How do thumb holes help under strong sunlight?
Even if you want to reduce the exposure of the back of your hands outdoors, it is difficult to judge how to utilize it just by the feature's name.
Inserting your thumbs into the thumb holes to keep the sleeves over the backs of your hands helps reduce the area of exposure under strong sunlight[1].
When you take your thumb out, you can leave the sleeve as it is, and when you insert it, you can hold the end of the sleeve so that it stays over the back of your hand. In the Umbra Sun Hoodie Aqua, these holes are applied to the sleeves[2]. When moving under strong sunlight, you can insert your thumbs to use them as an auxiliary means to reduce the exposure area on the backs of your hands.
UPF is an indicator of how well a fabric blocks ultraviolet rays. Since a thumb hole is a structure that holds the sleeve in place, it does not in itself guarantee the fabric's UV protection performance. It is better to check the fabric's UPF rating and also prepare separate sun protection methods for the fingers and gaps that the sleeves do not cover.
We have not yet confirmed exactly how far the back of the hand is actually covered when using the thumb holes.