Rain

The Tenth Doctor and Martha Jones caught in the rain in New New York. (TV: Gridlock)

You may be looking for the individual or living rain.

Rain, or aqueous precipitation, (TV: The Stones of Blood) was a form of precipitation that came from rain clouds, often connected to sadness. (PROSE: So Vile a Sin) Rain was formed from water at ground level, which then "goes all the way up into the sky, and then falls all the way back down on to me." (TV: Everything Changes) It was frequently accompanied by thunder. (PROSE: Frayed, Loving the Alien)

Rain on Earth and other planets

People rarely went outside while it rained. (PROSE: Pack Animals) Afterwards, stones would look "washed clean" (PROSE: The City of the Dead) and the dirt would turn to mud. (PROSE: The Shadow in the Glass, Loving the Alien) Carl Smithson found the sound of rain comforting "when you were tucked up and warm" in bed, but not so much from nearby. (PROSE: The Shadow in the Glass) Petrichor was the smell of dust after rain. (TV: The Doctor's Wife) Sarah Jane Smith loved "that fresh smell just after a rain shower". (TV: The Android Invasion)

According to Jack Harkness, contraceptives were in Earth's rain due to people flushing estrogen pills down the toilet and therefore into the water cycle. (TV: Everything Changes)

As Amy Pond implied, it rained a lot in Scotland; when it was raining in a car park, she deduced that it must be Great Britain. (PROSE: Apollo 23) It constantly rained in Cardiff, (TV: Everything Changes, Ghost Machine, They Keep Killing Suzie, Kiss Kiss, Bang Bang, Meat, Adam) as well as in Colchester. Once, Ian Chesterton spent "one of the worst weekends of my life" in Colchester with a girl; it rained without end. (PROSE: Byzantium!) On another occasion, rain in Cardiff forced Nettie Williams to hide in a bus stop. (TV: From Out of the Rain) The presence of rain clouds told the Second Doctor that he, Jamie McCrimmon and Zoe Heriot had arrived in England during the summer. (TV: The Invasion) Wellington was also well-known for its rain; hard, torrential rain was in fact dubbed "Wellington rain". (PROSE: World Game) Sun showers — "tiny droplets of cool rain falling down" — were found in rainforests. Io's Kibero Patera tried to imitate this in their rain garden, although Leabie Forrester was reminded more of Little Chalfont, England. (PROSE: So Vile a Sin)

The Joseph Conrad's food was produced hydroponically, and thus no precipitation was truly needed. Nevertheless, the ship's artificial sky rained "on special occasions". The Lacaillan homeworld, however, rained constantly, the species needing the rain to survive. (PROSE: So Vile a Sin) It rained a lot on Grått, forcing the Tenth Doctor to employ an umbrella. (COMIC: The Whispering Gallery) Rain was relatively uncommon on the planet Iwa and, even when it came, it did so extraordinarily briefly. As a result, the air remained fairly saturated and humid. The occasional downpours usually lasted only a few minutes, sometimes even a few seconds. Olmec's theory was that this was because the inhabitant's didn't grow much food; they got most of their food from cans and packages. Chac was Iwa's rain god, who supposedly "didn't visit" the planet often. (PROSE: Frayed)

Sally Sparrow first met Billy Shipton in the basement of Wester Drumlins while it was raining. After Billy got sent back by the Weeping Angels, she met an older Billy during the same rain. Billy died when it ceased raining. (TV: Blink)

Exceptional cases

An unnatural rain cloud (TV: Smith and Jones)

When the Judoon used an H₂O scoop to transport the Royal Hope Hospital to the Moon, the rain went up around the hospital, while no precipitation fell anywhere else. It rained again on the Moon when the hospital made the return trip. (TV: Smith and Jones)

When the TARDIS returned the planet Earth from the Medusa Cascade to the Sol system, it rained a lot for a while on Earth afterwards due to atmospheric disturbances. (TV: Journey's End)

The Doctor's sonic screwdriver was capable of causing rain via atmospheric excitation. (COMIC: Death to the Doctor!, Mudshock)

Clara Oswald's tears turned telepathic snowfall into salty rain on Christmas 1892. (TV: The Snowmen) Once, when Honoré Lechasseur set bottles of vodka free into the wind, it rained alcohol in London for a whole day. (PROSE: The Cabinet of Light)

Predictions

Several species and objects were able to predict rainfall. "Dark clouds" were a good indication that it would soon rain. (PROSE: Pack Animals, Byzantium!)

Bazoolium went cold before it was to rain. (TV: Army of Ghosts)

After the Pyroviles entered the Pompeii area, the soothsayers, the augurs and the haruspex inherited psychic powers and could predict crops and rainfall with absolute precision. The Tenth Doctor claimed that always knowing this would make life boring. (TV: The Fires of Pompeii)

The damages of rain

As Lucius Petrus Dextrus put it, "rain pleases the soil". (TV: The Fires of Pompeii) Rain was necessary to sustain plant life. The Tenth Doctor once said, "A little rain never hurt anyone." (TV: Gridlock) This, however, was most certainly not true.

According to Blon Fel-Fotch Passameer-Day Slitheen, while it was raining, "visibility was low" — this could lead to car accidents, a fatal misstep or loss of electricity. (TV: Boom Town, PROSE: The City of the Dead)

Excess rain could cause flooding, (TV: The Invasion) which in turn might drown someone. (TV: Small Worlds, PROSE: The City of the Dead) Flooding would also turn dirt to mud, which would make travel difficult, particularly on unurbanised soil. (PROSE: The Shadow in the Glass) Usually being incredibly cold, rain could also cause numbness. (PROSE: The Cabinet of Light)

Acid rain, rain that was unnaturally acidic, could "melt" forests. (TV: The Doctor, the Widow and the Wardrobe) A species resembling rain could easily cut through human flesh. When the human colonists arrived on its planet, they drank the water, and were consequently killed by it. The rain would keep on falling until at least one or two people were killed. (PROSE: Pitter-Patter)

References

"A little rain never hurt anyone" (TV: Gridlock) was not the only rain-related expression.

The Eleventh Doctor once used the expression, "It never rains but it pours" to express how drastic bad things always were, such as not only a sinking Soviet submarine, but also a murderous Ice Warrior. (TV: Cold War)

The First Doctor once told a "savage" that he'd have him "right as rain", meaning that he'd fix him. (TV: The Savages) Owen Harper said the same thing to Gwen Cooper after describing how he'd treat the Nostrovite wound that had impregnated her. (TV: Something Borrowed) Mike Yates likewise felt right as rain. (TV: The Green Death)

A period of light rain was dubbed a "soft day". "Anyone for tennis?" was, according to the Fourth Doctor, an English expression meaning "Is anyone coming outdoors to get soaked?" (TV: The Stones of Blood)

Legends

The Sun of Fire and Rain was one of the "Five Suns in the Sky" according to the Aztecs. (TV: The Aztecs)

According to the legends of the Gonds, when the Krotons invaded their planet, they "caused a poisonous rain to fall", killing hundreds of them. (TV: The Krotons)

The Night Travellers were said to "come out of the rain". Indeed, it always seemed to be raining when they attacked. (TV: From Out of the Rain)