The graph shows a linear decrease in temperature with altitude, which should seem odd to students after some reflection. At higher altitudes, the atmosphere is closer to the Sun and therefore should be hotter. Temperature decreases with altitude because the troposphere is warmed from below, through absorption and re-emission of incoming solar radiation by the Earth's surface, rather than being warmed from above by incoming solar radiation.
Clouds also play an important role.
The amount of incoming radiation is balanced by the amount of outgoing radiation. If the Sun were to decrease its solar output, the Earth would re-emit less radiation, and the atmosphere would cool. This process of cooling of the Earth's surface and corresponding decrease in longwave thermal radiation would proceed until the incoming and outgoing radiation budgets were once again balanced. And the atmosphere would be colder. Students should recognize immediately that no single set of measurements can represent air temperature, especially near zero altitude.