Method
Temperature and the reading
Cold shortens a flaccid reading noticeably and warmth lengthens it; erect readings are far less sensitive, which is one more reason to report erect.
Ambient and skin temperature move a flaccid reading by an amount worth caring about. They barely move an erect one. That single asymmetry is a small, practical reason the literature leans on erect and stretched-flaccid figures rather than flaccid ones.
Why temperature acts on flaccid state specifically
Flaccid size is governed largely by smooth muscle tone in the tissue, which is temperature-responsive in the same way skin visibly is elsewhere on the body in the cold - contracting in response to cold and relaxing in warmth. That is a state effect, not a measurement error, and it runs in a consistent direction: cold produces a shorter, tighter flaccid reading, and warmth produces a longer, looser one. The size of the shift is not something this site puts a number on without a checked source behind it, but the direction is well established and large enough to be obvious to anyone who has measured in a cold bathroom and again after a hot shower.
Erect state is governed by a different mechanism - engorgement that overrides the resting smooth-muscle tone that temperature acts on. That does not make an erect reading perfectly temperature-proof, but the effect is far smaller relative to the size of the measurement, which is one more reason an erect or stretched-flaccid reading is the more repeatable choice when repeatability is the goal.
The practical rule
Measure at room temperature, and not immediately after a cold environment or a hot shower or bath. A few minutes at a stable, comfortable temperature before measuring removes most of the effect. This costs nothing and takes no extra time - it is a condition to notice, not a procedure to add.
A cold bathroom in winter, a warm one after a shower, and an air-conditioned room in summer are three different starting states for a flaccid reading, even before anything else about technique changes. None of them is wrong to measure in - the point is not to chase a special temperature, it is to notice which one you were in and keep it roughly the same across sessions you intend to compare. A reading taken straight out of a cold outdoor walk and a reading taken after a hot bath are not the same kind of number, whatever the ruler says.
If you are taking a flaccid reading at all, which the literature itself treats as the least useful of the states it reports, temperature is one of the reasons why: a number that shifts with the room it was taken in is not a number that means much on its own.
Where this fits with the rest of the protocol
Temperature is one variable among several that should stay fixed across a set of readings if those readings are meant to be comparable to each other. Taking repeat sessions at the same time of day controls for a related but separate set of state variables that drift across the day rather than across the room. Both are cheap habits that remove noise before it enters the number, rather than statistical work done afterward to account for it.
For anyone who wants the fuller picture of everything that makes a flaccid reading wander - arousal, recent activity, time since last erection, and temperature among them - that list is covered in one place rather than repeated here. The general method for taking the reading itself, temperature aside, is the same one the published studies use.
What a warm room does not fix
Controlling temperature gets you a flaccid, stretched, or erect length that is comparable to your own next reading and, with the right method, to the published data. It does nothing for a separate question some readers actually have, which is how the whole thing is perceived by another person - that is a subjective judgement, the kind a human reviewer on a site like Rate Penis gives, not a length in centimetres. It also has no bearing on a photo-based estimate: a picture carries no ambient temperature information at all, and whatever an image-analysis tool infers about size from a photograph is working from visual cues rather than a controlled measurement. A composite rating built from images, the kind a photo-scoring site like Penis Rater produces, answers a different question again and was never claiming to control for room temperature in the first place. And if the number you actually wanted was a subjective score rather than a length, that is what a site like Rate Cock is for - a warm room will not turn a ruler into one.