It is not typical to have the frost fans operating as late as the 12th of November, but they are.
More interesting is the reason they are operating.
You see it is cold (approaching freezing), but the type of cold that can be warmed by a frost fan is very selective.
You see a frost fan relies on an inversion layer (see here and here) to work. It pulls down slightly warmer air from about 15m above the ground.
But what if there is no inversion layer? Well then you end up blasting your vineyard with cold air. Yes, that would mean making a bad situation even worse.
Now a strange thing happened yesterday. In Marlborough, a cold air mass moved in from the South and brought with it rain and low temperatures. So throughout the day, air temperatures were low and as a result the temperature of everything else was low.
A typical day in September/October in Marlborough sees sun warming the ground and the air. Then at night the warmth from the ground is radiated away as infrared radiation and the warm air mass traps the cold from the ground near the surface. Hence the effect of a rise in temperature with an increase in height.
Last night there was no warm air and the temperature dropped towards freezing. As a result the frost fans that were running were probably moving cold air over their vineyards. Creating exactly the effect they were designed to prevent. Needless to say, the residents they woke up were annoyed. It would be even more annoying to know that the fans shouldn't have been running at all because they were only making the situation worse.
4 comments:
Jack, as long as the region is not being invaded by sub-zero air from the south, an inversion layer will still form, even after a wet sun-less day. This is because the earth is still warmer than the sky and continues to radiates heat upwards.
Have a look at this weather snapshot from a Paynters Rd vineyard this morning ... there's a good inversion, and the wind machine is doing a good job.
http://www.harvest.com/w.cgi?hsn=3074
Well, I cant argue with the information from the site. You are correct! The data does indicate that the fans were doing an effective job of bringing down an inversion layer.
I also note that the temperature never went below 2.6 degrees and that the machine appeared to be operating mainly while the temperature was between 3 and 4 degrees. Is this normal? or do the machines normally trigger of the dew point temp?
Hello Jack , as you will see on the harvest unit concerened the temp A is the canopy, temp B the tower ,(top of the tower) subtract A from B and this will give you the inversion layer , it looks to me that this machine is on a 1 degree start up ,,
Very interesting. Thanks for the comments. You've explained it nicely. Do you see any reason why the machine started for the third time? The temperature seemed to be approx 2 degrees and climbing. I can't quite see how that piece fits the puzzle.
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