Why can Zhuge Liang calculate the east wind in Chibi, but not the rain in Shanggu?

From the novel level, first of all, these storylines are from the Romance of the Three Kingdoms by Mr. Luo Guanzhong, among which Zhuge Liang is full of skill, but some plots need Zhuge Liang to give way. After all, Sima Yi is the final winner. If Sima Yi dies, the history of the Three Kingdoms will almost be completely changed. Luo Guanzhong's novels can't be divorced from the fact that Cao Wei unified the whole country and wrote the Three Kingdoms. Sima Yi can't die because of the need of the plot. The rain in the upper valley is particularly important.

From some scientific points of view, it is an ancient law that it is easier to predict the wind direction at first than the rainfall. Battle of Red Cliffs, it is not difficult for Zhuge Liang to borrow the southeast wind. Zhuge Liang himself has learned some meteorological observations, and the wind direction along the river has a certain rule, which is known to everyone who lives by the river, so it is not particularly strange that Zhuge Liang can calculate the southeast wind.

The rain on the valley is not only a matter of time, but also may be affected by human factors.

Let's look at the topography of the upper valley. Shanggu's name speaks volumes. Its entrance is narrow, its hinterland is wide, its sides are high and its middle is low. This terrain is not conducive to air flow. If there is a fire in the valley at this time, the temperature will continue to rise, and then the air near the surface will be heated and expanded, while the cold air in the upper layer and nearby will begin to sink, thus forming a strong convection, and then there will be a strong wind scene.

When hot air rises and meets cold, it condenses into clouds. At this time, a large amount of smoke and dust produced by the Shu army burning firewood provided an ideal condensation nucleus for water vapor condensation, thus accelerating the condensation of water vapor. The small water droplets in these clouds collide and merge with each other, and the volume will gradually increase, eventually leading to the situation of torrential rain pouring down, which is called convective rain.

Therefore, even if Zhuge Liang knew astronomical phenomena again, these modern geographical knowledge could not be fully understood.