Wind Chill

[bnbmaplesyrup]

We have some nights coming up that are going to be in the 33 - 35 degree range - With the wind blowing will this be cold enough to cycle the trees to continue to give? Just hoping for a longer season!!!! Thanks Bill

[Clarkfield Farms]

March, 2012... We had 21 or 22 straight days of unusually high temperatures, including nightly temps throughout that entire time above freezing. My sap continued to flow the entire time. I do not understand it nor can I explain it with the currently held axioms and science but the fact remains -- sap flowed. Some days it was pretty scarce, other days it was strong. Those living close enough to me could vouch for this statement; however, most people tend to get raucous - then, and now - about how either the facts were fudged or my memory isn't what it used to be. Too bad for them that one of the things I live by is Ben Franklin's statement: "The faintest ink is still more clear than the best memory."

Anyways, that's here. I can't speak for Iowan maples. :)

[DrTimPerkins]

With the wind blowing will this be cold enough to cycle the trees to continue to give? Just hoping for a longer season!!!!

Trees do not experience windchill. If the temperature (of the tree, not the air) doesn't fall below freezing, then there will be no sap uptake.

[CampHamp]

Trees do not experience windchill.
Hmmm?? I would have thought that wind would cause tree temps to move more quickly to air temps. So if it were blowing at 30F you'd freeze a bucket of water faster than in still air...

[DrTimPerkins]

Hmmm?? I would have thought that wind would cause tree temps to move more quickly to air temps. So if it were blowing at 30F you'd freeze a bucket of water faster than in still air...

That is true (the trees will cool faster), but the temperature will not fall below the temperature of the air (33-35 deg F was stated), so the trees will not freeze and recharge.

It is sort of like my wife pushing the thermostat to 75 deg F because she wants the house to heat up faster.....it just doesn't work that way. :D

[DrTimPerkins]

March, 2012... We had 21 or 22 straight days of unusually high temperatures, including nightly temps throughout that entire time above freezing. My sap continued to flow the entire time.

This wasn't at all surprising under the circumstances, and occurred because the air temperature (and correspondingly the tree temperature) went upward in a series of steps each day. This causes an expansion of bubbles within the lumen of wood cells. Expanding bubbles take up more room, which push against the liquid in the wood, generating a small pressure that causes weeping flow.

Think of it this way. You have a balloon held under water in a glass by a screen or rod. The glass is full of water to the brim and starts out at just above freezing. As the system (balloon filled with air+water) warms, it expands a little, and some water will seep over the brim of the glass. As you continue warming it, more water will seep out....until the warming stops.

[maplestudent]

It is sort of like my wife pushing the thermostat to 75 deg F because she wants the house to heat up faster.....it just doesn't work that way. :D

Noooo Way!

[CampHamp]

That is true (the trees will cool faster), but the temperature will not fall below the temperature of the air (33-35 deg F was stated), so the trees will not freeze and recharge.

It is sort of like my wife pushing the thermostat to 75 deg F because she wants the house to heat up faster.....it just doesn't work that way. :D

Agreed - the wind chill effect will never bring an object's (or person's) temp below the actual air temps unless there is surface evaporation. OK, now there's some guy hauling his hose out into the woods to soak his maples. :lol:

My wife and my mother do that thermostat thingy your wife does. Funny thing is that our new Mr. Slim heat pump cranks out MORE HEAT if it is more than 2 degrees away from the set temp! She always wins an argument...

[MikeVT]

Trees do not experience windchill. If the temperature (of the tree, not the air) doesn't fall below freezing, then there will be no sap uptake.
Not to disagree with you, I know you have forgotten more about maple trees than I will ever know. than why is it when the air temperature is above 40 with high winds the sap doesn't run? and all the old timers tell me the wind shuts the sap flow right down, which I have experienced several times this month in Jericho with 40 degree air temp and windy days, the sap barely dripped.

[CampHamp]

On a sunny day, a tree's surface can get hotter than the air temps. However, when it is windy the tree's surface will have a harder time climbing above air temps because of the wind chill effect. Also, it's often cloudy on windy days...

[bnbmaplesyrup]

We were in the 50's and above freezing for the first time in 3 days and the wind blew 25-35 all afternoon - no sap ran - collected 15 gallons on 170taps - Will hope that the night time temps get down below 30 for several days in a row - thanks for all the input - been a very frustrating year trying to figure out why nothing is running - Thanks Bill

[MikeVT]

so after reading all of this, the trees do experience wind chill. everyone agrees that the trees thawing is affected by wind, so they are affected by windchill.

[DrTimPerkins]

so after reading all of this, the trees do experience wind chill. everyone agrees that the trees thawing is affected by wind, so they are affected by windchill.

Trees do not experience windchill the same as a rock doesn't experience windchill. The definition of windchill is:

Wind-chill or windchill, (popularly wind chill factor) is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air.

What that means is what temperature does it FEEL like when the wind is blowing at a certain temperature, and this temperature is below the temperature of the air.

While heat is removed more quickly by wind (if it is sunny and the tree warms up ABOVE air temperature), the temperature of the trees does not fall BELOW air temperature, thus trees cannot experience windchill. The effect of wind in this case is simply to remove excess thermal energy by reducing the boundary layer resistance.

[DrTimPerkins]

On a sunny day, a tree's surface can get hotter than the air temps. However, when it is windy the tree's surface will have a harder time climbing above air temps because of the wind chill effect. Also, it's often cloudy on windy days...

While it is true that a tree can warm up on a sunny day, and if there is wind it won't warm as much, this is NOT windchill. Rather, it is simply the tree absorbing energy and warming above air temperature. When the wind blows, the boundary layer resistance is lowered, and the tree temperature drops, but it will not drop below air temperature. In order for this to be windchill, the temperature of a tree on a windy day would have to fall below air temperature, which it does not....hence, no windchill.

[DrTimPerkins]

Not to disagree with you, I know you have forgotten more about maple trees than I will ever know. than why is it when the air temperature is above 40 with high winds the sap doesn't run? and all the old timers tell me the wind shuts the sap flow right down, which I have experienced several times this month in Jericho with 40 degree air temp and windy days, the sap barely dripped.

I do not disagree with the fact that trees can warm ABOVE air temperature on a sunny windless day. That is fine.....and you're still obeying the laws of physics (always a good thing). However if the wind blows on a cloudy day, the "perceived" temperature of the tree does not fall below air temperature. Thus, there is no windchill. What you're talking about is simply solar heating of an object above air temperature.....not windchill.

[CampHamp]

...but it will not drop below air temperature. In order for this to be windchill, the temperature of a tree on a windy day would have to fall below air temperature...

Wind will not bring a human's body or skin temperature below the air temperature either, so I don't think that is the defining characteristic.

Instead, it relates to the fact that we can sense the rate of temperature change to our skin. If it is cooling quickly then our body's alert system says "it's minus 20 out here!" because it's all about how much time we have before there is a problem (i.e. frostbite). Water feels colder than air for the same protective reason, that heat loss is occurring more rapidly than in a static air mass.

For our discussion here, it's probably just easier to say "the effects of wind" rather than "wind chill" because "wind chill" has popular connotations about human and animal perceptions which we obviously do not care about here.

Funny side-note is that Wikipedia and NOAA both say that wind chill is an effect only on humans and animals, but they also both say that there is a wind chill effect on inanimate objects. So, it's not surprising that we're also have this same argument...

It would be interesting to learn if any plants (like humans) sense and react to the rate of temperature change in their own biological decisions.

[happy thoughts]

It would be interesting to learn if any plants (like humans) sense and react to the rate of temperature change in their own biological decisions.

The one that comes to mind off the top of my head would be rhododendron. The colder it is the more tightly the leaves roll. When it got to -22F here, the leaves on mine were practically invisible :o.

[DrTimPerkins]

Wind will not bring a human's body or skin temperature below the air temperature either, so I don't think that is the defining characteristic.

Correct, but the perceived effect is that the temperature "feels" lower than what it actually is. Plants and inanimate objects do not have the same type of perception, and thus do not experience windchill. If inanimate objects did experience windchill, then your thermometer would read the windchill temperature directly.....they don't......they show the actual air temperature. The condition that causes them not to read the actual air temperature is when the sun shines on them, in which case they read above actual air temperature. If the wind blows it removes the solar heating effect to bring the temperature back closer to the actual air temperature, but the air temperature will never fall below actual air temperature no matter how hard the wind blows. Interestingly, to get the most accurate air temperatures, you have to use an aspirated air temperature gauge, which actually sucks air across a thermometer to avoid any solar heating effects.


For our discussion here, it's probably just easier to say "the effects of wind" rather than "wind chill" because "wind chill" has popular connotations about human and animal perceptions which we obviously do not care about here.

Semantics. The original post was entitled "windchill" and asked whether the temperature the trees experienced would be lower than air temperature. Obviously there is an effect of wind on temperature of objects under certain conditions (solar heating), but that is not what was being asked.

If we go back to the original post....


We have some nights coming up that are going to be in the 33 - 35 degree range - With the wind blowing will this be cold enough to cycle the trees to continue to give? Just hoping for a longer season!!!! Thanks Bill

The original question was asking if wind will cause the trees to go into a freeze "uptake" cycle even if the temperature doesn't fall below freezing. The answer is still NO....windchill will NOT result in a temperature BELOW the actual air temperature, so the trees will not freeze, and there will be no water uptake.

[CampHamp]

I think we're on the same page: The effect of wind on any object's temperature is that it will bring the object more rapidly toward the current air temperature than if there were no wind.

[CampHamp]

An old timer's poem about wind and sap flow was posted here (http://mapletrader.com/community/showthread.php?8960-Wind-direction&p=107195#post107195) by DanE a few years ago. Seems appropriate to re-post it with some analysis.

When I first read it, I dismissed it as soon as I read "Sap from the North, the sap Cometh Forth..." because I know that we get cold weather from the North. A "drunk old timer", I thought.

After this discussion, I dug up that post again and it now makes sense to me. If you want a good run, you need a good morning thaw. So...

Wind out of the North, the sap Cometh Forth
>>> The sunny-sides of the tree are shielded, but some wind swipes the east side and slows the morning thaw a bit (that's why it's not "best" direction).

Wind out of the East, the sap runs the Least
>>> The tree's "morning surface" (E) just can't climb any higher than air temps (air temps are low in the morning as well) and the mid-day side (S) get's a side-swipe.

Wind out of the South, there is a Drought
>>> Southerly wind slows the morning thaw with a side-swipe of cooler air temps and keeps the mid-day (South side) as cold as the air. Crappy.

Wind out of the West, the sap runs the Best
>>> Best morning thaw possible, the sun might break the thaw even in cold temps. Mid-day gets a side-swipe, but air temps are warmer now. End-of-day she'll run in the wind as long as it's above freezing.

So, this "wise old timer" has something that was not apparent to me at first glance!

[Brian Ryther]

While it is true that a tree can warm up on a sunny day, and if there is wind it won't warm as much, this is NOT windchill. Rather, it is simply the tree absorbing energy and warming above air temperature. When the wind blows, the boundary layer resistance is lowered, and the tree temperature drops, but it will not drop below air temperature. In order for this to be windchill, the temperature of a tree on a windy day would have to fall below air temperature, which it does not....hence, no windchill.
What if the tree has moisture on its surface? When the wind rushes past the wet tree doesn't it increase evaporation which would lower the surface temperature of the tree?

[CampHamp]

What if the tree has moisture on its surface? When the wind rushes past the wet tree doesn't it increase evaporation which would lower the surface temperature of the tree?

The evaporative cooling effect of water on the tree's surface temp occurs only above freezing and will cause the remaining water along with tree's surface temperature to decline (even below the air temperature). ...and "yes", the best conditions would be windy days because you get faster evaporation -- a low relative humidity is better, too. However, as the water temp approaches freezing evaporation declines to a stop (it is more than a linear relationship, unfortunately). The good news is that when we boil sap, this cooling effect also diminishes quickly... :D

[judgejp]

So why does the sap stop running on windy sunny days with the right temperature swings? What effect does wind have on the trees sap producing mechanism?

[DrTimPerkins]

What if the tree has moisture on its surface? When the wind rushes past the wet tree doesn't it increase evaporation which would lower the surface temperature of the tree?

A very slight amount, however that is not windchill, that is evaporative cooling.

[DrTimPerkins]

So why does the sap stop running on windy sunny days with the right temperature swings? What effect does wind have on the trees sap producing mechanism?

Because on a sunny day the stem and branches of the tree can warm up a fair amount above the actual air temperature. When the wind blows, this heat energy the tree absorbs is removed by the wind (the boundary layer resistance to heat movement is greatly reduced), so that the tree temperature will be closer to air temperature, and thus less likely to produce sap. Again, this is a case of the wind dropping the tree temperature closer to actual air temperature, but not reducing it BELOW air temperature.

In this case, wind can also increase transpiration that can occur through the bark of the fine branches, removing some moisture from the tree, which otherwise might contribute to head pressure and sap flow.