Tar softens when heat increases the movement of its oily and resinous parts. Fresh, thick, dark, or sheltered tar can soften more noticeably than thin weathered coatings.
Softening is one of tar’s most recognisable behaviours. On a warm day, a coating that seemed firm can become tacky or glossy. This happens because tar is not one rigid solid. It contains fractions that respond to temperature in different ways.
Sunlight often makes the effect stronger. Dark tar absorbs heat, so the surface can become much warmer than the surrounding air. That extra warmth can release smell, draw softer material toward the surface, and make dust or fibres stick.
Old tar usually softens less than fresh tar, but it can still respond to heat. Weathered surfaces have lost some lighter fractions and formed a harder outer layer. Under that layer, however, there may still be softer material, especially in thick coatings and sheltered crevices.
Softening is not always a defect. Flexibility helps tar move with wood and rope instead of cracking immediately. It becomes a problem when the coating smears, stains, traps dirt, creeps out of joints, or never becomes stable enough for the surface’s intended use.