Have you ever thought about where the vegetables you eat are coming from when it is too cold to grow them outside? They might have been shipped from a different country to your town—or they may have been grown locally in a greenhouse. Greenhouses are large houselike structures that are usually made mostly of glass (or clear plastic). How can they protect plants from the cold? In this activity you will find out—and create some extra heat from the sun.
Plants cannot grow everywhere. Their environment has to provide the right conditions for them to survive. Specifically plants need water, air, sunlight and suitable temperatures. In winter months cold temperatures are often a limiting factor for plant growth. This is why the ancient Romans created artificial environments so that they could grow their vegetables all year. Today many vegetables, especially tomatoes, are grown in greenhouses, which cover the plants inside.
Greenhouses work based on a physical principle called “the greenhouse effect.” In a greenhouse sunlight—which is made up of different wavelengths, some of which are in the visible and infrared spectrum—shines through the transparent glass or plastic roof and walls. Only the light in the visible spectrum can penetrate into the greenhouse whereas incoming infrared light, which is also known as heat radiation, is blocked by the glass or plastic. Inside the greenhouse the visible light is absorbed by the plants and soil and is converted into heat, which is then emitted by the plants and soil in form of infrared radiation. Because that heat radiation is blocked by the glass, most of it cannot escape, and the temperatures inside the greenhouse will steadily increase. Even in the winter, temperatures in a greenhouse can be warm enough for vegetables to grow. Want to see for yourself? Take a thermometer and explore the greenhouse effect in this activity! How high do the temperatures get in your “greenhouse”?
Two small outdoor thermometers
Glass jar with lid, big enough to fit one thermometer
A sunny workplace (outdoor or indoor)
Timer or stopwatch
- Find a work area that will have direct sunlight (no shadows) for at least 30 minutes.
- Take both thermometers and place them next to each other in the sun. Make sure that no shadows are cast onto the thermometers throughout the activity.
- Set your timer to five minutes and read the temperatures of both thermometers once the five minutes are over. How high is the temperature? Do both thermometers show about the same temperature?
- Put one of the thermometers inside the glass jar and close its lid. Place it next to the other thermometer again and make sure it is still fully exposed to the sun. How are the conditions inside the glass jar different from the conditions outside of it? What do you think will happen to the temperature inside the jar?
- Let both thermometers sit in the sun for about 20 minutes, then read the temperature again. Did the temperatures change? How? Are the temperatures the same inside and outside the jar? If not, which thermometer shows the higher temperature?
- For a better comparison calculate the temperature difference between the beginning and end of your test for each thermometer. (That is, the temperatures in the beginning versus those after 20 minutes). By how many degrees did the temperature change on each thermometer? Is the temperature difference higher inside or outside the jar?Can you explain your results?
- Extra: Do you think your results will be similar when you place your thermometers in the shade? Repeat the activity but this time put both of the thermometers (inside and outside the glass jar) in a shady spot. Do you get the same results? How do your final temperatures after 20 minutes compare with your results in the sun?
- Extra: Instead of just reading the temperatures in the beginning and at the end, make a time series and write down the temperatures every five minutes for about 20 to 30 minutes. How is the increase in temperature over time different inside the jar compared with outside? Which thermometer shows a faster temperature increase?
- Extra: Does the greenhouse effect work with other materials besides glass? Instead of a glass jar, try one of the thermometers in other kinds of containers. Make sure the containers are transparent so the sunlight can shine through. Does plastic work as well as glass?
- Extra: Have you heard about the greenhouse effect before in the context of climate change? Earth itself is similar to a greenhouse, which traps heat from the sun. Because of this we experience the warm temperatures that we have on our planet. Do some research to find out more about how this works. How is the greenhouse effect in a greenhouse related to Earth’s greenhouse effect?
Observations and results
As long as both thermometers are exposed to the same conditions, they should show the same temperature. There may be some slight variations between different thermometers, but they should not differ more than one or two degrees. Once you place one thermometer inside the glass jar, you place it in an artificial environment. Inside the jar the heat that is generated from the sunlight energy is unable to escape—glass does not allow heat radiation to pass through. Because the heated air inside the jar is trapped, there is also no airflow possible. This means the warm air cannot mix with colder air to cool it down. As a result, the temperatures inside the glass jar should have continued to increase over time. The actual temperature depends on how sunny it was at your workplace.
After 20 minutes you should have observed that the temperature inside the glass jar was significantly higher than that outside the jar. Although the temperature also increases outside the jar, the thermal energy generated by the sunlight escapes through the air when not trapped. Also, cool air from the surroundings can mix with the hotter air around the unenclosed thermometer, which slows the temperature increase outside the jar. Finally, you might have noticed the temperatures inside the glass jar can get pretty high. To avoid this in a real glasshouse, temperatures are often controlled by means of ventilation or even cooling.