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07-11-2014, 17:34
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#61
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Registered User
Join Date: Jul 2010
Location: Florida/Alberta
Boat: Lippincott 30
Posts: 9,904
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Re: Northwest Passage - 2014
Quote:
Originally Posted by hpeer
How often the sea 0, or -1 or whatever is a function of where you are. In that photo I posted above the water was 0C my air thermometer read 40F. I don't know the temp of that mist in the air, but it was f'ing COLD!
The temp of the Labrador current, which I was in will be right at -1.
I mean, essentially you have a bowl of water with ice cubes melting in it. It's all the same temp.
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Don't forget ocean salt water freezes at 28F. Icebergs are fresh water, and the difference in temps is what you see.
__________________
If your attitude resembles the south end of a bull heading north, it's time to turn around.
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07-11-2014, 17:50
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#62
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Registered User
Join Date: Mar 2006
Location: Winnipeg
Boat: None at this time
Posts: 8,462
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Re: Northwest Passage - 2014
The internal temperature of icebergs is much lower than 0 deg. c. and as the FLIR images show they show up very nicely on the screen.
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07-11-2014, 17:53
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#63
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Senior Cruiser
Join Date: Dec 2008
Location: Between Caribbean and Canada
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 8,503
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Re: Northwest Passage - 2014
Ok, in my example the water was 32f, bergs are fresh water at...32f.
No difference.
Look, I'm not saying it is of NO use, I'm saying that there are conditions when it likely will not work.
Also I went to the FLIR advert site and looked at their video. Although they SAY it works in rough weather they showed no conclusive evidence.
They showed lots of pics of big bergy bits at distance in calm water, and close ups of smaller bits. But none of growlers at any kind of distance or in even moderately rough sea, nothing with recognizable white caps.
That makes me skeptical.
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07-11-2014, 17:57
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#64
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Senior Cruiser
Join Date: Dec 2008
Location: Between Caribbean and Canada
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 8,503
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Re: Northwest Passage - 2014
Quote:
Originally Posted by DeepFrz
The internal temperature of icebergs is much lower than 0 deg. c. and as the FLIR images show they show up very nicely on the screen.
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I'm sure icebergs do. They show up on radar too.
What about a 10' diameter growler? In any kind of sea?
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07-11-2014, 17:59
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#65
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Registered User
Join Date: Mar 2006
Location: Winnipeg
Boat: None at this time
Posts: 8,462
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Re: Northwest Passage - 2014
Quote:
What about a 10' diameter growler? In any kind of sea?
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I guess we could both speculate on that but I really don't know.
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07-11-2014, 18:08
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#66
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Registered User
Join Date: Jul 2011
Location: USA
Posts: 1,006
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Re: Northwest Passage - 2014
Quote:
Originally Posted by LakeSuperior
My friend you are in my wheelhouse now. In the focal plane of your IR imaging system is an array of detectors that detect energy in a continuous band of wavelengths between either 3 to 5 or 8 to 12 micrometers. I am assuming 8-12 although it doesn't matter. Each detector corresponds to each pixel in the display.
The sensor/detector sees the sum of the thermal emission from the surface it is looking at. That emission is a function of it's physical temperature and the reflection of the background emissions. Where the energy from the background is a function of it's temperature and emissivity.
So if the emissivity of the surface is 0.5 the sensor sees half of the self emission plus half of the reflection from background.
If the radiative power or temperature of the surface and the radiative power or temperature of the background is the same then it doesn't matter what the emissivity of the surface is, the radiative power leaving the surface and incident on the detector will be unchanged no matter what the emissivity of the surface is. Higher emissivity is more self emission and less reflection. Lower emissivity is less self emission and more reflection.
Consequently, two objects with the same temperature but differing emissivities will be the same gray level or color (if a color display) in the thermal imager if the background they are reflecting is the same temperature. This example is a special case of basic radiative transfer in a scene.
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I don't pretend to understand these technical arguments but in the link mentioned above by cwyckham they assert that ice and seawater of the same temperature will appear different.
Quote:
Most of the ice in the sea surrounding Greenland originates from glaciers and therefore mostly consists of fresh water. The exact emissivity differs slightly depending on the circumstances, but generally speaking fresh water has a higher emissivity than the salty sea water. This means that even if the temperature of the ice and the seawater are the same temperature, there will still be a contrast between the two in the thermal image. Another factor is the movement of the surface. The surface of the seawater is ever moving, rippling and churning, while the surface of the ice is solid, still. Even when the amount of thermal radiation emitted to the thermal imaging camera is more or less the same, which means that the ice and the water have more or less the same color in the thermal image, the ice will stand out in the thermal image due to this difference.
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07-11-2014, 18:13
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#67
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Senior Cruiser
Join Date: Dec 2008
Location: Between Caribbean and Canada
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 8,503
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Re: Northwest Passage - 2014
Quote:
Originally Posted by DeepFrz
I guess we could both speculate on that but I really don't know.
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Yes, nor do I. But it would be really NICE to know.
I take you have sailed with bergs?
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07-11-2014, 18:43
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#68
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Registered User
Join Date: Mar 2006
Location: Winnipeg
Boat: None at this time
Posts: 8,462
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Re: Northwest Passage - 2014
Quote:
I take you have sailed with bergs?
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No, thank goodness. Most of my sailing was on the West Coast of BC.
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07-11-2014, 20:41
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#69
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Registered User
Join Date: Aug 2009
Location: California
Boat: Valiant 40
Posts: 431
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Re: Northwest Passage - 2014
Quote:
Originally Posted by hpeer
Yes, nor do I. But it would be really NICE to know.
I take you have sailed with bergs?
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I did the ice bucket test. Because it's a very small world, I'll be paddling with an engineer from a major thermal imaging company tomorrow morning. I'll ask him about the ice berg question. If anyone would know, he will.
JRM
Sent from my SAMSUNG-SM-G900A using Cruisers Sailing Forum mobile app
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07-11-2014, 21:46
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#70
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Registered User
Join Date: Jan 2008
Boat: Teak Yawl, 37'
Posts: 2,980
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Re: Northwest Passage - 2014
Quote:
Originally Posted by SailOar
I don't pretend to understand these technical arguments but in the link mentioned above by cwyckham they assert that ice and seawater of the same temperature will appear different.
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That would be my conclusion also. That is because of the cold sky in the infrared. In this case the sky thermal radiation that is reflected by the ice is generally much colder than the radiation emitted by the ice or liquid water.
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08-11-2014, 06:04
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#71
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Registered User
Join Date: Jul 2005
Location: Bellingham
Boat: Outbound 44
Posts: 9,319
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Re: Northwest Passage - 2014
Quote:
Originally Posted by LakeSuperior
The two items with differing emissivities will show up at different apparent temps only if the background radiative temperature is not equal to the item temperatures like with a clear sky.
If the sky is cloudy or low overcast then you may not be able to see/detect the temperature difference.
Additionally, the emissivities of ice and water vary with angle. For example, see sea water
Emissivity of the Ocean | The Science of Doom
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If the items have different emissivity, then they have different reflectivity. In practice in an environment like sea, clouds and ice I'd expect there to be enough radiation difference to stand out on a FLIR display. Fog does tend to block the IR.
__________________
Paul
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09-11-2014, 14:39
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#72
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Registered User
Join Date: Aug 2009
Location: California
Boat: Valiant 40
Posts: 431
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Re: Northwest Passage - 2014
Spoke to the engineer yesterday, and had him put it in layman's terms as much as possible.
The practical answer is yes, it works well. They do a lot with application specific wavelength filters, and have played around with this exact scenario. At one point, software for auto recognition was developed, but then not marketed. Their design market in this space was for larger commercial vessels that could better afford the technology and were watching for larger bergs.
JRM
Sent from my SAMSUNG-SM-G900A using Cruisers Sailing Forum mobile app
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11-05-2015, 20:42
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#74
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Senior Cruiser
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,888
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Re: Northwest Passage - 2014
Quote:
Originally Posted by DeepFrz
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Incredible:
"Watson’s team found that the record of sea level rise during the early 1990s was too high. ...The results revise downwards the average rate of sea level rise since the 1990s. The IPCC’s landmark report in 2013 found the sea had risen on average by 3.2 mm per year since 1993. Waston’s study found the rate was slightly slower, between 2.6 and 2.9 mm per year."
So SLR is now thought to be only 85% of that previously calculated. And that gets spun into more gloom and doom by the Grauniad.
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11-05-2015, 21:01
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#75
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Registered User
Join Date: Mar 2006
Location: Winnipeg
Boat: None at this time
Posts: 8,462
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Re: Northwest Passage - 2014
But the story is about the RATE of sea level rise Stu. The RATE is increasing.
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