THERMAL INVERSIONS

No Arboreal Dew

A.B. Fraser of Penn State takes on dew in the canopy. Arboreal dew is something new to most of us. Walks on dewy mornings results in wet shoes but it is rare that that the trees also are dew bedecked. How often can you remember the frozen form of dew (frost) on both the ground and on the canopy? Not often. The well-known nocturnal inversion mandates that it is coldest at the ground and it gets warmer with height above the surface. Consider a July night, say 10 PM, with dew on the ground the temperature gradient above the ground from 2.5 to 30 cm is 165 C/100 m and from 30 to 120 cm it is +36 C / 100 m. So if there is dew at 2.5 cm and the temperature there is the same as the dew point temperature, then at 30 cm above the temperature is warmer by 0.5 C and the relative humidity is then less than 100% and evaporation exceeds condensation and the dew either will not form or it will quickly evaporate. In the first 20 meters above the ground temperature at night can warm 6 to 7 C. So if dew is so hard to come by in the canopy how do you get it? First it could be pseudo-dew. If a fog or cloud passed through a canopy dew-like drops could be deposited on the leaves. The coastal coniferous forests of the West Coast would be a good place to find this pseudo-dew. To get real dew, drops coming into being right on the leaves, you first cool down the plant surfaces then advect a warmer, moister air mass over the area. Condensation and dew will form everywhere the surface is colder than the dew point of the advected air blown in! Sylvanshine is best searched for on herbaceous, small stature plants with the right kind of waxy surfaces. Else, as Fraser suggests, take a sprayer filled with water, select the right plant material, spray away, stand back and turn your headlights on your self-misted sylvanshine creation. This Jerry Franklin could do in his canopy without staying the night!

Photo: Vishesh Bajpai, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license

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