[SquarishTriangle (OP)]

Suppose I am wearing a long-sleeved jumper when the day starts to get uncomfortably warm, and a somewhat unsightly Bee Gees t-shirt underneath prevents me from removing the outer layer.

If I want to lose the maximum amount of heat while still wearing the jumper, should I roll my sleeves up to help lose some extra heat from my lower arms but make a thicker layer of jumper further up with possibly less ventilation, or should I leave the sleeves long and at an even thickness across the whole arm?

[LeeE]

Roll your sleeves up.  Remember not to wear the Bee Gees t-shirt.

[DoctorBeaver]

Remove the outer layer but remember to wear a sign around your neck saying "I won the bet by wearing this T-shirt"  []

If you consider that rolling your sleeves down on a cold day will keep your arms warmer then it is obvious that rolling them up will cause you to lose more heat. In fact, the more skin you expose, you more heat you will lose (do not try this in a crowded shopping centre unless you belong to the same Masonic lodge as the Chief Constable).

However, it should be noted that in very hot climates the inhabitants often wear full-length, light-coloured robes as the reflective power of the robe will keep you cooler than looning around starkers.

[Karsten]

I would say it also depends on how tight the long sleeve shirt is. A lose shirt may offer more air-flow with the sleeves rolled down than rolled up. If you roll up the sleeves on a lose shirt you will cut air circulation to the rest of the upper body since you effectively create a seal around each arm.

You may want to consider getting the shirt wet instead of rolling sleeves up. As in jump-in-the-water-and-get-out-again-wet. A wet, long-sleeve cotton shirt is an excellent cooling gown.

Karsten

[LeeE]

I can't see how the movement of air across the opening of even a very loose sleeve is going to provide cooling to the upper body, at least not without becoming a poncho/cape instead of a shirt.

In practice, there'll be a point where the sleeve opening becomes too large and simply collapses, so I don't think you can have an effectively arbitrary sized sleeve opening.  Also, if a sleeve is very loose, then why should it not still be relatively loose once rolled up?  Sure, it may be tighter once rolled up, but it'll still be looser than a tight sleeve.

[Karsten]

I looked at my post and I have to rephrase:

A) A loose shirt is better than a tight shirt.
B)Sometimes it felt to me that a loose shirt with sleeves rolled up is hotter than with the sleeves down.
C) A loose shirt with the sleeves rolled up allows more air flow to the lower arms but cuts circulation to the upper arms and body a lot. You are closing two ventilation holes and that may not be best.

But maybe I remember this all wrong. I will check this summer. Maybe someone from the Southern hemisphere can report results immediately?

Karsten