A: Any clothing that allows you freedom of movement is probably appropriate. However, there are a number of fabrics used to make clothing for physical activity which are not only comfortable, but have the advantage of being quick-drying, 'wicking', anti-bacterial and other characteristics which make them good choices.
Most people agree that although cotton is comfortable to wear, it is VERY absorbent. That means that in any situation where you are perspiring and want to dissipate that heat away from your body, cotton does not work well. The newer fabrics like Dri - line, Dri -lete, Coolmax, Coolmax-ultra, Dry-Fit, etc. are fabrics that are constructed to literally 'pull' moisture away from the body, acting like a wick.
In Hawai'i's warm, humid climate this is particularly useful, since the moisture in the air makes it more difficult for the perspiration to evaporate and naturally cool the body. These fabrics then not only absorb the perspiration, but move it to the outside of the garment. This keeps you cooler while you are active and helps to keep you from getting chilled as quickly, once you stop moving.
These fabrics are used to make shorts, shirts, jackets, sports bras, briefs, tights etc. In cooler climates, where layering garments may be required, remember that an under-layer made of a technical fiber overlaid with cotton or other non-technical fiber, will trap moisture and may cause you to chill faster.
Most garments have a great deal of information on the product tags. However, if you are in a store where the salespeople are knowledgeable, they should be able to explain the differences between various fabric types and construction.
A: The 20 degree rule is the best rule-of-thumb to apply. Dress as though it were 20 degrees warmer than the starting temperature. As you run your body will heat up and it will react as if it were 70 instead of 50. Also, I have found that dressing in layers that can be shed and tied around your waist (or discarded) is useful and works well. The 20 degree rule is applicable in any weather, although wind-chill and cold, driving rain may change that somewhat.
Remember also that there is a certain degree of heat loss from the head. Hats may be too warm, but ear-warmers (depending on temperature) may be very comfortable. Again, a technical fiber construction will be more comfortable.