Rain Belts and Deserts: A Latitude Video Exchange

Pen Pals Along One Line on the Map

1Mr. Farid Rahimi's sixth-grade class at Pinecrest Middle School joined a video exchange with students in five lowland towns. The towns are made up, but their data are based on real places. All five lie near the same line of longitude, spread from the equator to 60° north (60°N). Each partner class shared long-term yearly averages for its town.

2Near the equator, sunlight strikes the ground from high in the sky for most of the year, so its energy is packed into a small area. Closer to the poles, sunlight arrives at a low slant and spreads over a larger area. When air is warmed, it expands and becomes less dense, so it tends to rise. Cool, dense air tends to sink. Rising air cools, and some of its water vapor can condense into clouds and rain. Sinking air warms and becomes drier, so clouds seldom form in it.

3To model how uneven heating moves a fluid, student Noor Haidari and her group filled a clear tank, 60 cm long, with room-temperature water. An adult set a sealed cup of warm water against one end of the tank, and the group floated a sealed bag of ice at the other end. They put one drop of red food coloring near the bottom at the warm end and one drop of blue food coloring near the surface at the ice end. The red color rose, then spread along the surface toward the ice. The blue color sank, then crept along the bottom toward the warm end.

4Next, the group changed how warm the water in the cup was and timed how long the blue color took to travel along the bottom from one end of the tank to the other. They used the same tank, the same amount of water at the same starting temperature and the same size of ice bag each time, and they ran three trials for each cup temperature. On the real Earth, flows of air and water are also bent by the planet's rotation: in the Northern Hemisphere, air and water that travel long distances curve to the right.