Frost jackets are made from horticultural fleece, a non-woven material with varying weight ratings. These ratings, measured in grams per square meter (gsm) or ounces per square yard (oz/sq yd), determine the level of insulation and sunlight penetration. For example, medium-weight fleece (30-35gsm) offers moderate protection and allows 65% sunlight, while ultra-heavy fleece (70-100gsm) provides maximum insulation but blocks more sunlight.