The Beaufort Rain Scale (after Miles Kingston)

Force 0: Complete Dryness.

Absence of rain from the air. The gap between two periods of wet.
Associated Phrase: "it looks like it might rain."

Force 1: Scotch Mist.

Presence of wet in the air, hovering rather than falling. You can feel damp on your face. but if you supinate your hand, nothing lands on it.
Associated Phrase: "i think it's trying to rain."

Force 2: Individual drops.

Individual drops of rain falling, but quite separate as if they are all freelance and not part of the same corporate effort. If switched on now, windscreen wipers make an awful screeching noise. Spectacle wearers begin to grumble. A newspaper being read outside begins to speckle.
Associated Phrase: "it's spitting."

Force 3: Fine Rain.

Raindrops falling together now, but still invisibly, like the spray which drifts off a fountain with the wind behind. Ignored by all sportsmen except Test cricketers, who dash for cover. Spectacle wearers walk into oncoming traffic. Windscreen wipers, when switched on, make the windscreen totally opaque. If being read outside, a newspaper gets damp.
Associated Phrases: "is it worth putting the umbrella up?" and "another fine rain you've gotten us into."

Force 4: Visible Light Shower.

Hair starts to congeal around ears. First rainwear appears. People start to remember washing left out. Ignored by all sportsmen except Wimbledon players, who dash for cover. A newspaper being read outside starts to tear slightly.
Associated Phrases: "it's starting to come down now," "it won't last," and "it's settled in for the day now."

Force 5: Drizzle.

Shapes beginning to be visible in rain for the first time, usually drifting from right to left. Windscreen wipers are too slow at slow speed, too fast at fast speed. Shower-proof rainwear turns out to be shower-proof all right, but not drizzle-proof. First damp feeling inside either shoes or neckline. Butterflies take evasive action and begin to fly straight. A newspaper being read in the open starts to turn to pulp.
Associated Phrases: "it's really chucking it down now," "it's raining cats and dogs," and "nice for the farmers."

Force 6: Downpour.

You can see raindrops bouncing on impact, like charter planes landing. Leaves and petals recoil when hit. Anything built of concrete begins to look nasty. Eyebrows become waterlogged. Horse racing called off. Wet feeling rises above ankles and starts for knees. Butterflies fly backwards. A newspaper being read in the open divides into two. Gardeners watering the flowers begin to think about packing it in.
Associated Phrases: "it's coming down in stair rods," and "it's bucketing down."

Force 7: Squally, Gusty Rain.

As Force 6, but with added wind. Water starts to be forced up your nostrils. Maniacs leave home and head for the motorway in their cars. Butterflies start walking. Household cats and dogs become unpleasant to handle. Cheaper clothes start to come to bits.
Associated Phrases: "it's pissing down now," and "there's some madman out in the garden trying to read a newspaper."

Force 8: Torrential Rain.

The whole world outside has been turned into an en suite douche. It starts raining inside umbrellas. Windscreen wipers become useless. The ground looks as if it is steaming. Butterflies drown. Your garments start merging into each other and becoming indistinguishable. Man reading newspaper in the open starts to disintegrate. All team games except rugby, football, and water polo called off.
Associated Phrase: "jesus, will you look at that coming down."

Force 9: Cloudburst.

Rain so fierce that it can only be maintained for a minute or two. Drops so large that they hurt if they hit you. Water gets into your pockets and forms rock-pools. Windscreen wipers are torn off cars. Too wet for water-skiing. Instantaneous rivers form on roads, and man reading newspaper floats past. Rain runs UP windows.

Force 10: Hurricane.

Not defined inland - the symptoms are too violent and extreme (cars floating, newspaper readers lost at sea, people drowned by inhaling rain, etc.). So, if hurricane conditions do appear to pertain, look for some other explanation.
Associated Phrases: "oh my god, the water tank has burst - it's coming through the kitchen ceiling," and "i think the man upstairs has fallen asleep in his bath."

This was copied (and slightly modified) from a many-times-duplicated FAX-page which has been making the rounds. It appears to have originated with one of our Cousins Across the Pond, in courtesy to whom I have not entirely removed all references to things like windscreens and Test cricket. If that makes this document incomprehensible to you, check carefully -- it may be raining into your mouth, to your eventual detriment (see a dictionary of American slang for the definition of "frog-strangler").

I have recently discovered another level of rain, documented by Terry Pratchett (who is one of my favorite authors, largely because of his incredibly witty descriptions): "...merely an upright sea with slots in it."