Caves exchange air with the outside. In the Matienzo summer, when the outside temperature may be 30°C and cave air is at 10°C, certain entrances will blow cold air out and others will suck in the warm air. In winter and on cold nights, the reverse happens but the temperature difference is likely to be less than in summer months (i.e. outside 0°C, cave 10°C).
A cold, high-volume draught often indicates the potential for a large cave beyond any entrance constriction.
Infrared photography (picturing surfaces with a range of temperatures) should work to detect cave entrances and smaller holes which may be excavated to produce cave entrances. The supposition is that, on a warm day, cold air blowing out will cool the surrounding ground and vegetation and this effect will be recognisable using an infrared camera.
The camera used was a twin-lens (visible and IR) Flir One for Android which attaches directly to the micro-USB charging point on any Android device. (Flir web site). Android apps are available. The Android device used in Matienzo in the summer 2017 was a Samsung Galaxy Tab S2. A picture of the Flir One attached to a smart phone has been taken from the Flir One web site.
The Flir One for Android device currently has the following features: operating temperature range 0°C to 35°C; various palettes for viewing temperature differences; can detect 0.1°C temperature differences. The photo types produced are visible, infrared and a combined visible and infrared. The app also puts the low and high temperature figures on the screen and they are recorded on the photos.
The camera was first used on a powerfully draughting-out cave, Cueva Arenal, to get a feel for the Flir One's capabilities. The cooling effect of the river of cold air was immediately apparent including a cascade over the cliff outside the entrance and further ground and vegetation cooling at the base of the drop. (More details.)
Cueva-Cubío de la Reñada also provided definite, detectable differences in temperature around the draughting out, bottom cave entrance. The top entrance was draughting in and the extent of the warm air raising the temperature of the cave walls can be clearly seen in the photos. It was thought that a strongly draughting hole in the cave may have provided adiabatic cooling, but the effects cannot be seen using the Flir One for Android. (More details.)
When searching for new draughting holes on a warm hillside, it soon became clear that the rainbow palette being used would automatically show the coolest spot (whether draughting or not) thus providing many false promises. But it also became clear that a draughting hole could have a "signature": a dark blue spot where the draught emerged and a trail down from the hole progressively showing blue then green. This characteristic pattern was seen with most draughting holes, for example, see the IR photos from sites 0879, 1298, 3318, although site 4600 shows a less distinctive picture. A draughting shaft showed cooler edges to the top as the cold air spilled over, eg at site 4536.
It was found that putting a person in the picture generally expnded the temperature range, reducing the range of colours around the draughting area, eg site 4600.
In cold weather (March 2018), holes were photographed which were known to produce air warmer than the surroundings. The effect was noticeable, see sites 1185 and 1391 but there was no trail of warmer vegetation as the warmer air rises straight up. With holes surrounded by more vegetation, the warming effect on the surroundings may be visible.
The Android / Flir One combination is also capable of taking low resolution infrared video as seen for site 1298 (YouTube) and a walk up the track towards site 3318 (YouTube).
Of course, when holes draught out strongly, skin is a good detector of colder air! However, in dense undergrowth, where access is difficult, or where potential sites are some distance away, remote sensing using this IR method may prove of worth. More trials are required.
The Flir One/Android combination will detect temperature differences and has proved its potential usefulness by
1: distinguishing a range of temperatures with different colours
2: showing cold hollows with a dark blue spot
3: showing cold draughts with a dark blue area plus a trail of colder ground and vegetation (shown as blue and/or green) around and below the hole, depending on the volume of cold air being emitted.
4: Cold surface conditions are less useful due to the smaller temperature difference and the fact that warmer air rises straight up from the hole so having little, if any, warming effect on the surrounding ground and vegetation.
Juan Corrin 5/10/2017, 8/5/2018