Why Does Dew Form?

Aristotle, the Greek philosopher who had an explanation for most of the common place events that puzzled his fellows, was wise about dew. It is produced, he said, in serene weather and in calm place. Even thought it may seem that the early morning grass is always covered with shoe soaking dew, that isn’t so. Dew is fussy stuff. It comes only in certain conditions, and these usually promise fine weather.

For dew form, the day must be fine and warm, and followed by a still, cooler evening and night, in which the sky must be clear or almost cloudless. If u were to walk on a lawn or into a grassy meadow at twilight, and hold something cold – a hand mirror or tumbler of ice – just above the surface, you would coon find it coated with water droplets: condense vapour.

On a clear night, the ground radiates the day’s warmth into the skies. The ground becomes much cooler, causing vapour to condense. On an overcast night, clouds send that heat back to the ground, co that is never gets cold enough to form dew.

You may wonder why dew form on a lawn and not, say, on the soil of flower beds. It is not grass attracts dew, but that the plants in flower beds keep the ground warmer. You can check this for yourself by shielding a patch of grass from the night sky. You will need a piece of cotton cloth (a tablecloth will do) and some stick or canes. On an evening when conditions seem right, stretch the cloth over the canes about 30 cm (1 ft) above the lawn. Next morning, before the sun has dried things out, you will find that every ware, except beneath the cloth, the shown is covered with dew.

Why, sometimes, is the ground covered with frost? During the night, the air has got progressively colder. Cold air, heavier than warm air, has performed a layer at ground level, turning the water vapour that should otherwise be dew into frost.