Wink’s Blog

May Day, 2010.  Last night (Friday) was the second in what is forecast to be a series of four nights in which sub-freezing temperatures threaten our nascent fruit buds, potentially reducing our harvest. This is a strong possibility every spring as our peaches, apples, and grapes mature from bud-swell through full bloom to fruit set. Our two OrchardRite wind machines are our only defense against this threat. The closer a particular bud is to bloom, the more vulnerable it is to freezing. The critical temperature for both apples and peaches at the present stage is about 27 degrees. On Thursday night the thermometer went down to 25 and stayed there for several hours. I reproduce below my decision-process from April 30. Next I’ll reproduce my story from last night. Suffice to say I’ve not been getting much sleep recently.

April 30, 2010, 11:30 PM.  I’m awake, sitting by the warm wood stove, listening to my neighbors’ wind machines flog the air, struggling to find a modicum of warm air to stir into the frigid air mass that envelops Rogers Mesa. Why don’t I just turn mine on instead of belaboring this dilemma as I do every year at about this time.  I review the considerations:
1.  At each stage of development the fruit buds have “critical temperatures” at which 10% of buds will be killed (thinning the crop) and a lower temperature at which 90% will be killed (a disaster).  The further along they are, the less cold they will tolerate.  For our peaches in full bloom these temperatures are 27oF and 24oF, respectively.  My wireless thermometer stands at 27.3.  It will not be likely to rise again until 8 AM — this is a long period of cold.
2.  These values are gross generalizations.  Buds will withstand lower temps for shorter duration and vice versa.  Also, only some of our buds are at Stage 6, full bloom.  Other buds on the same tree which are at lower stages are good to lower temps: 26/21 or even 25/15.
3.  The wind machine is a giant fan on a tower which spins and rotates at the same time.  During spring frost events there is often a temperature inversion (warmer air above).  By stirring the air it is possible to warm the air at fruit level a couple of degrees.  Sometimes this is all it takes to save the crop.  Sometimes it is “fruitless.”  I have a wireless thermometer at crop height and one at the top of the barn in an effort to measure the amount of inversion.  Right now the spread is 26.4 x 27.8.  I’m looking for at least a 2 degree spread.
4.  Inversions occur under clear skies.  Cloud-cover acts as a blanket, preventing the relatively warm air from escaping.  Just when my neighbors turned on their machines a high cloud-cover moved in and the temperatures stabilized at 27 x 28.  Now the cloud has moved off and the temp is again falling — faster at fruit level than above.  This is an indication that radiational cooling is in effect.  The 2 degree spread will likely soon be reached.
5.  Absolute temperature is not the only effect.  When the dew point is reached, water vapor turns to liquid and gives off heat.  When the liquid freezes it gives off more heat.  When the dew point is close to the critical temperature, it is authoritatively said that there is nothing we can do that will be as effective at saving the fruit.  Dew point is calculated from humidity and temperature.  Tonight it is 25 oF.  Right now the temp spread is 25.5 x 27.3.   Would the wind machine simply dissipate this ephemeral heat?  How long does the heat last without wind?
6.  If the cold is the result of a cold-air-mass rather than radiational cooling under clear skies, there is no point in stirring the air around.  Tonight’s cold is on the heals of a cold front, but is there also radiational cooling taking place?
7.  So what’s the hesitation?  Cost vs effectiveness.  I’ve just outlined the effectiveness issues.  As for cost, the machine is driven by a propane-fired 440 cu in Chevy V-8, industrial engine.  It operates at full throttle and guzzles fuel.  It is not to be run unnecessarily.
8.  There is no point in running the machine if there is any wind: the wind is doing the stirring and, since the fan is a giant gyroscope that is constantly reorienting itself relative to the wind, it could blow apart under the stress of moving through too strong a wind with catastrophic results to the $20K machine and the farmer.  At the first hint of wind — indicated by intensification of the helicopter-throb of the rotor — I should hurry out and turn it off.

It’s 12:10 AM.  Spread is 25.2 x 27.1.  Sky is clear and calm.  I still have my doubts but I’m heading out to do my part to stir the air.  Call it giving-in to peer pressure.

Forty-five minutes, 2 wind machine start-ups, and a snow-sparkling walk under the full moon later and I’m back at the fire.  Dew point has certainly been reached as the towers and blades are coated in frost which blows off when spinning begins.  Checking the sky I see that another bank of high clouds is moving-in from the west.  The temp spread is now 25.3 x 26.2.  Is the narrower spread the result of the cloud cover or the wind machine?  I hear the rotors throbbing as they chop their way through the wind.  I’m surrounded by noise.  The cabin shakes in time to the throb.  The pitch changes with the orientation to the wind and the wind velocity.  Is there wind with the cloud bank?  Or is it the wind we are generating?  Should I contemplate turning off the machine?  Can I go to sleep with these questions on my mind?  Spread is now 25.5 x 26.6.  It’s warming-up?

Later.  The cloud cover is not unbroken.  In the West it seems clearer despite the forecast that calls for 60% chance of snow.  Precip would obviate the wind machine.  Dilemma.  I think I’ll crawl my weary body and mind into the warm bed and try to forget that there are no certain answers.  Any major change will likely wake me — that is if am able to sleep at all in this racket… and with all these questions…

Morning after.  I managed four hours sleep.  In the dreary dawn there is an unbroken thin overcast blanket.  The temp spread has all but disappeared and stands at 25.5.  There’s no reason to keep the machines sucking down fuel.  At 6:15 I begin the process of turning them off.  By the time I get back to the cabin half-an-hour later the thermometer is at 27 and trending upward.  This raises the question: Did the wind machines, in the absence of a temperature gradient, actually suppress the temp?  Worse than for naught, actually counter-productive.

This entry was posted on Saturday, May 1st, 2010 at 5:25 pm and is filed under Mesa Winds Farm. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.