Where does Dew Water Come From?

If this were a multiple-choice test it might look like this.

Where does dew water come from?

  1. from the soil
  2. from the plants
  3. from the atmosphere
  4. from all the above
  5. from none of the above

It brings back memories?

The correct answer is “from the atmosphere”.

If it comes out of the plants we would call it gutation.
If it comes out of the soil we call that distillation.
If it comes out of the atmosphere we call that condensation.

To paraphrase Steve Martin – Those scientists have a word for everything.

Gutation is the exudation of drops of xylem “sap” on the tips or edges of leaves of some vascular plants, such as grasses.

Condensation occurs on surfaces (plants or otherwise) when the surface temperature falls to the dew point temperature. At that point condensation exceeds evaporation.

What about dewfall? The thickness of water on the leaf surface is measured in mm, e.g. 0.06 mm. It may get that thick in about 1 hour. So some scientists use the term dewfall to record growth rate of the layer liquid water on the surface, for example, 0.06 mm/hour. It is not a velocity.

If this were a beauty contest, gutation would win hands down! See the photo above.

How does frost get to be frost? It is not frozen dew. It skips the liquid phase. Atmosphere water is condensed on a surface colder than the dew point. Some use the term freeze point. If the temperature is above 0 C it is dew point. If temperature is 0C or colder it is the freeze point. Your super cooled liquid water freezes to get frost. Freeze point is used by scientist who study ice nuclei in a samples of air.

Skipping the liquid phase is sublime! Grade = A+ Frosty!


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