In hot weather, tar can soften, smell more strongly, become glossier, move slightly, pick up dirt, or feel tacky, especially if it is fresh, thick, or exposed to direct sun.
Heat wakes tar up. Many tars are mixtures of heavy and lighter compounds rather than one fixed substance. When the temperature rises, the softer fractions move more easily. A coating that felt firm in cool weather may become tacky or glossy on a hot day.
The effect is strongest on dark exposed surfaces because black and brown tar absorb sunlight. A tarred fence, roof edge, boat part, or outdoor object can become much warmer than the surrounding air. That extra surface heat can release smell and make the coating more prone to marking.
Fresh tar is usually more sensitive than old weathered tar. Thick applications are also more likely to stay soft because the inner material is shielded from air and takes longer to settle. Dust, leaves, insects, and fibres can become stuck in the surface while it is warm and tacky.
Hot-weather behaviour is one reason thin, controlled coats often work better than heavy applications. Tar needs enough material to protect, but too much can stay soft, creep, and collect dirt long after the surface looks finished.