The Romantics, Tate Britain, London

Nowadays, being romantic is saying it with flowers or dropping off a couple of diamonds in the middle of the night.

Not so long ago, the word was used disparagingly: being romantic was to be flamboyant, self-obsessed, unconventional and a dreamer. Tate Britain's new display, The Romantics, makes it clear that the originals weren't limp-wristed fops or bohos wearing frilly shirts, but radically disaffected young men trying to break free from the constraints of their fusty forefathers and the stoic rules of high classicism.

The very first Romantics were 18th-century writers: Europeans such as Goethe, who applied the label to "everything that is dark, absurd, confused, incomprehensible", or Rousseau, who pined for "torrents, rocks, woods, mountains, steep roads to climb or descend, abysses beside me to make me afraid". Among the poets and composers who followed suit, it was our painters who perhaps best epitomised Wordsworth's "I wandered lonely as a cloud ...". Man's insignificance in the face of nature became a central touchstone for Romantic art, indeed this exhibition makes visible the power and primacy of landscape in biblical deluges, shipwrecks and storms every bit as apocalyptic and prophetic as anything in Al Gore's An Inconvenient Truth.

The Clore galleries at Tate Britain have always been dedicated to the dramatic, if occasionally rose-tinted, visions of that landscapist extraordinaire, JMW Turner, but Tate has ripped up its own rulebook to mix in some of his kindred spirits, beginning with his great rival, John Constable, who described painting as "another word for feeling". Constable's attempts to recreate picture-perfect idylls of the Stour Valley in his studio resulted not so much in mirrors of his own mind or moods, but in hyper-detailed and contrived confections of dappled light and shade. While he often laid his particular brand of Terry's All Gold or Cadbury's Milk Tray on with a trowel, Constable was more exciting (and romantic, in a robust way) when sketching en plein air: a frosty morning over a field or women gleaning corn forcefully suggesting the truth to nature that he set out to distil. And nobody painted lonely clouds quite like him.

At least Constable was consistent (if a little dull); Turner could be a raging contradiction – romantic visionary and canny marketeer – ruining a riotously romantic sunset by plastering a lacklustre figure of Napoleon on top and pandering to Victorian tastes for drowning lovers and other sentimental tosh. Despite a few duds, Turner still forms the spine of these nine rooms at Tate Britain and there's never a bad moment to revisit his late ocular feasts, where sunrises blast holes into his canvases, tearing the visual asunder. Forget Romanticism, this is Abstract Expressionism, Minimalism and Colour Field painting, all in one.

There is much of the "ism" to wade through, of course: disparate journeymen and second-string Romantics who continued in the wake of a movement that was all but spent by the 1850s (I'm thinking here of mad Richard Dadd, the boy from Bedlam, and Thomas Webster's sickly street urchins among other makeweights). If the famous painting of Thomas Chatterton's suicide by Henry Wallis was literally Romanticism's last gasp in 1856, at least its still-beating heart is also well represented here. Poet and engraver William Blake, a contemporary of the tragic Chatterton, is still a hero to disenchanted artists everywhere, having shunned industrial progress and turned inwards to reveal his unfettered freedom of expression: "I must Create a System, or be enslav'd by another Mans," he raved. Blake was a moralist and contrarian, who clearly loved the use of capitals in his many pronouncements: "Copiers of Nature are Incorrect, Copiers of the Imagination Correct". Tate shows off its eight recently acquired drawings (found in a railway timetable), each a blaze of colour, chastisement, birth, pain and hellfire. The shackles were well and truly off.

The closest we get to an heir to Blake, visually, is in the Edenic country views of Samuel Palmer, but the 20th century had its fair share of Neo-Romantics as well, including such towering figures of landscape art as Graham Sutherland and Paul Nash. However, the final room in The Romantics is reserved for some oddly downbeat black-and-white photographs, depicting an age "after the picturesque". Presumably Keith Arnatt's sad take on the demise of the cult of nature – a series from 1982-84 ironically titled A.N.O.B. (Area of Outstanding Natural Beauty)– is a poignant endpoint to the idea that an earthly paradise can reflect man's highest attainments in creativity. Certainly it leaves us feeling forlorn, under a familiar grey sky.

Ossian Ward is visual arts editor of Time Out