toward the center of the planet as it goes around. 67 00:03:58,950 --> 00:04:01,160 But it's going to be going north/south and 68 00:04:01,160 --> 00:04:04,000 it's not actually going to feel any rotation 69 00:04:04,000 --> 00:04:06,260 at all at the equator. 70 00:04:06,260 --> 00:04:08,620 And then if you have a pendulum somewhere 71 00:04:08,620 --> 00:04:12,120 in between you get an amount of rotation, which is 72 00:04:12,120 --> 00:04:14,450 in between the full rotation of the 73 00:04:14,450 --> 00:04:18,320 planet of the pole, and no rotation at the equator. 74 00:04:18,320 --> 00:04:23,805 Less than 360 degrees in 24 hours, or one full rotation in more than 24 hours. 76 00:04:24,900 --> 00:04:31,770 The way that physicists visualize this 77 00:04:31,770 --> 00:04:37,670 and calculate this is by drawing a line. 78 00:04:37,670 --> 00:04:40,270 A vector, a line that has a direction, which is the 79 00:04:40,270 --> 00:04:43,950 rotation axis, and so that would be like the North Pole. 80 00:04:43,950 --> 00:04:47,900 And then comparing that to a line that points 81 00:04:47,900 --> 00:04:50,200 toward the center of the Earth at any latitude, 82 00:04:50,200 --> 00:04:52,310 which would be like a flagpole. 83 00:04:52,310 --> 00:04:56,830 And the amount of rotation you get is proportional to what 84 00:04:56,830 --> 00:05:01,860 they would call the projection of the flagpole on the rotation axis. 85 00:05:01,860 --> 00:05:04,930 At the North Pole, they're both going in the same direction and so 86 00:05:04,930 --> 00:05:09,420 you get the full rotation, but as you get to the equator they're orthogonal 87 00:05:09,420 --> 00:05:11,766 to each other, and so 88 00:05:11,766 --> 00:05:14,220 you get no rotation at the equator. 89 00:05:14,220 --> 00:05:17,642 And then in the Southern Hemisphere, the rotation works 91 00:05:17,642 --> 00:05:19,846 the same way but in the opposite direction.