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.
你有没有想过，当你吃的蔬菜太冷，不能在外面种植的时候？ 它们可能是从一个陌生的国度运到你的城镇，又或者它们是在温室里种植的成果 温室是通常由玻璃（或透明塑料）制成的大型房屋式结构。 那么它们如何保护植物免受寒冷？ 在这项活动中，你会发现并创造一些来自太阳的额外的热量。
植物不能生长在任意的地方，它们的生长环境需要给它们提供恰当的条件以供存活。 特别的是，植物需要水， 空气，阳光和适宜的温度。 在冬季，寒冷的温度往往是植物生长的限制因素。 这就是古罗马人创造人工环境的原因，这样他们就可以全年种植蔬菜。 今天很多蔬菜，尤其是西红柿， 生长在庇护着植物的温室里。
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
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.