frost question, what caused this?

[Bennz]

My place is usually frost free, mostly because of the coastal hillside situation which reliably keeps the temp above freezing. Before the present rain it had been unusually dry, enough moisture to keep the grass green and dew on the ground every morning, but dry enough I still needed to irrigate.

So what happened is I set up a rotary garden sprinkler on an area of my orchard, towards the lower part of a slope. Around dawn there was ice formed on the grass in the area watered by the sprinkler, but none anywhere else despite the presence of water from dew (to run-off point) on all the vegetation surfaces.

I don't understand the physics of this. I would have thought that the area where an irrigator was actively working would be the last place for ice to form, rather than the only place. If it was cold enough for ice there, why did it not form as frost on any of the other wet leaf surfaces? It was not a microclimate factor, there was a basin a little down hill where all the cold air should collect, and should have been the first place for frost to form. It appears there was something about the act of irrigation that caused some localised freezing event. But if so, why does this never occur during rain?

 

I'm no scientist, it would be great if someone could explain this for me. 

[Ed in Houston]

I suspect that frost formed in the irrigated area due to evaporative cooling. The dew that formed in the non irrigated area was due to radiational cooling and although it to has some evaporative cooling, it is not as strong as when the whole area is saturated with the irrigation. There are other factors that could also be at play. The irrigated area being more subject to wind would be another factor that would increase the evaporative cooling.

 

Ed in Houston

 

 

 

[tropicbreeze]

The only thing that I can think of is that the very fine spray from a sprinkler has a huge surface area, much greater than you'd have with rain drops. That great surface area exposed to dry air would result in rapid evaporation and rapid cooling. The air around rain drops would already have high humidity slowing evaporation so temperature drop to that degree wouldn't happen. That's my 2 cents worth.

[Tyrone]

I could be wrong but by irrigating you increased the humidity of the air, then cold air drainage froze the now moist air. Further up slope would be warmer anyway, but the air further up slope was drier so no or little frost would form up there.

[Bennz]

Thanks for all the suggestions. I wish NZ still had 2c coins so I could post a picture! Except my answers aren't even worth 2c.

 

I don't think any of the answers are right though. The air was not dry, the dewpoint had been reached at ground level and the air humidity would have been close to 100% even at 1.4m. If it was evaporative cooling the leaf surfaces should have frozen too, but frost did not form even downhill from the sprinkler wet area, which would have been colder due to  simple cold drainage to the basin downslope. The sprinkler was of a very low tech kind and had very large droplet size, the blobs of ice were all in the 5-8mm  size, and tear-drop shaped.

 

Still don't understand! But at least it wasn't cold enough to damage even young tamarillo plants which are usually frost-sensitive.