Do cold temperatures affect the Down Low Glow?
We have had some customers use our product for snow and ice biking in the northern states. The temperature does affect the performance, but only in the time it takes to warm up the light. Once it's warmed up, the low temperature is OK. Also, using a full battery is more important when the weather is cold. A battery that is less than half-charged may not be able to bring the lights to their 'superbright' state in cold weather.

Comments
Don't Freak Out!
This is very true. Don't freak out thinking your DLG's are broken just because they won't light up as brightly in cold weather. I found what helps is bringing the tubes to warm temperatures before use, like running them under warm water or the like. A heater is a great method of priming them too.
Through experience, I've found that any tubes that are facing the same direction that the wind will be coming at you will get colder faster during a commute, and get dull during your ride. Tubes that are parallel with the ground will not get cold during a commute as fast as a tube with full exposure to the wind coming at it, and will tend to keep lit longer