When I set out for Lyonnesse,
A hundred miles away,
The rime was on the spray,
And starlight lit my lonesomeness
When I set out for Lyonnesse
A hundred miles away.
So begins the lovely poem that Thomas Hardy wrote after the death of his wife Emma, remembering the winter trip to Cornwall where he first met her. Hardy was an acute observer of the natural world. But what exactly was it that he saw on the twigs and branches that winter morning? He calls it rime. I always assumed that rime was a dialect word, or an old-fashioned country word, a synonym for what is more often called hoar frost. But that is not so. Rime and hoar frost are different. Hoar frost happens with deposition: water vapour (gas) turning directly into ice crystals (solid). It tends to happen in bright, clear, still weather and as soon as the sun shines on it, it melts and vanishes. Rime occurs as a result of freezing: liquid water droplets turning to ice when they come into contact with a cold, solid surface. It typically happens on very cold, foggy mornings, often when there is a breeze, so the spiky crystals are clustered more thickly one one side of the branch or twig.
A glazed frost happens when rain falls onto cold surfaces and freezes on contact. If the glaze is hard enough, entire trees become coated. Too heavy to support their own weight, they splinter. Whole forests can be brought down this way (as happened in Slovenia in 2014), not to mention power lines. Streets and paths become ice rinks. In the garden, though, each plant is cased in a sleek, transparent carapace of ice, magnified and beautified.