How hot do cacti get?

[Pattock]

It's clearly not measuring the temperature of the sun's reflected rays, otherwise the temperature would be reading 5500C.

That would only be true if the reflection from the surface was perfect and if the camera had no polarised light filters to cut down on interference from reflections.

There's a good explanation of thermal cameras here:
https://talkingphysics.wordpress.com/20 ... mperature/

As they write on that page:

Students tend to think that tools that are more high-tech tend to be better than their lower-tech cousins. This isn’t true and frequently the lower-tech devices are better. To measure a temperature I’ll take a thermistor or thermocouple any day of the week.

I dont think spines will fool the camera. Its pixels are are far too big to be picking up individual spines.

The light received by an individual sensor will be from both the spines and the stem surface. The individual pixel will report both without discrimination so that pixel shows an average of all the incident radiation over that area of the sensor.

I think your pictures show how well adapted cacti are by selection for infrared reflection. They can tolerate very high temperatures but they prefer to ameliorate those extremes.

[ralphrmartin]

That would only be true if the reflection from the surface was perfect and if the camera had no polarised light filters to cut down on interference from reflections.

IR needs special lenses and optical components made from quite different materials to visible light. Normal thermal cameras don't use polarisation filters.

[FredG]

I did a project measuring root temperatures in some of my plants, ie. Darlingtonia, some years ago. Digital (with probe) and analogue thermometers did the job perfectly well. After all it's the temperature of the medium that counts not the outside of the pot.
As said previously, it's only the outside rank of pots that get the sun exposure on the side. The main exposure to sun on the inner ranks of pots is the top layer of medium and the plant itself, therefore the colour of the pot in these is irrelevant.

[Apicra]

I think reaching 48C is concerning, since this is only April and there is scope for another 15C rise in outside temps at the peak of summer. I'm not sure what to make of the variation in the results, but more could be learnt by investigating further.

The approach should be to try and separate heating/cooling processes from each other. So the compost should be completely dry to remove any possibility of evaporative cooling. Obviously having all pots of the same manufacture but just different colours is ideal. A "control" for comparison should be in the greenhouse nearby but in the shade, for comparison with the pots heated directly by the sun. Pots should be away from vents to avoid any cooling wind. It all gets complicated!

Best wishes,
Derek

[FredG]

I'm puzzled as to why you're concerned Derek. Pots have been heating up just the same for many many years without problems. We just haven't been measuring it like this.

[el48tel]

I think we may have overlooked the physics / meteorology. (and year 10 science)
Soil / land / rock heats up very quickly from solar gain. But it also cools down very quickly once the sun is no longer shining on it.
Water (and cacti / succulents are mainly water) heat up very slowly and cool very slowly.
There's about an at least 6x difference in the heat capacity values in terms of how much energy you need to raise the temperatures of each. Then factor in the surface characteristics for the exchange of energy at the surface because of its colour etc.
And now the botany ................ only the plants which can tolerate extremes of temperature change will survive. (Spines and hair and external characteristics etc) By evolutionary change or by selection over millions of years, only the tolerant survive.
I like FredG's comment "I'm puzzled as to why you're concerned Derek. Pots have been heating up just the same for many many years without problems. We just haven't been measuring it like this. "