2 00:00:10,311 --> 00:00:13,670 How does the Coriolis acceleration affect flows? 3 00:00:13,670 --> 00:00:17,560 It's a really interesting phenomenon known as geostrophic motion. 4 00:00:18,640 --> 00:00:23,080 Imagine that you have some push force. 5 00:00:23,080 --> 00:00:29,320 This could be high pressure on one side of a column of gas, and 6 00:00:29,320 --> 00:00:31,390 low pressure on the other, or it could be 7 00:00:32,560 --> 00:00:35,660 wind blowing over the surface of water and tending 8 00:00:35,660 --> 00:00:38,690 to push the water in some direction. 9 00:00:38,690 --> 00:00:41,490 But what the water does initially, if you push it 10 00:00:41,490 --> 00:00:43,640 in some direction, is it goes in that direction. 11 00:00:45,060 --> 00:00:49,730 In the same direction as you push it, and when it starts going in that direction, 12 00:00:51,230 --> 00:00:55,800 you produce a Coriolis acceleration, which is always going at 90 13 00:00:55,800 --> 00:01:00,700 degrees to the direction that it's moving. What this is acting to do 14 00:01:00,700 --> 00:01:06,590 is pull the direction of the flow around, to rotate it. 15 00:01:06,590 --> 00:01:09,370 The same way as the direction of the ball 16 00:01:09,370 --> 00:01:14,020 in the merry go round was deflected by the Coriolis acceleration. 17 00:01:14,020 --> 00:01:18,570 After a little while, the flow is not going the 18 00:01:18,570 --> 00:01:21,740 same direction you were pushing it anymore; it's rotated. 19 00:01:21,740 --> 00:01:25,890 Off to the side, and the Coriolis is always following the flow here, 20 00:01:25,890 --> 00:01:30,660 and eventually you end up with the equilibrium state, which 21 00:01:30,660 --> 00:01:34,670 is what it relaxes to and then stops changing. 22 00:01:34,670 --> 00:01:40,400 It can stay in this equilibrium state forever, where the flow is actually 23 00:01:40,400 --> 00:01:44,230 astonishingly going at 90 degrees to the 24 00:01:44,230 --> 00:01:46,180 side of the way you're actually pushing it. 25 00:01:47,500 --> 00:01:51,100 And in this way, the Coriolis, which is 90 degrees 26 00:01:51,100 --> 00:01:57,060 to the flow, is now pushing back, exactly counteracting the push force. 27 00:01:57,060 --> 00:01:58,830 And so that's why this is equilibrium, 28 00:01:58,830 --> 00:02:01,230 because those forces can cancel each other out. 29 00:02:02,250 --> 00:02:03,390 You can't even look at a weather 30 00:02:03,390 --> 00:02:06,530 map without seeing the affect of geostrophic motion. 31 00:02:06,530 --> 00:02:10,920 A hurricane is basically a hole in the atmosphere where the 32 00:02:10,920 --> 00:02:16,210 pressure is lower, the atmospheric pressure, than it is in the surrounding air. 33 00:02:16,210 --> 00:02:19,890 You remember that the pressure is just the result of the 34 00:02:19,890 --> 00:02:23,010 weight of the air on top of you head in the atmosphere. 36 00:02:23,970 --> 00:02:26,180 This low pressure at the ground at the center of 37 00:02:26,180 --> 00:02:29,380 a hurricane just means that there's less air piled up there, 38 00:02:29,380 --> 00:02:32,920 and the difference in the air pressure is leading 39 00:02:32,920 --> 00:02:37,190 to a push force going from high pressure to low pressure. 40 00:02:37,190 --> 00:02:39,440 You would think in a non-rotating frame, 41 00:02:39,440 --> 00:02:41,510 if you had higher pressure of gas here and 42 00:02:41,510 --> 00:02:44,000 lower pressure there, it would just go from the high 43 00:02:44,000 --> 00:02:47,140 pressure to the low pressure and fill in the hole. 44 00:02:47,140 --> 00:02:51,990 But due to the geostrophic motion, you have the 45 00:02:51,990 --> 00:02:57,540 flow in the atmosphere is going at 90 degrees to the way you're pushing it. 46 00:02:57,540 --> 00:02:59,890 It's going around like this. 47 00:02:59,890 --> 00:03:05,840 And then the where, where the flow is going in this direction. 48 00:03:05,840 --> 00:03:06,970 The Coriolus is 49 00:03:06,970 --> 00:03:11,370 pointing at 90 degrees to that and so the coriolus is reaching 50 00:03:11,370 --> 00:03:13,400 the state of equilibrium, where it's pushing 51 00:03:13,400 --> 00:03:18,400 back against the push force, in the geostrophic equilibrium. 52 00:03:18,400 --> 00:03:22,610 It is as if you made a hillside. 53 00:03:22,610 --> 00:03:26,360 Or, a dip in a membrane or something like that. 54 00:03:26,360 --> 00:03:28,160 And you set a marble on it. 55 00:03:28,160 --> 00:03:32,230 And in a non rotating frame, it would just roll to the bottom of the hill. 56 00:03:32,230 --> 00:03:35,630 But in a rotating frame, it doesn't go the way you're pushing it. 57 00:03:35,630 --> 00:03:38,310 It goes sideways to it, and so it goes around 58 00:03:38,310 --> 00:03:42,290 and around and around, only very slowly does it go down. 59 00:03:42,290 --> 00:03:45,850 And this is why hurricanes last as long as they do. 60 00:03:45,850 --> 00:03:50,820 There's a feature on Jupiter called the great red spot. 61 00:03:50,820 --> 00:03:53,600 Which, as far as we know, has been there for centuries. 62 00:03:53,600 --> 00:03:54,790 And it's just one of these sorts of 63 00:03:54,790 --> 00:03:57,250 systems with a pressure imbalance and the flow 64 00:03:57,250 --> 00:04:00,400 going around and around and around in geostrophic motion. 65 00:04:00,400 --> 00:04:02,920 And that's why it can last for so long. 66 00:04:02,920 --> 00:04:08,860 You also see the effect of geostrophic motion in ocean flow. 67 00:04:08,860 --> 00:04:14,580 The gulf stream is a giant river of water in the North Atlantic. 69 00:04:16,240 --> 00:04:22,380 And the sea level is about one meter higher on one side of the Gulf Stream 70 00:04:22,380 --> 00:04:23,940 than it is on the other. 71 00:04:23,940 --> 00:04:29,040 It's very wide feature so you never see a wall of water a meter high. 72 00:04:29,040 --> 00:04:31,590 It's too spread out to be that obvious. 73 00:04:31,590 --> 00:04:34,340 But, you can measure it, and if you measure also 74 00:04:34,340 --> 00:04:40,560 the pressure at some level underneath the ocean level, 75 00:04:40,560 --> 00:04:44,100 you have higher pressure here where the sea level is higher because 76 00:04:44,100 --> 00:04:47,380 you have more stuff over your head than you do here 77 00:04:47,380 --> 00:04:48,800 where the sea level is lower. 78 00:04:48,800 --> 00:04:51,250 You have a pushing force going in 79 00:04:51,250 --> 00:04:54,800 this direction, and then in geostrophic motion, you have the Gulf 80 00:04:54,800 --> 00:04:57,340 Stream which is an arrow going into the board 81 00:04:57,340 --> 00:05:00,530 here, 90 degrees to the direction that you're pushing it. 82 00:05:00,530 --> 00:05:06,674 In order to simulate, the way that the atmosphere and the ocean carry heat 83 00:05:06,674 --> 00:05:12,818 from the low latitudes to the high latitudes, we have to understand about 84 00:05:12,818 --> 00:05:16,317 how flow occurs on a rotating sphere.