If one were to analyze the heart, Meng Jingnian was undoubtedly the least suitable subject.
Lin Qin fell silent for a moment, then proactively changed the subject: "Little uncle, have you finalized your thesis?"
"I originally planned to revise it, but left it for two days and couldn't be bothered to touch it again. I'll deal with it after the pre-defense."
"You procrastinate too?"
"Why wouldn't I?"
"I always thought you were a perfectionist."
"Perfectionism isn't a virtue." Meng Jingnian smiled faintly. "When energy is limited, you can't afford to worry about perfection."
"I was worried that coming to see you before your defense would waste your time. Hearing you say that puts my mind at ease."
"It's fine. It's only an hour or two at most—I can always find the time no matter how busy I am. First, tell me—what kind of project is it exactly?" The last stack of books was unwrapped, and Meng Jingnian joined in the sorting, placing some rarely-read volumes on the top shelf of the black walnut bookcase.
The boy who had blocked Lin Qin at the classroom door on Thursday was named Yan Mingxuan, a junior in the School of Atmospheric Sciences. As he had said, their team competition project was halfway done when one of the members bailed.
After Lin Qin had some free time, she met with him once, looked through their source code, and ultimately took over the mess.
"It's a small program whose main function is a weather visualization system. The part assigned to me is the weather prediction module."
"How far along is it?"
Lin Qin placed the few books in her hands onto the shelf, brushed the dust off her palms, took out her laptop from her backpack, set it on the desk, opened the developer console, and pulled up the user interface to show Meng Jingnian.
Meng Jingnian set down his book, walked over to her side, rested his palm on the edge of the desk, and leaned down to look at her screen.
A faint warmth, along with a clean scent, drew near. Lin Qin's fingers curled slightly, then casually relaxed again.
"Can I interact with it?"
"Yes."
She nudged the laptop toward Meng Jingnian, and he tapped the touchpad, testing the basic functions.
Lin Qin couldn't help but stare at his hand—the distinct knuckles, the faint blue veins visible beneath the fair skin of his hand.
She couldn't recall exactly which year it was when Meng Jingnian became her big friend she could talk to about anything.
But realizing that her dependence on him had shifted into something else came in her senior year of high school.
Even to this day, she still tried to hypnotize herself into believing that her feelings for Meng Jingnian were perhaps merely an unfounded closeness toward a friend or family member.
But toward a friend, she wouldn't want to hold that hand to test whether it was cold or warm; she wouldn't want to press her forehead against the fabric of his shirt; and she certainly wouldn't break into a sweat across her palms just from catching his scent.
"The visualization is well done—clean and straightforward." Meng Jingnian asked, "What kind of help do you need from me?"
Lin Qin snapped back to attention: "…For the weather prediction model, I'm not sure where to start."
"How are you currently obtaining your data?"
"We scrape data from several websites, including the meteorological network and satellite cloud imagery sites."
"The meteorological network provides corresponding forecast results. Adding those into your existing module shouldn't be difficult, should it?"
Lin Qin shook her head. "Our project is registered under the artificial intelligence track. Just scraping data won't meet the registration requirements."
Meng Jingnian thought for a moment. "What kind of functionality do you hope to achieve?"
"Building my own algorithmic model based on neural network algorithms."
"Do you have accuracy requirements? Is it short-term or mid-term forecasting?"
"Forecasting results within one day to one week. Naturally, the higher the accuracy, the better."
"Short-term and mid-term forecasting are actually two different research directions." Meng Jingnian chose his words carefully. "Yiyi, I absolutely believe in your abilities, but to achieve what you're describing—independently constructing an AI algorithm and achieving weather forecasting as accurate as possible—might be beyond the scope of an undergraduate. In fact, this is a difficult problem that the world's top AI teams are currently tackling."
Lin Qin wasn't discouraged. "Can you tell me specifically where the difficulties lie?"
"Explaining it might be a bit dry."
"That's fine. The more I understand, the better I'll know how to approach it—if it's not taking up your time."
"It won't be." Meng Jingnian said with a smile.
He closed the laptop, handed it back to Lin Qin, and picked up a stack of A4 paper and a red ballpoint pen from beside him.
"The atmosphere we usually talk about is actually composed of seven physical quantities: pressure, density, wind speed, temperature, and humidity." Meng Jingnian wrote down the abbreviations of these physical quantities on the blank paper.
"Seven?"
"Wind speed has three directions—z, v, and x—so that brings the total to seven."
Lin Qin nodded, indicating she was following along and he could continue.
"The equation of state, thermodynamics, the moisture equation, the continuity equation, and the equations of motion based on fluid dynamics—this is a vector equation that can be broken down into three separate directional formulas—that makes seven equations in total, forming the fundamental system of equations for atmospheric motion. Theoretically, as long as you know the initial values of the seven physical quantities mentioned above and plug them into the system, you can calculate the state of the atmosphere at any given moment."
Lin Qin stared intently at the board work Meng Jingnian was writing on the blank paper. Perhaps to keep up with the pace of his explanation, his handwriting was a bit more hurried than usual, but it was still elegant.
"Is this the principle behind modern weather forecasting?" Lin Qin asked.
"Not entirely. Have you studied calculus?"
"Yes. Mathematics is one of our core courses."
Meng Jingnian nodded. "Of these seven equations, five are nonlinear partial differential equations. Solving nonlinear partial differential equations is a recognized academic challenge."
Lin Qin nodded. "The three-body problem is a nonlinear partial differential equation."
"The three-body problem can't be solved even with just three point masses, and atmospheric motion is an extremely complex chaotic system with countless point masses."
"The so-called butterfly effect?"
"You could think of it that way."
"Then how is weather forecasting done today?"
"Currently, the mainstream approach to weather forecasting is numerical simulation—discretizing continuous space and time, dividing space into grids according to latitude and longitude…"
Perhaps worried it might be hard to understand, Meng Jingnian took another blank A4 sheet. "Take the heat conduction equation as an example. The traditional method is to find the solution, plug in the time, and you can get the temperature at any given moment. But the numerical simulation approach selects a finite number of points and calculates subsequent temperatures by stepping forward at time intervals."
While demonstrating the calculations on paper, Meng Jingnian glanced at Lin Qin.
During his PhD, he naturally had plenty of opportunities to serve as a teaching assistant for his advisor—especially since his advisor was also the dean of the department, juggling administrative duties and other non-academic work on top of teaching, stretched far too thin. When Meng Jingnian had been a TA, he'd observed the students in the department; roughly only a third were actually paying attention. He'd been a student himself, so he wasn't surprised, nor did he care to interfere with who was sleeping or who was openly scrolling through their phone.
Though he would inevitably end up on the teaching track himself, he knew deep down he wasn't really suited to be a teacher, because at his core he harbored a certain distaste for laziness and stupidity, while the fundamental quality of a teacher was to teach without tiring. He wasn't a patient person—he was just good at pretending to be one.
But Lin Qin was more attentive than even the most focused student in his classroom, as if what he was saying was truly that fascinating.
Meng Jingnian's mind wandered slightly, until Lin Qin looked up at him, and he continued: "…To achieve accurate numerical simulation weather forecasting, you need an enormous amount of computation. The IFS system at the European Centre for Medium-Range Weather Forecasts takes three hours for a single forecast, requiring 1.5 billion operations."
Lin Qin naturally understood what that meant—to do the same computation on her laptop would probably take a hundred years.
"Limited by computational power, artificial intelligence is indeed the direction for future breakthroughs. Nvidia just released an AI weather model recently—have you been following it, Yiyi?"
Lin Qin nodded. "FourCastNet. But from what I've read, although it has high spatial resolution and fast computation speed, its accuracy isn't great. The reason seems to be that weather forecasting is a three-dimensional system, while FourCastNet was only trained on two-dimensional data. I looked up some materials, but I still haven't figured out where the difficulty of the three-dimensional aspect lies."
"Perhaps it's because the vertical levels in three dimensions are isobaric surfaces rather than constant-height levels. Isobaric surfaces are affected by latitude and specific altitude without a fixed pattern, making them difficult to correct once converted into data. The challenge is figuring out how to use algorithms to eliminate such errors. Of course, that's just my assessment—on the algorithmic side, that's the domain of AI, which isn't my strength."
Meng Jingnian set down his red ballpoint pen.
Lin Qin looked at the two full pages of content. She was someone who barely listened even in her own major's classes, yet she'd listened to Meng Jingnian's entire lecture from start to finish. "…I really did oversimplify the problem."
"I believe you'll definitely solve this challenge in the future and build a perfectly accurate AI algorithm model."
"When I publish in Nature, I'll let you be a co-author."
"Then I'll look forward to it." There was a smile in Meng Jingnian's voice.
Lin Qin let out a small laugh as well.
Meng Jingnian caught that fleeting smile and couldn't help but feel a bit surprised. Lin Qin wasn't someone who smiled much. Though she wasn't hard to get along with, her perpetually expressionless face was enough to deter anyone who tried to approach her.
She was beautiful when she smiled—her eyes curving slightly, carrying an extra touch of youthful innocence, as if this was how she ought to look at her age: like most young women, vibrant and light.
The sunlight outside had gradually taken on a caramel hue, the prelude to a sunset.
Meng Jingnian glanced at the time—it was almost five o'clock—and only then realized he had been a poor host. "We've been talking for a while, and I haven't even asked if you'd like something to drink."
Lin Qin blurted out, "Wasser, bitte. Danke." (Water, please. Thank you.)
Meng Jingnian paused.
"…Did I say it wrong?"
"No, it was perfectly standard. You've studied German?" Meng Jingnian said as he walked toward the kitchen.
Lin Qin unconsciously followed him. "Earlier, Didi mentioned she was going to visit you in Germany, so I picked up a few simple phrases on the fly."
"What else can you say?"
"Tschüss, ja, nein… Kaffee, Wein, Brot, and some basic beginner vocabulary, and also…" That sentence ran through her mind, searing her nerves like a red-hot iron, and she immediately stopped speaking.
"And also what?" Meng Jingnian pulled open the refrigerator door.
Lin Qin shook her head. "…Nothing. I can't remember anything else."