Taking the temperature, who'd have thought it could be so difficult?
The "official" temperature is the temperature of the air, away from solid objects which could heat or cool it so strictly the temperature of the air at about head height. To measure this in your greenhouse, you'll have to protect the thermometer from the heating effect of the sun. It will need to go into a shaded location and not one right next to another object that will be heated by the sun. I have mine inside a white polystyrene enclosure. You could test your enclosure by taking the temperature in the sun out in the middle of your yard. If you get something close to the official air temperature for your location then the enclosure is working, usually it will read quite a bit warmer.
A thermometer in the sun only tells you how hot the thermometer is and that's fairly pointless because it isn't the same temperature as anything else like you or a plant. It isn't even the same temperature as another thermometer right next to it which tells you how pointless it is. One possibility is to place the thermometer not in the shade but in a strong stream of air such as in front of a fan. If there is enough air movement, then the thermometer will read fairly close to the actual air temperature even if it is in the sun. Remember not to place it in an air stream coming from outside if you want to know the temperature of the air inside the greenhouse
The next question is whether you actually want to measure the temperature of the air. After all, the temperature of the plants and soil is what really matters and it could be quite a lot higher than the air. Or lower. The effects of heat directly from the sun and reflected from objects including the greenhouse glazing are what is really damaging in a greenhouse and you can scorch plants even when the air is quite cool. For example, air at 50C but without any sun would be quite tolerable for most cacti, but air at 50C with the sun shining through glass would kill almost any cactus quite quickly. Still air is more harmful than moving air even at the same temperature because moving air takes heat away from objects sitting in the sun. And objects in the sun under glass heat up more than objects in the open, because the sky is cold believe it or not (read up on radiative cooling if you want to know more, its why your car windscreen freezes over before anything else and before the air temperature drops to freezing, its also why the sun feels much hotter through glass even when the air is the same temperature). You could experiment with probes in pots or even in plants if you like, but generally I think it is impractical to measure "the temperature in the sun" in any useful way. Worth thinking about though.
So you measure the temperature of the air, but where in the greenhouse? Does it matter? After all, do you really know the temperature at which your plants will be damaged in the conditions of your greenouse? And it will vary wildly depending on the amount of air movement and the strength of the sun anyway. Play around measuring temperatures in different parts of the greenhouse to see how much variation there is and then pick a spot and leave it there. Call that the temperature even if bears no relation to anything else, because at least it will be consistent for you and you'll be able to compare it with "temperatures" the plants have experienced before.
Taking the temperature, who'd have thought it could be so difficult?
Would the large size of the GH help to keep the hot air far enough away from the plants? With the sides rolled up it should keep the bench plants at a reasonable temp.
