Colorful Convection Currents
Demonstrate convection currents in a very colorful fashion
Cool, crisp, clean mountain air has long been an important reason why people move to colorado. Unfortunately, the air in the city of Denver isn’t quite so clean. By the 1970s, the pollution hanging over the city had a name—the “brown cloud.” Denver’s location at the foot of the Rocky Mountains makes it prone to temperature inversions in which warm air higher up traps cooler air near the ground, preventing pollutants from rising and escaping into the atmosphere. However, the phenomenon of temperature inversion is not unique to Denver, as evidenced by numerous reports of similar brown cloud sightings over major cities throughout the world. This demonstration provides a great illustration of what’s really happening in the atmosphere as hot and cold air masses meet.
- Four empty identical bottles (browse the juice aisle at the grocery store to find bottles similar to those pictured in the book. As a general rule of thumb, the mouth of the bottle should be at least an inch in diameter.)
- Access to warm and cold water
- Food coloring (yellow and blue) or Fizzers coloring tablets
- 3 x 5 inch index card or an old playing card
- Masking tape
- Paper towels
Fill two of the bottles with warm water from the tap and the other two bottles with cold water. Use masking tape and a pen to label the bottles with the words “HOT” and “COLD.”
Use food coloring to color the warm water yellow and the cold water blue. Each bottle must be filled to the brim with water.
The next step can be a little tricky, but with practice you’ll have no problem. Your first observation will be what happens if the bottle with warm water rests on top of the bottle filled with cold water. To accomplish this, place the index card or old playing card over the mouth of one of the warm water bottles (remember, it’s the bottle with the yellow water). Hold the card in place as you turn the bottle upside down and rest it on top of one of the cold water bottles. The bottles should be positioned so that they are mouth to mouth with the card separating the two liquids. Just make sure to have towels close by in case everything doesn’t go exactly as planned.
Carefully slip the card out from in between the two bottles, making sure that you are holding onto the top bottle when you remove the card. Take a look at what happens to the colored liquids in the two bottles.
For the second variation of the experiment, you need to have the warm water on the bottom and the cold water on top. Repeat steps three and four, but this time place the bottle of cold water on top of the warm water. Carefully remove the card and watch what happens.
How Does It Work
Hot air balloons rise because warm air is lighter and less dense than cold air is. Similarly, warm water is lighter in weight or less dense than cold water is. When the bottle of warm water is placed on top of the cold water, the more dense cold water stays in the bottom bottle and the less dense warm water is confined to the top bottle. However, when the cold water bottle rests on top of the warm water, the less dense warm water rises to the top bottle and the cold water sinks. The movement of the water is clearly seen as the yellow and blue food coloring mix, creating a green liquid.
The movement of the warm and cold water inside the bottles is referred to as a convection current. In our daily life, warm currents can occur in oceans, like the warm Gulf Stream moving up north along the American Eastern Seaboard. Convection currents in the atmosphere are responsible for the formation of thunderstorms as the warm and cold air masses collide.
Although the bottles whose colored liquids mix are more interesting to watch, the other set of warm and cold water bottles helps to illustrate another important phenomenon that occurs in the atmosphere during the winter months. During daylight hours, the Sun heats the surface of the Earth and the layer of air closest to the Earth. This warm air rises and mixes with other atmospheric gases. When the Sun goes down, the less dense warm air high up in the atmosphere often blankets the colder air that rests closer to the surface of the Earth. Because the colder air is more dense than the warm air, the colder air is trapped close to the Earth and the atmospheric gases do not mix. This is commonly referred to as temperature inversion.
What are the results of temperature inversion? Air pollution is more noticeable during a temperature inversion since pollutants such as car exhaust are trapped in the layer of cool air close to the Earth. As a result, state agencies in many parts of the country oxygenate automobile fuels during winter months with additives like MTBE in an attempt to reduce the harmful effects of trapped pollution. This trapped pollution is what causes the brown cloud effect. Wind or precipitation can help alleviate the brown cloud effect by stirring up and breaking up the layer of warm air that traps the cold air and pollution down near the surface of the Earth.