"Can you guys actually do that too?" Hermione blurted out. She couldn't take her eyes off the window, where the rain was slamming against the glass with brutal force. She had memorized every single page of her first-year books, but nothing like this was mentioned anywhere. Modifying the weather on that scale, without a wand and without shouting a Latin incantation, was insane. What Harry had just explained... sounded way more like her muggle primary school science classes than fairy tale magic. "It just doesn't make sense. It's pure thermodynamics, Harry, but mixed with... I don't know, something that breaks all the rules."
Susan and Hannah looked at each other and then shook their heads, their mouths still slightly open in shock.
"Are they going to teach us this at school?" Hermione asked, sounding almost desperate. If Hogwarts didn't teach stuff like this, she felt like her precious new books were completely useless.
The other two shook their heads again, wearing frustrated grimaces.
"No way, Hermione, forget about it," Susan sighed, shifting in her seat. "At Hogwarts, you just get the usual: charms to make feathers float, basic transfiguration, potions... What Harry did is completely outside anything the Ministry approves of."
Hermione took a breath, ready to unleash another question from her endless arsenal, but the compartment suddenly went dark. The sliding door creaked open and two older students stood in the doorway. They wore pristine robes and shiny prefect badges on their chests; she was Ravenclaw and he was Hufflepuff. They looked dead serious, scanning the place as if looking for trouble.
The silence became super awkward. Hermione shrank back into her seat, but Susan relaxed the second she saw their faces clearly.
"Anna? Elias? Hey," she greeted, letting out her breath.
The drill-sergeant act the prefects were pulling instantly crumbled. They grinned ear to ear at the niece of the Head of Magical Law Enforcement.
"Sue!" they both said at the same time.
They stepped in quickly, and Elias, with a casual flick of his wand, closed the door and cast a silencing charm. It was obvious they didn't want anyone seeing them there.
"Come on, Sue, introduce us, won't you?" Anna asked, crossing her arms but wearing a curious little smile.
"Well, you already know Hannah," Susan said, pointing to her braided friend. "And they are Hermione Granger... and Harry Peverell."
Both prefects froze, the air leaving their lungs. Elias went completely rigid, staring intently at Harry's ring finger. In the center of the ring was a raven carved into black stone. Suddenly, the silhouette's red eyes seemed to flash, and Elias's mind went blank. For a second, a terrifying chill gripped his chest and a smell of blood so real hit him it made his stomach turn. He snapped out of the trance with a gasp as his twin sister delivered a criminal elbow right into his ribs. Elias blinked, pale, rubbing the back of his neck.
Out of pure survival instinct to avoid trouble with an Old Family, they both bowed their heads.
"Lord Peverell," they said, swallowing hard. At their age, the title felt heavier than usual in their mouths.
Elias cleared his throat, trying to get his prefect posture back, and looked at Susan with an urgency he couldn't hide.
"Susan... please tell me you know who caused that storm outside. The prefect carriage is a total mess, everyone's looking for whoever did it. That weather magic... well, it would be amazing for our family's greenhouses. We're willing to make a formal trade: five thousand galleons and a hundred types of rare magical seeds if the creator passes us the method."
Susan stayed quiet. She looked at Harry, waiting to see what he'd do, while Hermione's eyes went wide as saucers as she did the math in her head. 'Five thousand galleons? That's nearly 25,000 pounds sterling.' She almost fell off her seat right there in the Express.
Harry nodded, a short, quick gesture.
"Harry, their family is a pretty big deal around here," Susan whispered, leaning toward him. "They grow rare magical plants and supply almost everything to Diagon Alley."
Harry stood up, stripping all the mystery away from the situation.
"Hi, I'm Harry," he said with a light smile, extending his hand. He gave Elias a firm handshake and then greeted Anna with a polite nod, skipping any over-the-top ceremony.
"The rain outside is mine," he continued, sitting back down and shrugging his shoulders. "I'm down for the trade, but what exactly do you need?"
"Just the arithmancy is more than enough," Anna jumped in, her eyes shining with pure studious excitement.
Harry got comfortable, pulling a crumpled piece of parchment and a pen from his jacket. He stared at the paper for a few seconds, figuring out how to put into words what he did purely by instinct.
Then, he started writing. Anna, Susan, and Hermione immediately crowded around him to spy, practically pushing each other.
"Looks like they're gonna finish before I even get back," Elias muttered, pocketing his wand. "Go ahead and review that. I'm heading to the prefect carriage to grab a magical contract to lock this in."
Hannah just nodded. She pulled a chocolate frog out of her bag and watched the map of letters unfold as Harry wrote in a clear but hurried handwriting:
Controlled Atmospheric Condensation
Core Variables:
M = Available Mana η = Channeling Efficiency H = Ambient Humidity T = Surrounding Temperature A = Target Area C = Condensation Level Ccrit = Precipitation Threshold
Harry paused for a split second and dropped the first mathematical line:
Cost = (A * H) / η
(Mana cost depends directly on the area and available humidity. Higher efficiency means less waste).
The pen moved faster now:
C = M * η * (H / T)
(Condensation can be forced by increasing vapor density. Lowering the temperature increases the process's stability).
Harry tilted his head, reviewing what he'd put down with a slight frown, and added:
Rain = Θ(C - Ccrit)
(Once the critical point is passed, precipitation happens inevitably. You don't "create" rain. You allow it to happen).
At the bottom, he drew a perfect circular diagram representing the core of the spell.
(The runic circle doesn't execute the effect. It only stabilizes the variables and maintains the relationship between them).
(Common mistake: trying to increase M instead of improving η. Result: wasted energy and loss of control).
Harry left the pen on the table and looked at the final equation:
Rain(A,t) = Θ(((M * η * H) / T) - Ccrit) * e^(-λ * A) * (1 - e^(-κ * t))
Anna leaned over the paper, her eyes wide with awe just as her brother walked back in with the contract. Hermione and Susan were practically on top of her.
"Wait, Harry," Hermione blurted out, covering part of the parchment with her finger. "Does this Θ function act like a switch? Meaning... the magic doesn't do anything until the physical environment reaches its absolute limit?"
"Exactly," Harry said, crossing his arms and leaning back into his seat. "If you try to force rain without dropping the temperature (T), the mana just dissipates. It's basic applied physics, nothing more to it."
"And what about this λ letter down here?" Susan asked, sparked by curiosity. Her cheeks were slightly flushed with excitement. "They teach us such boring arithmancy, none of it has this level of detail."
"It's the dispersion limit," Harry explained calmly. "The larger the area (A), the harder it is to keep the vapor dense. If you don't calculate the efficiency (η) right, the storm falls apart before it even hits the ground."
"This is brilliant!" Anna gasped, devouring the formula with her eyes.
Just as Hermione was about to ask something else, the compartment door banged open for a second time.
A pale, sharp-nosed blonde boy marched in arrogantly, flanked by two large boys who looked like gorillas. Draco Malfoy had a smug smirk on his face, ready to deliver some rehearsed speech about who his father was.
But he didn't get a single word out.
The moment the door opened, Anna, Susan, and Hermione snapped around at the exact same time, glaring at him with looks so fierce they could kill for interrupting the explanation. The Ravenclaw prefect narrowed her eyes dangerously; Susan looked at him with a coldness no one knew she had; and Hermione looked ready to use the heaviest book she owned as a blunt weapon. Even Hannah stopped mid-bite into her chocolate frog to give him a nasty look.
Draco froze dead in his tracks. He looked at the three girls who looked ready to make him disappear, looked at Harry who hadn't even moved from his seat, and suddenly all his bravery evaporated. He swallowed hard, took a rather clumsy step back, and slammed the door shut, rushing down the corridor with his two shadows hot on his heels.
Outside, the loud, clear laughter of the Weasley twins could be heard as they walked past the hallway.
The rest of the trip absolutely flew by. Harry spent it explaining the doodles, leaving Hermione with a notebook full of notes and a massive grin.
When the Hogwarts Express finally began to brake, signaling their arrival at Hogsmeade station, the Silvaris siblings stood up to say goodbye. They signed the magical document that sealed the trade of the seeds and the money. Right before stepping out, Anna turned around and, with a quick movement, slipped a small, satin card with silver edges into Harry's hand.
On it, only one name was written in strange, gothic letters: Grau Zauberer.
"What's this?" Harry asked, flipping the card between his fingers.
"It's an independent group, Lord Peverell," Anna murmured, lowering her voice as the train came to a final stop with a metallic screech. "We dedicate ourselves to studying real magic—the kind they don't teach in class. If you're interested... look for me at the castle."
With a quick nod and a secretive smile, the Ravenclaw prefect vanished alongside her brother into the crowd of students beginning to step out onto the dark platform.