"The forecast predicts heavy rain tonight. I've closed the windows."
Mrs. Hudson explains to John as he walks up the stairs. He nods his head, but he knows he will disregard the kindness she has shown to his flat. He cranks open the window in his bedroom. (The front windows are a pain to open ever since Sherlock had them replaced)
John loves when it rains because when it rains, the world makes sense. When the wind is howling full of agony, he understands its pain. The noise makes sense in this desperate dull world.
John falls asleep and wakes to find that it never rained. The forecast was wrong. The world doesn't make sense. He leaves his window open, hoping it will rain another day. It needs to.
Sherlock hates rain… a lot.
It ruins his suits. It ruins his hair. It ruins his train of thought with it's drip-dripping.
So it surprises him one day when he is in New York City (hunting down one of Moriarty's men) when it downpours on him. It surprises him because it was supposed to be sunny all day, but what's the point of meteorologists being good at their jobs. That would be a waste.
He doesn't duck into a restaurant to stay dry or doesn't dart off like tourists to find some stupid tourist place to buy an umbrella or those dreadful ponchos. He stands in the rain and lets it wash over him because for a moment it feels wonderful.
For a moment, it washes away his pain, agony and anything he has felt in the last six months. It cleanses him of the scraps and bruises from Buenos Aires and Hong Kong and the one bullet hole he got in Munich.
There is a crack of thunder and people are scurrying away or opening an umbrella, but Sherlock just laughs.
The rain eventually stops and Sherlock walks calmly into the nearest establishment that would sell suits. It turns out to be a Brooks Brothers. He throws down his credit card and declares he's in need of new suit. Store employees immediately flock to him.
His wet suit is discarded to the floor and Sherlock goes about buttoning his burgundy top and flexing his arms in his new black suit jacket. He looks in the mirror and frowns. The rain didn't wash away everything. He straightens his jacket. It'll have to do. It's not like Sherlock believed in magic anyway.
Rain was just an atmospheric condition after all.