2. Greenhouse effect and cooling effect of the atmosphere

Fig. 2: The distribution of solar short wave radiation.

The energy of the sun is transmitted to the surface of the Earth through short wave radiation, 2% of which is ultraviolet light and other radiation harmful to the human body. It is absorbed by the ozone layer located at the upper region of the stratosphere; another 20% is absorbed by water vapor, clouds and other particles in the troposphere. After this, yet another 35% of the sun's energy is reflected back into space by the surface of the land, the atmosphere or clouds. As a result, only 43% of the energy reaches the land directly and is absorbed (Fig. 2).

The ground will radiate the absorbed solar energy to the sky in the form of long wave radiation. The water vapor, clouds, and other particles in the atmosphere can absorb this kind of long wave radiation so that the surface heat of the land will not be totally lost. Especially when the sky is covered with clouds, the atmosphere can keep ground heat energy from dissipating. This heat-trapping effect of the atmosphere is similar to that of planting in a glass building during cold weather. We call this characteristic of the atmosphere the "greenhouse effect" (Fig. 3).

During Taiwan's winter season, when a cold surge arrives, it comes from originating areas of the continental cold air mass with very low temperature, such as Siberia or Mongolia. The air temperature of the current is much lower than the temperature of the region it has now reached. The temperature decline caused simply by the arrival of colder air is called advective cooling. Furthermore, since there is no solar short wave radiation at night, the long wave radiation on the surface of the land will dissipate heat and further reduce the surface temperature. This is especially true when the sky is clear or covered with sparse clouds. During the period between nightfall to daybreak the radiation effect will cause continuous heat dissipation from the surface of the land to the sky creating a rapid decrease of temperature near the ground. This phenomenon is called radiative cooling. Sometimes the duel effects of advective cooling combined with radiative cooling will result in a very low temperature. The best example of this is during arrival of a cold surge in which the atmosphere is influenced by the cold northwest flow and becomes clear. After sunset the temperature will drop drastically, and on the morning of the next day there will be an extremely low temperature.

Fig. 3: Greenhouse effect. The short wave radiation of the sun can pass through the glass surrounding the greenhouse while the indoor long wave radiation cannot escape. The atmosphere has the same characteristic which we call the greenhouse effect.

Fig. 2:The distribution of solar short wave radiation
Fig. 3:Greenhouse effect