please help

[kristina221]

This is not a homework assignment, but I didn't know where to post it.

I found this question in a book about physics, but there was no explanation. Q: Why do fruit fruit growers set car tires on fire in their orchards in the clear winter mornings?

I imagine te answer would be to stop frost from forming, but I don't understand how smoke or soot could do that. The chapter is about wave-like properties of electromagnetic radiation and particles. Could soot act as a black body at 32°F (273 K)?

[Blahah]

"Fog or smoke Clouds and fog are well-known for their ability to reduce radiative heat loss from the surface. Smoke from smudge pots or burning tires or refuse and mist from fine water nozzles have been used in attempts to reduce this heat loss. Since it is difficult to maintain the smoke over the sensitive crop area and to produce droplets the optimum size to intercept the long-wave radiation, this method is not very effective. In addition, our environmental laws now prohibit the use of this method, where smoke is involved."

from http://www.omafra.go...5-116.htm#freez

 

and...

 

"A smudge pot (also known as a choofa or orchard heater) is an oil-burning device used to prevent frost on fruit trees.

Condensation of water vapor on particulate soot prevents condensation on plants and raises air temperature very slightly."

From the Wikpedia Smudge Pot article.

Edited February 23, 2011 by Blahah

[kristina221]

"Fog or smoke Clouds and fog are well-known for their ability to reduce radiative heat loss from the surface. Smoke from smudge pots or burning tires or refuse and mist from fine water nozzles have been used in attempts to reduce this heat loss. Since it is difficult to maintain the smoke over the sensitive crop area and to produce droplets the optimum size to intercept the long-wave radiation, this method is not very effective. In addition, our environmental laws now prohibit the use of this method, where smoke is involved."

from http://www.omafra.go...5-116.htm#freez

 

and...

 

"A smudge pot (also known as a choofa or orchard heater) is an oil-burning device used to prevent frost on fruit trees.

Condensation of water vapor on particulate soot prevents condensation on plants and raises air temperature very slightly."

From the Wikpedia Smudge Pot article.

 

thank you, that helped

 

other ideas are also welcome...

[michel123456]

Q: Why do fruit fruit growers set car tires on fire in their orchards in the clear winter mornings?

I imagine te answer would be to stop frost from forming, but I don't understand how smoke or soot could do that. The chapter is about wave-like properties of electromagnetic radiation and particles. Could soot act as a black body at 32°F (273 K)?

 

I don't think it is to protect from frost or to reduce heat loss. Frost can happen at night as well, and temperature loss can happen anytime.

IMHO the "clear morning" indicates that at this particular moment radiation from sunrise can burn, or at least produce a thermal shock.

Edited February 23, 2011 by michel123456

[kristina221]

in that case i think the soot could act as a black body to protect the plants from radiation(probably UV)

[Blahah]

Err, no, fruit growers burn tyres to protect the crop from frost (as I already explained and referenced). Seriously, google.

 

Plants don't experience heat shock unless the final temperature is higher than ~40°C (although it varies with climate), and even then most species will not until considerably higher temperatures. A clear winter morning means there is no cloud cover, so frost is more likely.

 

Frost is a major concern for fruit growers, growing ice crystals damage cell membranes and make fruit soften and bruise, becoming unsellable. You can lose a whole crop overnight. So, on cold nights, or clear nights which indicate a possible cold morning, they heat the susceptible areas of their orchards. I work at the Royal Botanical Gardens and our estates team were doing this all through winter (with a piped heating system, not tyres) in the orchards. I would be surprised if the person writing the physics book knew that fruit growers used to burn tyres but did not know why, and based a question around a misunderstanding. Thus, the answer is the one I gave earlier.

 

But there might be more interesting explanations as to how thick smoke can help prevent frost.

Edited February 23, 2011 by Blahah

[lemur]

I don't think it is to protect from frost or to reduce heat loss. Frost can happen at night as well, and temperature loss can happen anytime.

IMHO the "clear morning" indicates that at this particular moment radiation from sunrise can burn, or at least produce a thermal shock.

Maybe the smoke acts as a black body to absorb solar radiation into a warmer air-blanket around the trees. I don't know if this would have some benefit greater than letting the sun warm up the trees and ground more gradually, though.