Q: During a recent hard frost, I noticed that holly leaves tended to have thick frost along their edges but hardly any across their faces. My father suggested that it had something to do with the radius of curvature. Is this true?
A: For water droplets to form in saturated air they need a centre of nuclei upon which to condense. This is provided by dust particles or sharp leaf tips, forming mist or dew respectively. During the winter, when temperatures are much colder, water will condense and freeze at the same time, changing from water vapour to frost. The holly leaf, having sharp edges, provides the nuclei for the water vapour to freeze on, leaving thick frost along their edges but little on top.
A: The upper face of a holly leaf is covered by a thick layer of a wax-like substance, which prevents water loss from the inside the leaf. The wax molecules carry no charge and repel water molecules, which are charged. So any dew drops condensing on the leaf face would run to the edges. Frost, which is simply frozen condensation, would therefore form only at the leaf edges when temperatures dropped below 0掳 C.
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A: The edges of holly leaves, particularly near the spikes, are less vascularised than the faces. This means that they contain less water and therefore cool more quickly and conduct heat less well.