If you want to keep your hands warm, you need to keep your arms warm. I find that in temperatures down to near freezing, I don't need a top when I'm running except for the problem of cold hands. Gloves obviously help, but if your arms are cold, then you have chilled blood flowing to your hands. Worse, most of the muscles that work your hands are in your forearm and if they get too cold, you can't use your hands. Therefore I started looking for some warm arm warmers, and being unable to work out which were the warmest, I ordered a few pairs.
Nearly all of the arm warmers I ordered seemed to be designed for cycling, not running. They were made of dense material, because air movement is higher when cycling, but were not thick enough. More annoying is that they were not comfortable when the elbow is bent to the right Arm Position for running. (Cycling typically has a far straighter arm than running). Therefore the best arm warmer turned out to be a converted hunting sock rather than an 'arm warmer'.
1 The Best - Sock Conversion
This was an act of desperation having found all of the arm warmers I ordered were insufficient. I got a pair of Merino Wool hunting socks from the local Bass Pro store. I used hunting socks as they were the only ones long enough to come down over my wrist. The ankle bend works great at the elbow and they stay up the upper arm surprisingly well. These are not a cheap, and the socks were more expensive than all but two of the purpose made arm warmers.
2 Recommended - Asics
The Asics arm warmers are quite thin, but they are unusual in that they are long enough to come down over your hands. The even fold over and have a thumb hole for partial coverage. They are quite long and are available in a number of colors. They are flexible enough to fit well over the upper arm.
3 Worth Considering – Smartwool
The Smartwool arm warmers are quite different to the others here in that they are far less dense and are more flexible. This thinness gives good flexibility, and even without a tacky band at the top they stay up well. They are seamless, quite long and not made for different left/right arms.
4 Pearl Izumi
It seems that Pearl Izumi tried harder than other manufacturers. The arm warmers are different between left and right, with separate panels over the bicep. I'm not sure if the bicep panel is different material from the rest of the arm warmer, but I couldn't detect any difference. The cut at the top of the arm warmer is not straight, but angled to fit a little better. They are much bigger at the top than the other arm warmers, so these may work for runners with more arm muscle, but for me they were quite baggy.
I had high hopes for the Craft arm warmers, as the other clothing I have from them is well made and highly functional. These arm warmers a different for left and right arms, but the difference is subtle. They fit okay, but because they are not flexible enough they are a little loose at the top of my arm and tight in other places. Runners with more arm muscle would probably find the fit worse. The length was okay, but not great.
6 Sugio SubZero
With a name like 'SubZero' I was sure these would be warm enough. They are quite thick compared with most of the other arm warmers, but not thick enough to provide enough warmth. They fitted quite well, though they were one of the least flexible arm warmers which cut into the circulation even after a short time.
7 Sugio MidZero
The MidZero managed to combine thin material for a lack of warmth with inflexibility for poor fit. They were also too short – much shorter than the SubZero.
8 Mizuno Breath Termo
I did not try the Mizuno Breath Thermo arm warmers because I do not like the material. The idea of Breath Thermo is that it generates heat, but it only does so when you are sweating, which is when you don't want the extra heat.