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Results Purpose To use a rotating tank to simulate the circulation of Hadley cells and eddies in the global atmosphere. Procedure Experiment Part I 1.  Fill 1/3 of circular tank with water and use a thermometer to measure the temperature inside the tank. 2.  Use a level to ensure equilibrium of the rotating table. 3.  Add a drop of food coloring and observe the motion without rotation. (See Figure 1) 4. Turn the knob slowly to start the rotating table at the rate of 3.5 rotations/minute . 5.  Add a drop of food coloring into the tank and observe the dispersion patterns. 6.  Place plastic floating disks on the surface to ensure the water rotates at the same rate as the tank. 7.  As soon as the water reaches equilibrium with the rotating tank, add food coloring into the solution and observe the movement. (See Figure 2) 8. Increase the rotation rate to 5.5 rotations/minute and observe the movement of the coloring. 9. Stop the turntable and empty out all the water in the tank. Part II Experiment 1.  Fill a tub with room temperature water. 2.  Place a heating pad and an ice pack on the opposite sides of the tub. 3.  Add one drop of food coloring on the side of the tub with the ice pack. 4.  Add a different colored dye on the side of the tub with the heating pad. (See Figure 3) Experiment Part III 1.  Fill 1/3 of circular tank with water. 2.  Fill a metal cylinder with ice and place it in the middle of the circular tank. 3.  Add food coloring near the cylinder and observe the results. 4.  Add a different color of food coloring near the edge of the circular tank and observe the motion of the two colored food colorings. 5.  Turn the knob slowly until the rotating table reaches the rate of 4 rotations/minute. 6.  Add drops of food coloring in the water after the table has reached equilibrium, and observe the results. (See Figure 4) 7.  Turn the knob slowly to a new rate of 16 rotations/
minute and observe the food coloring. Abstract The Hadley cell is an atmospheric circulation system that features rising warm air near the equator that flows poleward at a 10-­‐15 km altitude to 30°latitude. The air then descends and forms a high pressure system which flows back to the equator as the trade winds. We used a rotating tank in order to model both the Hadley circulation and the eddies, which form due to instability. By setting the rotating table at different speeds and inserting a bucket of ice (a mock North Pole), we were successfully able to create environments where these eddies and small-­‐scale Hadley cells could form. Each formation developed as a method of heat transfer, reducing the temperature gradient from the cooled center to the warmer edges. (Figure 1) Gravitational force moves the food coloring vertically; the surface tension of the water stretches the coloring horizontally along the surface. Underwater, the food coloring slowly isotropically disperses throughout the tank. (Figure 2) The centripetal force caused the water near the edges of the tank to rise taller than the water at the middle. The food coloring moved vertically and was dragged along the edge of the tank due to frictional forces from the bottom, forming a circular shape. (Figure 3) The temperature gradient across the tub caused the water to circulate and mimic a Hadley cell. The cold water at the bottom of the tub traveled across towards to the heating pad. The heating pad warmed the water, causing it to rise and travel across the tub near the surface where it cooled and fell to the bottom to complete the circulation. (Figure 4) The rotation of the turntable caused the food coloring to move both vertically and horizontally. The temperature gradient, combined with the rotational instability of the tank caused the food coloring to form several small eddies that rotated in different directions near the edge of the tank. Due to turbulence in the tank, these eddies gradually grew together and formed larger eddies. Acknowledgements
We would like to thank Dr. Joel Norris and Timothy Myers for their generous help on our project. (Figure 5) A model of the Hadley cell.
Experiment Part I Before we started the rotation of the turntable, we observed that the food coloring dispersed horizontally. Once the rotation reached 3.5 revolutions per minute, the food coloring spread vertically. In addition, the coloring formed a large eddy. When the rotation sped up to 5.5 rotations per minute, the eddies were much smaller. Experiment Part II Due to differences in density, the ice pack, on the left, caused the food coloring to sink to the bottom of the tank while the heating pad, on the right, caused the food coloring to stay on the surface of the water. The food coloring slowly began to mimic a Hadley cell with the cold water flowing towards the heating pad on the bottom of the tank and the warm water flowing near the surface towards the cooling pack. Experiment Part III With the ice placed in the middle of the turntable, the food coloring dropped near the container circled around the cylinder and slowly dispersed to the edge of the tank. The food coloring dropped near the sides of the tank moved towards the ice. When the turntable reached 4 rotations per minute, the food coloring added near the cylinder and the food coloring added near the sides were moving counterclockwise, in the same direction as the turntable. The food coloring added near the edge remained on the edge of the tank. When the rotations sped up to 16 rotations per minute, the food coloring formed 7 different eddies in the tank. More eddies are formed when the turntable moves faster. Conclusion
The rotation of the turntable mimics the rotation of Earth. The ice placed in the middle of the tank represents the North Pole while the edge of the tank represents the equatorial region of the world. The eddies created by the rotating table and the food coloring show the instability of the fluid circulation when the table is rotating quickly. The eddies, much like Hadley cells, work to bring warm air from the equator to the poles. In our experiment, we observed the different effects of the food coloring with and without rotation. Without rotation, the gravitational force causes the majority of the food coloring to sink to the bottom of the tank while surface tension causes some food coloring to remain on top. Relating this to the real world, the food coloring that sank represents cold air in the atmosphere whereas the coloring that stayed on the surface is the hot air in the atmosphere. Cold air is denser than hot air, therefore the cold air flows below the hot air. Although this is true, one huge factor is missing: Earth is always rotating. In order to try and more accurately model the Earth, our group performed the same experiment again, but with the rotating table set to rotate. With rotation, the food coloring formed eddies in the water. Based on the hot and cold air explained previously, the eddies formed helped transport hot and cold air between the tropics and the poles. This allowed the hot air from the equator to migrate toward the polar region. When the rotation of the turntable was increased, more eddies formed around the container filled with ice. This represented the Earth’s air circulation system. The Hadley cell brings the less dense, warmer air from the tropics and moves it toward the polar region.