Long against short sleeves in the sun
Short answer: A loose long sleeve in a thin fabric can be cooler than a short one, because it keeps the sun off the skin. A tight long sleeve is warmer. What matters is whether the sleeve is loose and the fabric thin enough to let air through.
What the sun does
Direct sun heats the skin faster than the air does. A thin, loose sleeve shades the skin and still lets air through, which is why the arm feels cooler. The skin does not get the direct heat, and air can still move between fabric and skin. That is why a long sleeve can actually be cooler than a short one.
The sleeve as shade
Think of a loose long sleeve as a small shade with air underneath. It keeps the sun rays off the skin, while the movement of your arms swaps the air. It is the same idea as the clothing worn in hot countries, which is often loose and light. The effect needs thin fabric and a sleeve that does not sit tight.
When short is best
A short sleeve is best in the shade, in high humidity and when you move a lot. In the shade there is no sunlight to keep off, and in humid air moisture struggles to evaporate under a long sleeve. With a lot of movement, the arms need free air around them. See also bamboo leggings for women.
Loose or tight
Whether the sleeve is loose or tight matters a lot. A tight long sleeve lies directly on the skin, leaves no air between and becomes warmer than a short one. A loose sleeve gives air, and that air makes the difference. Check the fit at the elbow and wrist before you choose a piece for warm days.
Thin or thick fabric
A thin fabric lets air through and can shade without getting too warm. A thick fabric holds the heat, and then the long sleeve quickly becomes warmer than a short one. Choose a light fabric if you want long sleeves in the sun. A bamboo long sleeve t-shirt for men is an example of a garment with sleeves.
When both are met
If the sleeve is loose and the fabric thin, the long sleeve works like a shade with air underneath. You get protection from the sun and air against the skin. That combination gives the coolest solution on a sunny day, and it is the one to look for when you choose clothes for a longer time outdoors.
When it becomes warmer than short
If the fabric is thick or the sleeve tight, the long sleeve becomes warmer than a short one. It then holds heat and moisture close to the skin, and you get no benefit from the shade. If you cannot feel air between fabric and arm, the sleeve is too close for warm days.
Dresses and other pieces
The same thought applies to dresses with sleeves, where a loose, thin long sleeve can be a pleasant choice in the sun. The sleeve shades the arms while the rest of the dress lets air in. See bamboo dresses to see how sleeves and cuts vary when you want something that works in summer.
How to choose
Choose long sleeves in direct sun when the sleeve is loose and the fabric light. Choose short sleeves in the shade, in humid weather and for lots of movement. If you are unsure, try both on one day and notice the difference. Your own feeling is a good guide, because weather, skin and activity vary.
Frequently asked questions
Is long sleeve always warmer
No. In direct sun a loose long sleeve can be cooler than a short one, because it shades the sun and lets air through. That applies especially if the fabric is thin.
What about tight long sleeves
They are warmer, because there is no air between fabric and skin. You then get neither the shade nor the air, and the heat stays close to the skin instead of escaping.
What is best in the shade
Short sleeve, because there is no sun to shade from. Then it is better that the arms have free air around them and the moisture can evaporate without a sleeve in the way.
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