Introduction

There are two "ingredients" needed for weather on Earth:

1) Temperature Gradient

The Earth receives more sunlight at the equator than anywhere else. Hence, the equator receives the most net energy gain and loses the energy at the Poles. Temperature gradient is created through this process of the heat transfer. 

2) Rotation

Rotation can have an effect on the circulation of the atmosphere. In the tropics (Equator), the Coriolis force (due to rotation) is small, so the Earth's rotation has less effect on the tropics. On the other hand, Coriolis force has more effect in the the middle and higher latitudes. 

Hadley Circulation

The process of the gain and loss of solar heat sets up a general circulation pattern where the air rises near the equator, flows away from the equator until the 30 degrees latitude, sinks near the poles, and the flows back to the equator.This process is called Hadley Circulation. 

The air is not able to move beyond the 30 degrees latitude due to the conservation of angular momentum, especially the zonal velocity of the air must increase. The Coriolis effect is small. 

(Source: http://www.cotf.edu/ete/modules/elnino/cratmosphere.html)

Tank Experiment of Hadley Circulation

In order to visualize the Hadley Circulation, a rotating tank experiment was set up. We define Hadley Circulation as a "low rotation" experiment since the effect of Earth's rotation is small. Place a bucket of ice in the center of the tank, which will represent the cold poles. Make sure to measure the mass of the ice beforehand (Our mass of ice was 675 grams). The equator will be represented at the edge of the tank, since it is warmer than in the center of the tank. Then, place eight thermometers in the tank to get data on temperature gradient inside of the tank. The set-up is shown as below:

(Top-view of the tank set-up)

(Side-view of the tank and thermometers placement) (Christopoulos, 2014)

The tank was rotated at 1 revolution per minute (rpm). Place permanganate potassium crystals and the blue dye in the tank to observe the motion of the dye and crystals. The crystals are used to observe the motion of the fluid at the bottom while the dye is for motion within the fluid. Place black paper dots on the surface of the water to observe the motion at the surface. Use the overhead camera to record the evolution of the tank.

Note that the tank moves in counterclockwise motion in the image below. The crystals move in clock-wise motion, opposite of the tank rotation, but the blue dye moves in the direction of the tank rotation. The crystals represent "Easterly wind" and the blue dye the "Westerly wind." 

Note how the cold water would slowly move out from the center at the bottom (such as Blue #1, #2) and the eventually, the temperature of the outermost thermosisters would drop. The trend of the graph clearly decreases over time. There are minor "bumps" in the graph, which could be attributed to the rotation of the tank. 

Then, we used the thermal wind equation for water, since water in the tank is an incompressible fluid:

where Ω = 0.1046 rad/sec, α = 207 * 10^-6^ K^-1, and dT/dr = 35.7 C/m. du/dz ended up to be 0.34 s^-1 from those values. The surface velocity was 0.054 m/s. The Hadley circulation is axisymmetric, especially in due to thermal wind balance with the radial temperature gradient. 

Atmospheric Examples

Eddy Circulation