All the shirts are the standard "coldgear" series from Under Armour, which should be the heavyweight material for cold weather activites (55 degrees or less according to UA). The UA "metal" coldgear seriesalso features aneven thicker material, except for a small portion under the armpits which has a lighweight material for more breathability and movement.

I believe thelightweight and medium-weightyou are referring to are the UA "heat gear" and "all-seasons gear", which are thinner lighter material for warmer temperatures. But please correct me if I am mistaken.

I hope this helps!