Curiouser and curiouser...

[MountainDon]

The roof on the cargo trailer is some sort of galvalume steel, greyish in color, somewhat mottled. I have an IR thermometer. Yesterday it was in the 60's outside. The sun shining on the white aluminum skin gave me a surface reading of 89 F. When I climbed a ladder and pointed the IR thermometer at the roof surface the reading jumped. Not up but down to 17 F. What!!    So what is going on?     I've never seen the IR thermostat drop like that when pointed at something that feels warm to the touch. It felt as warm or warmer than the white aluminum sides.

[Redoverfarm]

Don maybe something like radar.  There are some surfaces and material that absorb more rather than reflect or as least a portion thereof.  So actually you are not getting a true reading.   

[Don_P]

A stealth trailer... I like it 
does angle of incidence or distance matter?

[UK4X4]

Definitely to do with the reflective color, messing with the IR

Put a a piece of black duct tape on the roof,,,,you'll soon see some action !

Oilfield wise in desert - high heat locations all the pipework is painted silver to reflect the heat

mind you on the hot days even chrome still absorbs a lot of radiant heat.

In venezuela and Oman you'd often see us with buckets of water being used as tool holders !

Heat and heat transfer is a science in its self, in my field we don't use IR measurement for issues like you are seeing.

Standardly we'd use thermocouples or PT1000 sensors for measuring exterior temperatures

I sell design and install fiber optic temperature measurement systems for oilfiled and pipeline monitoring.

The tech is pretty interesting lay down a fiber anywhere you want and I'll tell you the temperature every 1.6ft along its length, up to 15KM from the measurement unit.

With nothing but light and a fiber, no sensors or detectors.

So I do and have similar issues in lots of jobs once we install and test.

1 job I had we had'nt even installed the fiber yet it was just on the roll, I had huge spiky data 60oC down to 15oC.

I thought the machine was busted, but it was just radient heat, early in the morning the drum was still cold underneath but hot on top from the sun, the bottom had'nt caught up with ambient temps and was still at night times temps

So I had a hot spike every time the cable hit the top of the reel and a cold spike every time it reached the bottom.

We had to wait 4 hours till the client would believe me ! 
we had to rotate the drum every 15 mins till temperatures evened out and he'd let us install the fiber

[flyingvan]

   We'd use them a lot when I was on the SDFD HazMat team and there were certain reflective surfaces that would read cool.   I talked to the rep from MicroTemp and this was his explanation.
   Some surfaces reflect IR at the very surface, then just below the surface--typically two sides of the same coating.  The spacing is just right that the IR photon reflects of the surface and just below the surface and interferes with itself (somewhere in the 800nm range)
   Same thing makes the pretty rainbow colors youe see in a gutter when a hydrocarbon is spilled on water.  White light reflects off the surface of the oil and water at the same time.  Slight variations in the thickness of the oil (or gas, whatever) cause a different area of the spectrum to be cancelled out when the peaks and valleys of the same wavelength match.  (Opals do this too, BTW)
 

[MountainDon]

Thanks FV and UK

Here's something that helps explain the issue.

http://www.thermoworks.com/blog/2012/03 ... ermometry/

Myths about infared thermometers
Not all surfaces are created equal. Depending on what you're pointing your infrared gun at you're likely to get variations in emitted infrared energy. This variation is called emissivity. Emissivity is a measure of a materials ability to emit infrared energy. It is measured on a scale from just about 0.00 to just below 1.00.
Generally, the closer a material's emissivity rating is to 1.00, the more that material tends to absorb reflected or ambient infrared energy and emit only its own infrared radiation. Most organic materials, including the byproducts of plants and animals, have an emissivity rating of 0.95. These are ideal surfaces for accurate temperature readings.
Emissivity 3 Three common misconceptions about infrared thermometersSubstances with very low emissivity ratings, like highly-polished metals, tend to be very reflective of ambient infrared energy and less effective at emitting their own electromagnetic waves. If you were to point an infrared thermometer with fixed emissivity at the side of a stainless steel pot filled with boiling water, for example, you might get a reading closer to 100°F (38°C) than 212°F (100°C). That's because the shiny metal is better at reflecting the ambient radiation of the room than it is at emitting its own infrared radiation.
What is fixed emissivity?
Fixed emissivity is a setting in some infrared thermometers (usually of 0.95 or 0.97) that attempts to simplify their operation while leaving them suitable for most material surfaces, including almost all foods. Other infrared thermometers come with adjustable emissivity settings, so you can more accurately prepare your thermometer for the type of surface being measured, particularly when measuring non-organic surfaces

[flyingvan]

I like that explanation better.  The emissivity could be low to space, but the heat transfer to something solid, like say to your hand, could still be high