What's the weather like where you are? Chances are there's a cloud somewhere on your horizon - a collection of millions of microscopic water droplets formed as moist air rises, cools and expands. If the cloud is dense enough, it rains. If it's cold enough, it snows. And if it's low enough, we call it fog. But until 200 years ago, people didn't know what to call them and the fluffy things that passed overhead were known by their resemblance to things such as mare's tails or mackerel's scales.
In 1783, huge volcanic eruptions in Iceland and Japan produced spectacular sunsets across the northern hemisphere. Eleven-year-old Luke Howard was entranced by these displays and became a keen student of the young science of meteorology. Twenty years later, he made the first internationally recognised classification of clouds.
Suddenly the sky was full of cirrus (high, threadlike cloud) and stratus (low, layered cloud), nimbus (rainclouds) and cumulus (low, puffy cloud). By combining these Latin names and grouping them by the height at which they occurred, Howard came up with a 10-point system for identifying clouds.
The best known of Howard's top 10 is cloud nine, cumulnimbus, the second highest, which became 20th-century slang for a state of euphoria after it was coined in a 1950s American radio show. Lesser sung but just as uplifting to the eye are banner clouds, which fly stretched out by the wind from mountaintops; wave clouds, common in eastern Scotland, which gather their rippled effect while passing over mountain ranges; and noctilucent clouds, illuminated at night by the moon.
We may be sick of the sight of them after experiencing the wettest spring on record, but drought-stricken countries are crying out for a bit of cloud cover. Scientists in the US and Russia have attempted to create clouds by cloud seeding - dropping condensation agents such as dry ice from aeroplanes to encourage nascent clouds to form.
Artists, too, have found inspiration in the skies - Wordsworth wandered lonely as one, Shelley named a poem after "the daughter of earth and water and the nursling of the sky", while Turner and Constable captured their fleeting likenesses on canvas.
Whether you are under one, on top of one or have your head in one, clouds, like our moods, are ever changing. And perhaps that is the beauty of them.