LONDON taxi drivers who are worth their salt don't just drive taxis. They consider it their duty to entertain their clients. This is done with chat. Funds of chat they are - of anecdotes, wise sayings, advices, political opinions, and jokes.Since all chat in Britain starts with the weather, taxi chat is no exception. You: "Terrible day!" He: "Terrible." You: "When d'you think the rain'll stop?" He: "Not this week. And it'll rain all next week as well 'cos I'll be on 'oliday. Just like it. Always rains when I'm on 'oliday." Pause. (He still:) "You know - I've 'eard all the jokes there are about the English weather, but not this one." (You sit forward on your back seat ready not to miss the punchline when it comes). "I 'eard this one just aft er the war." (Why, you wonder, do London taxi drivers, of whatever age, all remember "just after the war"?) "This," he goes on, "was a joke of Bob 'ope's. (What number Duke Street did you say?) 'E was entertaining the Allied Troops 'ere in England. He asked 'em: 'So - wot d'yer think of the weather 'ere? 'E paused and then 'e answered 'is own question: 'Incredible - when the fog clears enough, you can just about see the rain! Both you and he guffaw with suitable immoderacy. "That's very good," you exclaim. "Good, eh?" he says. Actually, it is fairly good. But today it is only half true. The fog half is long part of the past, give or take a day or two each year: It's a measure of Britain's cleanup of industrial pollution that the pea-souper fogs that used to envelop this small island, particularly in its urban concentrations, through long stretches of winter - well, yes, just after the war - have now virtually vanished. British fog is now a thing of nostalgic fantasy. Or Hollywood dry ice. But that fine old British institution, rain, is as present and correct - and wet - as ever. What would a British summer be without much rain, and often? The answer is - unusual. The month of May can be a heat wave. September can be a warm and comfortable "Indian summer." But June, July, and August, which are theoretically known as "summer," are more or less (generally more) months for the rain to show off its special powers. Ralph Waldo Emerson, though American, understood British summer rain perfectly w hen he wrote: "The good rain, like a bad preacher, does not know when to leave off." Dipping into a new edition of Dorothy Wordsworth's "Grasmere Journals," I've been struck with the daily importance of weather - and, inevitably, of rain - to the poet's sister. Other diaries and journals - those of Samuel Pepys or Francis Kilvert, for instance - scarcely seem to notice rain. But Dorothy - and after all Grasmere is in the British Lake District, which is famous for high rainfall - was something of a rain specialist. She doesn't mention it for poetic effect; it carries no romantic symbolism. It's just matter-of-factly there, a kind of weather report, accurately defined, in all its nuances. In the summer of 1800, for example, she notes on June 1: "Rain in the night - a sweet mild morning"; on June 7: "A very warm cloudy morning, threatening to rain"; and again on June 10: "cold showers with hail & rain." The 13th was "a rainy morning"; Thursday the 19th, evening, "There was a gloom almost terrible over Grasmere water & vale - a few drops but not much rain. No Coleridge whom we fully expected." In between the rains are plenty of bright and "fine clear" times. Things were what today's TV weat hermen call "changeable." There is something quietly appealing about the certainty that Dorothy experienced, day by day, just the same sort of unreliable British weather that we have today, 191 years later. It gives you a similar kind of cosy fraternity, with wet days remembered from childhood, days when, trapped indoors with friends, you sat on the windowsill and looked hopelessly out through running window panes and decided the afternoon would have to be spent indoors, playing one mammoth and never-ending game of Monopoly. Even today I can think of nothing to compare with it as a satisfactory way of passing (or wasting) a wet afternoon... . It's chucking it down - drenching - now, with a vengeance, as I write here in Glasgow. Quite beyond the limits of reasonableness, it dripped and soused its way through all of last evening, paused momentarily about breakfast this morning, resumed about 10 and has fallen ever since. It's now almost 3 o'clock and the glass roof of our dining room, under which I am sitting dryly, continues to splatter and stream noisily with a downpouring that sometimes increases to a crescendo and then unbelievably doubles and even trebles its ferocity and fury. But mostly it just goes on at the same pitch an d doesn't let up. The leaves of the magnolia twitch irregularly as the drops bombard them. The curving spears of the flag-iris leaves trickle unceasingly with spangled drops. The pond surface bubbles and rings like a pan of hectic, boiling soup. ACTUALLY, I am not one of those who find rain automatically depressing. Given a choice, I don't go and stand in it. And I'm not as keen as one of our neighbors is to work in the garden under its beneficent descent; she actually prefers it to sunshine. All the same, it does have a certain drippy charm. This seems to be particularly true if you happen to be a duck or a plant. At this moment our three ducks are behaving with a splashy and abandoned relish that a mere human finds a little hard to understand. "Water off a duck's back" is quite literally accurate: It runs off in instantly repelled droplets like beads of mercury. Umbrellas should be made of duck-feathers. Hot sunshine just makes ducks sleepy; dry hard ground yields very little grub, worm, an d beetle food for their foraging, blunt-ended beaks. But let it rain for a day, and happiness is theirs. In the lush mud, their feet slap and slop, their beaks slurp. And there are garden plants that come into their own in the rain Lady's Mantle" (Alchemilla), for example. Raindrops gather and sit around enchantingly on them no less than dewdrops; they are like pearls. Tall summer grass, bending and dripping with rain, also sparkles and drips deliciously. Some flowers go soggy like old dish mops, their colors wash out, and their petal sag and drown. But, apart from being weighted down temporarily by excessive wetness, most foliage simply receives endless amounts of rain with a kind of pleasure. Shakespeare of course had a great appreciation of rain in all its moods, way before the 19th century Romantic poets and their sisters. He knew its endlessness the rain it raineth every day ... with a heigh, ho, the wind and the rain... ." He knew its tempestuous face, soaking to bare skin, Lear and the fool, pitiable or raging. He knew its gentleness - the "quality of mercy" being a two-way metaphor in "The Merchant of Venice." It "droppeth as the gentle rain from heaven," which is as much a depiction of that forgiving art as of a particular kind of amiable raininess. It's intriguing to read Virgil's description of autumn rain in a translation of the "Georgics"; he calls it an element of great force reaking havoc on the farmer's harvest. Hardly a northern view of it. In north Europe, rain is pervasive rather than invasive. It may be muttered about, but essentially it is tolerated. We don't want it desperately like desert people. In the Bible, rain is often the longed-for answer to the drought-parched prayer. My favorite mention is in the King James Version's rendering of one of David's songs in 2 Samuel. It has to do with the character of light that occurs when the rain has cleaned and freshened the air, and then stopped: "And he shall be as the light of the morning, when the sun riseth, even a morning without clouds; as the tender grass springing out of the earth by clear shining after rain." The writer of that knew about sunshine - and he knew about rain. He may have been translating ancient Hebrew, but he was unquestionably British!