Understanding How Greenhouses Maintain Suitable Temperatures: A Guide to Heat Energy Flows

One of the more useful ways to understand scientific concepts is to think of processes not in terms of static amounts but in terms of flows. Let’s examine this idea through the analogy of a river moving through a canyon and how greenhouses manage temperature despite releasing heat energy through their glass walls.

An Analogy of a River Flowing Through a Canyon

Imagine a river flowing through a canyon. The river may flow quickly or it may flow slowly, but as long as the water moving out of the canyon is the same as the rate that water flows in, the water level will be fairly stable well below the edges of the canyon. This is a state of equilibrium.

Now imagine someone builds a tall dam in that canyon. The dam resists the flow of water, making it more difficult for water to get out of the canyon. However, the water arriving is constant. This means the water will build up behind the dam, increasing the height and volume of the river, potentially up towards the edges of the canyon, while the river's flow through the area slows down.

At the base of the dam, at the bottom of the lake, there is a relatively small conduit letting water out through the dam. As the water in the lake rises, there is more pressure to force water through the conduit, and it sprays harder and harder. Eventually, this forms a strong jet, despite the conduit being narrower than the river. This high-pressure state will continue until the rate of water moving out through the conduit is the same as the rate of water moving in to the lake, as long as the river continues to flow at a reasonable rate.

Heat Energy and Temperature: An Analogy to the River and Dam

Heat energy is like this too. Temperature is a measurement of heat energy, much like the amount of pressure building up in the flow of heat through a material. If there is a lot of heat in an object, that heat will be more driven to escape, resulting in a higher temperature reading.

Translucent glass in greenhouses behaves similarly to the dam in the river analogy. It allows energy to move in as visible light but blocks some of the radiative heat, acting as a selective insulator. When sunlight strikes a surface and warms it, the infra-red radiation bounces off the glass, which is not translucent to infra-red. This blocks heat energy from escaping, raising the temperature inside the greenhouse.

Temperature Dynamics in a Greenhouse

Gradually, as the greenhouse resists the flow of heat escaping, the temperature or heat pressure inside the greenhouse rises. Some heat is still escaping, but not as quickly as it is moving in. As the temperature in the greenhouse rises, more heat tends to escape.

If the temperature outside the greenhouse is just below freezing and the sun is shining, the temperature inside the greenhouse will remain above freezing. When the sun sets, the heat takes longer to escape through the glass, so the temperature will still be above freezing by morning.

Factors Affecting Heat Energy in Greenhouses

In the summer, the greenhouse can become very hot, potentially damaging plants. In the winter, if the outside temperature drops well below freezing and sunlight is inadequate, the temperature inside the greenhouse can drop, but it will take longer and not reach as low as the outside temperature. If the night lasts an indefinite amount of time and the temperature outside the greenhouse stops changing, the temperatures will even out, approaching zero difference between the inside and outside of the greenhouse.

A typical backyard greenhouse in a cold seasonal climate might only keep temperatures about 2.5°C (5°F) warmer than the outside temperature by morning. During the day, with strong energy flow into the greenhouse, temperatures can be 12.5°C (25°F) higher. In tropical places with intense sun, the difference could be even higher. However, even a few degrees can make a significant difference for plants sensitive to cold if the greenhouse is in a climate close to what they prefer.

Conclusion

Understanding the dynamics of heat energy flow in greenhouses is crucial for maintaining the optimal temperature for plant growth. By recognizing the similarities between the principle of a river flowing through a canyon and how greenhouses manage temperature, one can better appreciate the intricacies of temperature regulation within these structures.