does rain, clouds, wind effect sap flow?

[Starting Small]

Assuming the temperature is correct for sap flow does rain, cloudy days or wind significantly effect sap flow?

[Cake O' Maple]

Yes, I've noticed increased flow in low atmospheric pressure days like that.

[shane hickey]

Absolutely it does

[HyeOnMaple]

Kind of like a woman in her ninth month. If a big storm roles through, expect the water to break! It's the sudden drop in barometric pressure that makes it pop.

[PerryW]

yes, clear weather generally produces the conditions for warm days (sunny) and cold nites (radiational cooling). Having said that, there are some days when the sap runs like crazy on a cloud, windy day with temps bairly above freezing. No one knows why; but I think, sometime the trees are just ready to run and even marginal conditions will allow them to do so.

[killingworthmaple]

Oh yes it does, but it is hard to pin down if it will run or not. some days you just know it's gonna run and it does not. I have thought it would be neat to study it by taking several seasons daily sap flow and compair it to temp., wind, high or low pressure and make a chart to see how it looks but I don't have good sap flow records.

Nathan

[johnallin]

It runs like crazy when it rains but, for me anyway, if it gets windy it shuts down. Then the bags are empty and blow all over the woods. Trying different spouts this year to try and stop that.

Boiling is best on a clear day/night. If it gets overcast, it slows down.

[maple flats]

Generally I get the best runs when it was cold overnight, warm and rain or very cloudy that day.

[Tweegs]

I have a theory that I can’t really prove, but here it is anyway.

I think good sap runs have more to do with a change in barometric pressure rather than a steady high or low pressure system, and not only the change in pressure, but the rate of change.

I have a hunch that a pressure difference between inside and outside the tree either aids or hinders the tree’s ability to move sap, depending on whether the pressure outside the tree is higher or lower than inside. The difference in pressure may have to do with gasses expanding and contracting in relation to the barometric pressure, and the tree’s need to equalize the pressure against the resistance of the sap.


It starts to explain why a rain or snow shower (a transition to low pressure) might trigger a run, while the wind coming up (transition to high pressure) might end one. Or why we have a crisp, cold night followed by a bright warm sunny day, and nothing happens (steady high pressure).

To help further support the theory, if the tree froze on a clear night (high pressure) and rain moved in during the morning (warm, low pressure) there should be a significant pressure difference since the tree couldn’t equalize while it was frozen. Therein lay one good run.

Lots of loose ends in that theory, admittedly.

[happy thoughts]

tweegs- I think your theory is the generally accepted one as I have read more than a few explanations along those same lines. When sap flows the pressure inside the tree is higher than the pressure outside the tree. Putting a tap hole in the tree allows a route for the pressure to equalize- flowing from high pressure inside the tree to low pressure outside. Low barometric pressure increases the difference so flow will be greater. Whenever I have seen long continuous sap runs over 24 hours and longer, especially if night temps did not go below freezing, it has always been during rainy weather when barometric pressure was low .

[Vermonner]

I agree with what your offering. Isn't pressure differential what we're all after with vac? We're trying to create an environment where sap will flow and basic hydrodynamics says it flows from an area of high pressure (tree at given atmospheric pressure) to area of low pressure (tap hole in tree exposed to vac). Broad brush strokes there, but that's my understanding

[Tweegs]

That’s the way I understand it. The pressure inside the tubing will always be lower relative to the tree and therefore produce sap as long as the tree is thawed and the tap holes are open. This isn’t the case with natural collection types where the rise and fall of barometric pressure has a direct influence over sap flow.

It is my contention that it takes a falling barometer to produce good sap runs, that a rising barometer will kill a run, and a steady barometer will produce light to moderate runs depending upon the pressure system in place…sound about right?