Right outsideWeather
Hoarfrost: ice crystals growing on twigs, wires and weed stems
When: November to February
On the right cold morning, water vapor turns straight into ice without ever being liquid, and it builds interlocking crystals on whatever is thinnest and most exposed. Fence wire furs over while the ground stays bare.
Why this time of year
It needs air whose dew point is below freezing brought to saturation by cooling, with the object itself below freezing. Late autumn and winter clear, still nights are when that lines up here.
How to notice it
- Go out early, before the sun gets on things. It goes fast.
- Look at the THIN stuff first: wire fences, bare twig tips, dead weed stems, railings. Small diameter and freely exposed to the air is the rule.
- Get close enough to see that these are interlocking crystals with structure, not a dusting of white.
- Compare a thin twig against a fence post or the ground beside it. The thin thing wins, every time, and that is the whole tell.
Try this with kids: Find the thinnest thing in the yard. Is it furrier than the fat things next to it?
sublimation is water vapor turning straight into ice without being liquid first, which is how these crystals build
Sources
- forecast.weather.govNational Weather Service: hoar frost is a deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires and poles; it develops when air with a dew point below freezing is brought to saturation by cooling, and the temperature of the frosted object must be below freezing. Frost generally is thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers or fans
In the guide for November, December, January, February