On Friday, we talked about why there is uneven heating on the earth and what affects wind. There are 3 main reasons that the earth is heated unevenly:
- The rotation of the earth
- The earth rotates around an axis, and it completes one full rotation each day. At different times of the day, different areas of the earth receive sunlight, which actually brings me to the next factor:
2. The earth’s tilt
- The uneven heating of the earth’s surface also has a lot to do with the tilt of the earth’s axis.
- Since the axis is tilted, different parts of the earth are facing the sun more directly at different times, depending on where the earth is in its rotation around the sun.
- During our summer, the earth is tilted so that the northern hemisphere gets more heat because the sun hits the hemisphere more directly. The southern hemisphere isn’t facing the sun as directly, so it is colder down there…and visa versa for our winter.
The picture above shows how the earth’s rotation and the tilt work in conjunction with each other to cause uneven heating on the earth’s surface.
REMEMBER: DISTANCE DOES NOT AFFECT HEATING!!! The distances are not shown in the diagram; however, during our summer, the earth is actually further away from the sun than in our winter. The distance has nothing to do with how much heat the surface actually gets though; it just depends on the tilt and rotation of the earth.
3. The other factor that causes uneven heating is the difference in heating between land and water. The land heats up quickly and stores heat for a shorter amount of time, while the water takes a longer time to heat up but stores the heat for a much longer period of time.
In class, we demonstrated all 3 factors by putting two beakers with a globe behind them on the counter with a huge lamp shining on them. One beaker had water, and one had soil, and the globe had three temp. readings on it: one at the equator, one at the temperate zone, and one at the polar zone. Everything started out at room temperature. We left them out for the whole class, and at the end of the period, we found that the soil was warmer than the water, and the three zones on the globe all had different temperatures. The equator was 84 degrees Fahrenheit, the temperate was 74 degrees, and the polar was 70 degrees.
The picture is pretty blurry, but you can see how the equator would get the most direct sunlight, and the polar area would get the least direct sunlight. Also, the beakers are a little difficult to see, but they are in front of the globe.
After talking about uneven heating, we moved on to CONVECTION and wind.
In the atmosphere, hot air rises because the heat causes the volume to increase, which makes the density decrease, and the air therefore rises. Cold air does the opposite: the volume decreases because there is less heat, causing the density to increase, and the air sinks. The lateral movement between the hot and cold air is wind, and the entire unit is called a convection cell. We demonstrated this in class by putting a candle under one tube and ice under another tube, and when we connected the two tubes and put the lit stick down the cold air tube, the air sank and moved through the connecting tube into the hot air tube, where the air rose again.
There are 3 main types of convection cells that govern the earth’s wind:
- Hadley Cells–these control the tropics–move counter-clockwise
- Ferrell Cells–control the temperate zones–move clockwise
- Polar Cells–control the polar zones–move counter-clockwise