When Messages Can't Be Read, It's Always Raining

"It's raining again."

Yuki stared out the window. Raindrops were tapping against the glass.

"We probably can't read Mira's message today either," Riku muttered.

Aoi tilted their head. "What do you mean?"

"Well, Mira's doing remote research from overseas, right? When it rains, messages somehow get corrupted."

"Corrupted?" Yuki showed interest.

"Yeah. Normally they're perfect, but when it rains, they suddenly become strange."

Aoi pondered. "That's because channel noise is increasing."

"Channel noise?"

"Radio waves, optical fiber—whatever communication method is used, it's affected by the physical environment. Rain absorbs and scatters electromagnetic waves."

Yuki wrote in the notebook. "So weather affects communication quality?"

"Exactly. In Shannon's theory, a channel has capacity. This is the maximum amount of information that can be sent per unit time without errors."

Riku asked. "When it rains, does that channel capacity decrease?"

"Precisely, when noise increases, capacity decreases. Shannon's formula is C = B log₂(1 + S/N)."

"C is channel capacity, B is bandwidth, S/N is signal-to-noise ratio," Aoi wrote on the whiteboard.

Yuki realized. "When rain increases noise, S/N decreases, and C also decreases."

"Exactly. And when the sender tries to transmit information faster than this capacity, errors increase."

Riku looked at his phone. "Mira sends at the usual speed, but on rainy days the capacity isn't enough?"

"Highly likely. When the communication protocol isn't adaptive, that happens."

"Adaptive?"

"A mechanism that changes transmission speed and error correction strength according to the environment. Many modern systems are adaptive, but not perfect."

Yuki thought. "So how can we read messages even on rainy days?"

Aoi listed three methods.

"First, reduce transmission speed. If kept below capacity, theoretically the error rate can approach zero."

"But it becomes slower," Riku pointed out.

"Yes. Second, strengthen error correction codes. Increase redundancy to withstand noise."

"This also reduces effective information rate," Yuki understood.

"Third, use multiple channels. If one fails, send via another route."

Riku's eyes lit up. "I see. But they all have tradeoffs."

"That's the essence of information theory. Capacity is determined by physical laws. No matter how clever, you can't exceed the Shannon limit."

At that moment, Yuki's phone rang. A message from Mira.

"Oh, I can read it."

Aoi looked at the window. The rain was lightening.

"The noise level decreased. Channel capacity recovered."

Riku laughed. "Who knew weather forecasts would predict communication quality."

"Actually, in satellite communications, transmission rates are adjusted using weather data," Aoi explained.

Yuki read the message. "'Sending today's experimental results. But compressed,' it says."

"Smart," Aoi nodded. "Mira's adapting too. Compressed the information to reduce required bits."

"Compression?"

"Entropy coding. Remove redundancy and send only essential information. This fits within channel capacity."

Riku was impressed. "Information theory is practical."

"Theory is born from reality and applied to reality. Shannon created the theory to solve actual communication problems."

Yuki suddenly thought. "Human conversation might be the same."

"What do you mean?"

"On rainy days, people's speech somehow becomes harder to hear. Because environmental noise increases."

Aoi smiled. "Sharp observation. Acoustic noise also reduces S/N ratio. So on rainy days, people speak louder or slower."

"Strengthen the signal or reduce speed," Riku understood.

"We adapt unconsciously. Biological communication ability embodies information theory principles."

Outside the window, the rain had completely stopped.

Another notification came to Yuki's phone. A detailed data file from Mira.

"Received perfectly."

"Channel capacity fully recovered," Aoi said.

Riku stood up. "Alright, I'll reply now too. Before it rains again."

"Weather is both an enemy and teacher of communication," Aoi said philosophically.

Yuki closed the notebook. "When messages can't be read, it's always raining. But now I know the reason."

"When you know the theory, countermeasures become visible. That's the power of information theory."

The three quietly looked out the window. Light was streaming through the clearing. The channel was open again.