In this episode, Dr. Dave Brown discusses frost mitigation best practices with Bill and Nick Anderson, cherry growers in the Dalles, Oregon region. They talk about the practical meteorology of frost events, how buds experience these weather events, and wind machine best practices.

This transcript has been edited for flow and clarity.

Brad Newbold (Host):

Hello and welcome to POMETA Coffee Break, a podcast produced especially for tree fruit and nut producers.

In today's conversation, Dr. Dave Brown will be discussing frost mitigation best practices with Bill and Nick Anderson from Anderson Fruit Cherry Growers in the Dalles, Oregon area.

Dr. Brown served for 20 years as a faculty member at Montana State University and Washington State University, where he pursued research on soil sensors, spatial data science and digital agriculture. At both universities, he served in many leadership roles for major research projects, academic programs, and most recently as director of the WSU AgWeatherNet program. In this capacity, he hired and supervised a team of meteorologists who pursued research and extension activities focused on evaluating and improving the quality of weather data used for agricultural decisions. I'll hand it over to Dave, Bill and Nick.

Bill Anderson:

Right now, a big one is frost mitigation for us. That's pretty critical. We don't farm a lot of acres. We have under 170 now, and we have 11 wind machines. So we have some cold areas.

Could you explain how you feel about dew points in your critical temperatures? What I tend to do, as an old school rule of thumb, is check temperatures around 9:00 PM, mainly to give me an idea of what I think the night's going to do if it's already clear out. If it's above 30, generally that's a pretty good number. If you're in the twenties and dropping, we know it's getting colder.

But if you wouldn't mind explaining a little bit about dew point versus critical temperatures?

Dave Brown:

In general, the lower the dew point, the more risk you've got. The more humid the air, the higher the dew point, and the more water you have in the air. And that acts as sort of a brake on how fast the air temperature can drop.

So when dew points drop down in the single digits, that means that there's not much stopping temperatures from dropping. Now, that doesn't mean you're going to go there, because it also depends on the cloud cover and the wind speed – basically how much mixing you've got. All those things matter too, and the ground temperature matters as well. So when I see a really low dew point, it means, “Okay, I don't have a guardrail tonight.”

But one of the things I did want to bring up is that it's not quite right to think of dew point as the bottom. The dew point isn't necessarily the lowest your temperature can go because the dew point can drop during the night, and you can go lower than that under certain conditions. It can continue to dry out, and you can actually drop during the night and go significantly below your critical temperature.

Bill Anderson:

Wind has everything to do with it too. A few weeks back, we had really low, single digit dew points, and we were having an invective weather event until the wind stopped. And then it changed to a –

Dave Brown:

We call it a radiative event at that point.

Bill Anderson:

– radiative, yeah. That was what I was trying to get at.

Dave Brown:

So to clarify for folks, I'll take a second and explain that. It means that you’ve got a cold air mass that's blowing in. What it also means from a fruit grower's perspective is there's not a lot you can do, because there's not any warmer air above mixing in. The air's going to do no good because it's already blowing. The rule of thumb is that when you've got four or five miles per hour wind with a cold air mass blowing in, you're fairly helpless to mitigate.

Bill Anderson:

No sense in running a wind machine at that point.

Dave Brown:

Yeah. Certainly not a wind machine. Even things like water are less effective because it gets in the way of some other things, too. We’ll talk about that later.

Bill Anderson:

So, a little bit more on this weather. I'm not sure if everybody understands this – we've been misinformed before. Could you talk a little bit about inversions, how they happen and how it works?

Dave Brown:

Sure. The first thing you need is some warm air. That happens when the sun hits the ground and the trees and the buds and warms up the air during the daytime. It all starts with warm air from sunlight. So if you have a cloudy day, then you tend not to get as strong of an inversion at night because you haven't warmed up the air to begin with.

I guess I should take a step back and explain an inversion. It means there's warm air above and cold air at the surface, which is not the normal way of things. Normally you go up and it gets colder. Anyone who's hiked a mountain has experienced this. But sometimes it flips around and it starts out by warming up the air during the daytime warming up the ground and then the ground is in contact with the air and warms up.

Then at nighttime, you get the reverse of what happens in the daytime – but if you don't have any clouds, everything that was warming up during the daytime starts emitting heat. It was emitting during the daytime, but it was much less than the sun’s heat.

So it's emitting radiation back out, and if there are no clouds to catch it, it goes right out into space. So the ground and the trees and the buds all start cooling off really fast, and then they in turn cool off the air. The important thing is that what's driving all this is that the surfaces are what warm up and cool down. The air is always reacting to the surfaces. The sun doesn't really warm up the air directly, it's more warming the surface and the surface is in contact with the air, so it warms the air.

And at night it's the opposite. Those surfaces cool down the ground, cool down the trees, cool down the buds – indirectly, the air is cooled down by being in contact with those things.

Then cold air is heavier, so it sits there and you get an inversion – a cold air layer down low and  warm air higher up. The other thing that happens with an inversion is that once that cold air starts pulling up, it can start running around, right? It's heavier. So if there's a low spot, it'll run. And I know you guys have a low spot, you're down in that valley –

Bill Anderson:

We experience that on a regular basis.

Nick Anderson:

It seems a lot easier to find that in places like our canyon or most of the draws around here. It's pretty easy to get a slight inversion, enough to pull some heat off of the surrounding hills, mix enough warm air in.

Dave Brown:

Yes, you're going to have a lot of warm air up above because of that pooling effect. That's the other thing to say about the inversion – it's going to be stronger when there's a clear night. It's going to be stronger when there's a clear day too. So it's both, right? A clear day, you get a lot of sun warming things up. A clear night, and everything cools off faster. Then the inversion forms. But then the third piece is that it needs to be calm, with no wind to mix it back in again.

Bill Anderson:

That's one nice thing that we found with POMETA – being able to actually chart those inversions. I mean, yes, we might think we're having an inversion, but previously we've never really had a way to measure that, but now we can actually chart it.

Dave Brown:

It was pretty cool watching that inversion on February 23rd, where you guys were down in the valley, and turned on your wind machines. We measured the inversion and it was only a degree or two difference between the top of the wind machine and what was in the canopy, but when you turned on the wind machine, the temperature went up, both in the canopy and at the top of the wind machine, which meant there was a lot more warm air way above the wind machine that you were pulling down and drawing the temperature up. It was pretty cool to watch that event.

Bill Anderson:

POMETA is super helpful for doing frost. It's now a thing – anytime we're getting ready to roll out, I click on the laptop and go, “Yep, okay, we've got a couple degrees.” It has been pretty cool to be able to watch that. I can go back home, check my laptop and watch the rising temps.

I had a question, how do dew point, relative humidity, and temperature all relate to our critical temperatures? Is our relative humidity in correlation with our dew points?

Dave Brown:

Well, relative humidity and dewpoint are two different ways of reporting the same thing essentially. They're based on the same principle – how much water is in the air in terms of your critical value. As far as I know – I haven't seen any research – that says the bud absorbs more moisture if it's more humid.

Bill Anderson:

On the wind aspect – do buds actually experience wind chill?

Dave Brown:

Well, no. Let's explain how humans feel wind chill. The fundamental idea is that we are warm-blooded mammals, so we're a lot warmer than the air. When the air blows past us, convection carries heat away, and the faster the wind blows, the faster it removes heat from our body. This only happens because we're warmer than the air. At night, the buds are colder than the air, but during the daytime, they're absorbing all the radiation from the sun, making them warmer than the air. The ground, trees, leaves, and buds warm up during the day because of sunlight. At night, they radiate energy into the atmosphere, and if there are no clouds to catch and push it back down, they cool off quickly. Everything that makes sense in the daytime is flipped at night. Therefore, wind chill is not a concern, and it's tough to damage a wind machine unless the wind is blowing too much.

Bill Anderson:

So no matter what, unless you've got an invective event happening, run the wind machines.

Dave Brown:

Pretty much, you can't do any harm.

Bill Anderson:

Some people have said that you're going to freeze buds by running wind machines in the cold, but if you're moving air past those buds –

Dave Brown:

On a cloudy day or night, you won't get as much benefit, but generally, the buds are almost always going to be at least slightly colder than the air, even with some radiative loss. So mixing the air around them might warm them up a bit. It's possible you might not get much benefit, but you won't do any harm either.

Speaker 4:

You think we should be running it a little sooner to start mixing air? Maybe start a couple degrees earlier?

Dave Brown:

That's an active research question we're investigating this year, even in your orchard. One of the questions we want to ask is, what are the benefits of running the wind machine early? It's not entirely clear. First, in terms of the time it takes to mix the air, that's about half an hour. It's pretty well mixed in about half an hour, but keep in mind that after you mix the air, it takes a certain amount of time to actually warm the buds with that mixed air because the buds are colder than the air, and you need some time for that. You're probably looking at roughly an hour. When you're talking about starting an hour earlier, I think what you're probably considering is, can I warm up my ground? I'm curious to see the answer to that. I've heard of some people turning the wind machine on at 8 o'clock at night and running it forever. I'm not sure that the science supports a real benefit to that necessarily because, at the end of the day, whatever the wind is doing, the ground is still radiating out that energy. It would be interesting to see how much benefit you get, but there's probably some benefit.

Bill Anderson:

What people have done for quite a few years on the wind machines down here is start them approximately two degrees above critical. But what I've always looked at is, first of all, where is my dew point and how fast is the cold air moving in? We have certain areas where the cold air moves in slowly, and we have another area where our home place is located, where I've seen the temperature drop three degrees in 45 minutes. So there, we usually set it about four degrees above critical.

Dave Brown:

I'll say that basically, when growers set the wind machine at three, four, or two degrees above critical, they're essentially trying to make a forecast. The idea is to start the wind machine an hour before crossing the critical point, and on the fly, make a forecast of how much the temperature will drop and set the temperature accordingly. However, a better long-term solution is to have a nowcast model that uses machine learning AI to analyze the trends of the air temperature and dewpoint and predict when the critical point will be reached. The decision to start the wind machine can then be made based on this forecast, rather than a set point. Using a set point can be problematic, as people often have to experiment with different set points each night.

Brad Newbold:

Alright, our time's up for today. Thanks, Bill and Nick, for being with us today, we really appreciate you spending time with us. We'll continue with more frost mitigation discussion in part two of the series.