chapter  13
17 Pages

Energy and climate

During daylight, a growing plant is warmed by energy in sunlight. At night, the plant will cool, because heat flows spontaneously from a warm object-the plant-to a cooler one-the air surrounding the plant. The plant loses heat by convection and by radiation. An energy balance equation applies to this plant, just as to any other system:

EIN – EOUT = ESTORED

Here, EIN is the energy provided by sunlight, and EOUT is the energy by convection and radiation. A plant in a greenhouse will also be warmed by the energy in

sunlight. The sun emits energy in many regions of the electromagnetic spectrum. Much of the energy received on the Earth is in the visible region of the spectrum. Ordinary glass is, of course, transparent to visible light; it would be much less useful to us if it weren’t transparent. Energy in the sun’s light helps to warm the contents of the greenhouse. When daylight hours are over, the plant will cool. In this case, though, two factors intervene to retard the cooling process. First, the roof of the greenhouse helps to trap

convection currents of warm air, and keep them inside the greenhouse. Second, and more importantly, the glass helps trap infrared radiation. Any object hotter than its surroundings will attempt to come to

some equilibrium temperature by transferring heat to those surroundings. The region of the spectrum in which a hot object radiates depends upon its temperature. Some objects radiate heat in the visible region, e.g., the heating coil on an electric stove set on ‘high.’ We can see this, and speak of such an object as being ‘red hot.’ Objects only slightly warmer than their local environment do not radiate in the visible region, but instead radiate in the lower energy (longer wavelength) infrared region of the spectrum. The essence of operation of a greenhouse is that, while ordinary glass is transparent in the visible, it is nearly opaque in the infrared. Radiated heat is trapped inside the greenhouse. In a real greenhouse, convection also helps retain heat inside because the roof keeps currents of warm air from rising out of the greenhouse. The energy balance equation also applies to the system of plant plus greenhouse:

E 0 IN – E

0 OUT = E

Here the ‘prime’ signs (0) only signal that we’re writing the equation for a different system. The energy coming in is the same in both systems; it’s the energy

in sunlight. But, E 0 OUT is smaller than EOUT, because some of

the heat has been trapped in the greenhouse by its roof and by the glass being opaque to infrared radiation. If the energy coming in is the same in both cases, but the energy going out has decreased for the greenhouse, then inevitably E

0 STORED must be larger than

ESTORED. Therefore, energy stored in the greenhouse is larger than the energy stored in the plant not in the greenhouse. The observable effect of the larger value of ESTORED for the

greenhouse relative to the plant outside is that the greenhouse is warmer. More energy is stored as heat. We sometimes do greenhouse experiments ourselves. Almost everyone has left a car parked in direct sunlight on a hot summer day with all the windows up. After several hours, the interior of the car becomes hot enough to be painful to touch. The car’s roof and windows trap much of the

heat provided by the sun, by limiting heat loss via convection and infrared radiation, and the temperature inside soars. The ability of the greenhouse to maintain an artificially warm temperature inside is why we build greenhouses in the first place-to provide an environment for growing fresh flowers, fruits, or vegetables in the winter, or, in regions of cool climates, for growing plants that normally require much warmer climates. The sun’s energy reaching the surface of the planet is mostly in

the visible and infrared regions of the spectrum. This radiation is absorbed by the Earth, warming the continents and oceans. The average temperature of the planet, about 15°C, is much higher than the –270°C of outer space. Consequently, the Earth will try to transfer heat spontaneously to the frigid surroundings of outer space. Our atmosphere, like glass, is transparent to visible radiation.