Historical Miscellany #23 – Dew, Aristotle’s Explanation [c.350BC]

dew

To fit this scorching summer weather I’ve picked a meteorological snippet for this week’s miscellany. In the fourth century BC the Greek philosopher Aristotle wrote a fascinating tract, Meteorologica, explaining a wide variety of atmospheric phenomena, as well as earthquakes, shooting stars and other mysterious curiosities. His theories held for centuries after his death, although today almost all of them have been discredited. This is what he had to say about the formation of dew.

Any moisture evaporated during the day that does not rise far because the amount of the fire raising it compared to the amount of water that is being raised is small, falls again when it is chilled during the night and is called dew or hoar frost. It is hoar frost when the evaporation is frozen before it has condensed into water again; this happens in winter, and more readily in wintry places than elsewhere. It is dew when the vapour has condensed into water and the heat is not so great as to dry up the moisture that has risen nor the cold so intense as to freeze the vapour, either because the district or the season is too warm. Dew tends to form rather in fair weather and mild districts; hoar frost, as said, under opposite conditions. For it is obvious that vapour is warmer than water, as it still contains the fire that caused it to rise, and so needs more cold to freeze it. Both dew and hoar frost form in clear calm weather, and no condensation is possible in wind.

(More more miscellany here)

Image credit: Matias Erhart

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