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... What
is the origin of the
dew ?
"Dew is
made of water. It forms during the night, when the sky is clear". Everything is summarized in these few words.
Dew is the
nearly magical result of the transformation of water vapor. This
vapor, contained in our atmosphere, metamorphoses itself on a cold support
into droplets
of liquid water. The support can be colder than the ambient atmosphere
because it yields its heat to the interstellar space by diffusing its light, the infra-red waves.
The disordered
thermal agitation of the vapor molecules in the air is transformed into liquid order thanks to the lower temperature and to the organized
structure of the substrate: it is the phenomenon of condensation. The temperature
at which supersaturated water vapor is transformed into liquid water, is
called the "dew temperature".
Drops of rain and fog are born, also, on
substrates, but
on microscopic substrates: dust, sand grains from the desert, spray
of sea water… which are the various germs favoring condensation. We can say that rain and
fog are the dew of the sky.
The importance of the radiative exchanges in the temperature of
the surfaces
where dew forms was suspected to contribute very early in the formation of dew
; this underlying hypothesis is found in the first book on this subject, by William Charles Wells
"An essay on dew, and several
appearances connected with it", published in 1814 in London.
The radiative exchanges have, to simplify
things, two
antagonist actions: heating, by solar radiation, and cooling, mainly by infra-red
emission, that is, the radiation emitted by a material at ambient temperature.
During day, direct or indirect heating by the sun
is greater then radiative cooling. During night, without solar heating, the substrate cools. Obviously, the
greenhouse gases present in the atmosphere, like carbon
dioxide and especially water vapor, limit the infra-red cooling. Cooling can even
be canceled out when the cloud cover is dense : it is for this reason that dew
forms only during the clears nights.
If dew is appreciated the freshness it brings to
the early summer morning, and for the moisture it gives to the plants, even in dry periods,
whilst white frost is
less appreciated when it
appears, generally in October, and makes us scrape the windshields of
our cars before going to work.
If white frost exists, this is because the temperature of
the surface
goes down below zero degree Celsius and that the dew droplets have frozen. These cold droplets continue to grow bigger in the shape
of dew needles of ice
which form the white frost, a true "crystal" of dew. Dew and white frost however
both arise in the same manner and require the same conditions: a clear
night sky, strong radiation, then a fall temperature, a quiet wind and strong
air moisture.
The dew droplets grow according to simple rules, that lead to an astonishing order. When small droplets are born on a surface, they are of very small size, of
order of some millionth of millimeter. They grow bigger by agglomerating the vapor molecules around
them: drops are real pumps of molecules. While increasing
their size, they touch and fuse together – it is the phenomenon of coalescence – and form a new drop, larger and of the same form,
but occupying less place than the two preceding drops. The result is surprising, but it is simply due to the fact that the drops "grow" in a dimension different from
that of the surface, the third dimension. Whatever the sizes of the
droplets, whether they are very small, of dimensions comparable to the
atomic lengths used during the elaboration of thin layers for use in nano-electronics,
or a million times larger such as the drops which we observe with the naked
eye on leaves in the garden or the kitchen window pane, almost half of the surface will remain dry permanently.
This results in "universal" properties for dew, which we will not detail here, but
which makes this model very popular among the scientists who study the statistical properties of matter. And one can even make "jump"
the drops of
dew ! If the temperature of the solid on which dew grows is close to its melting point,
the (latent) heat that the
solid must evacuate so that the disordered vapor molecules "calm down" and
organize themselves in liquid, is sufficient to liquefy the substrate.
The drop truly "jumps" the dance of Saint-Gui.
The dewdrops can even "speak": when they touch and
fusion, they emit a small "cry" which can be made audible.
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Drops of dew on glass
Coalescence of the dew
drops
A coalescing drops of dew
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