The Greenhouse Effect

In Woodstock College, Md., there were two green houses: one made of wood and another of glass. The wooden house was painted green and named the Green House. In winter, the trees and bushes outside the glass house had no leaves, but inside the plants were green. Inside vegetables and flowers were grown in winter. The greenhouse effect resulted in temperate temperature inside the glass house in freezing weather outside.

The sun equally sheds light and heat on the greenhouse and around it. But while heat absorbed by the surrounding ground is radiated into the sky, the energy entering the greenhouse is partially trapped. Glass transmits electromagnetic radiation (light and heat) but partially blocks infrared or heat. What enters the greenhouse is energy in the form of the visible, infrared and ultraviolet. This energy heats the material in the greenhouse.

The energy that enters the green house is the energy radiated by the sun which has temperature of 6000 degrees Kelvin. The material in the greenhouse is heated to about 300 degrees Kelvin. Thus the spectrum of the energy it radiates is that of a 300 degree Kelvin body. The spectrum of solar energy peaks in the yellow while the spectrum of 300 degree Kelvin peaks in the red.

The ground outside the greenhouse also gets heated, but the energy is radiated to the sky. This radiation to the sky goes on during the night. Glass absorbs infrared and then radiates it in all directions. Thus part of the infrared is transmitted to the sky and part remains in the greenhouse. At night little radiation leaves the greenhouse. These processes keep the temperature in the glass temperate while it is winter outside.

The earth system is a natural greenhouse. The solid earth is surrounded by the atmosphere which contains greenhouse gases (GHG). Like glass, GHG does not transmit all of the infrared. Thus the earth does not cool down drastically at night, and the planetary temperature is 15 degrees Celsius. The variation between daytime and nighttime temperature is about a degree or so in the Philippines. Without the greenhouse effect the earth’s temperature would be about thirty degrees cooler.

The moon has no atmosphere and so at night all the energy received in daylight is totally radiated into space. In addition, the lunar “day” is about 14 earth days; its surface temperature varies from 400 degrees Celsius in daytime to -400 degrees Celsius at nighttime. Mercury has no atmosphere and the day is very long. Its temperature varies from 90 K to 700 K. Mars has very little atmosphere. Its temperature varies from 27 C to -133 in the poles.

Venus is an example of a runaway greenhouse effect. It has a dense atmosphere composed mostly of carbon dioxide. Hardly any heat escapes at nighttime. The temperature is 480 degrees Celsius.

Car owners have greenhouses without knowing it. Temperature in a car parked in the open under the sun with windows closed get very hot. If the owner left the windows open, he would find it cooler, if it had not been stolen.

The largest greenhouse is found in Eden Project in Cornwall, England. Giant geodesics which use a special plastic instead of glass house the world's largest rainforest in captivity with steamy jungles and waterfalls.

Energy can be conserved if architects made buildings that do not absorb heat. The UP administration building is like an oven because it has a north-south orientation. This would be good where there is winter. But in the Philippines it is best to have buildings with an east-west orientation. It is good also to have small windows. And infrared reflection film should be attached to the glass surfaces.

Ninety-nine percent of the earth’s atmosphere is nitrogen and oxygen. GHG are in the remaining one percent. In descending order, the most abundant GHG in the Earth's atmosphere are: water vapor, carbon dioxide, methane, nitrous oxide and ozone.