What does this paragraph mean? Why is the air sinking over the desert and how does it rise with rain systems thousands of kilometers away?
In fact, it has been calculated that the Sahara Desert actually loses more infrared radiation than it gains solar radiation from the sun. This net loss of radiant energy is balanced by the sinking air over the desert, which warms as it is compressed. The air over the desert is, in turn, being forced to sink by rising within rain systems hundreds or thousands of miles away. Thus, in some sense, rain can be considered the cause of deserts(!).
The paragraph is explaining the process of why air sinks over the desert and how it rises with rain systems far away.
To understand this, it's important to know that air rises and falls due to differences in temperature and pressure. In the case of the Sahara Desert, it has been calculated that it loses more infrared radiation than it gains solar radiation from the sun. This means that the overall energy balance of the desert is negative, resulting in a net loss of radiant energy.
This loss of energy is balanced by the sinking air over the desert. When air sinks, it gets compressed and warms up. So, the sinking air over the desert is warmed because it is getting compressed as it descends towards the surface.
However, the paragraph also mentions that the sinking air over the desert is being forced to sink by rising air within rain systems hundreds or thousands of miles away. This is where the connection between the desert and rain systems comes into play.
Rain systems, such as thunderstorms or larger weather systems, often involve rising air. As the air rises in these systems, it creates an area of low pressure. This low-pressure area then draws in air from the surrounding regions to fill the gap, including air from the desert.
As the air from the desert moves towards the low-pressure area, it starts sinking because it becomes compressed. This sinking air then warms up and contributes to the warming of the desert. So, in a way, rain systems act as the mechanism that forces air to sink over the desert.
In summary, the sinking air over the desert warms up due to compression, which helps balance the net energy loss of the desert. This sinking air is caused by rising air within rain systems far away. These rain systems create low-pressure areas that draw in air from the desert, causing it to sink and warm up. Thus, rain systems can be considered as a cause of deserts.