What weather condition is indicated by the code 'HZ'?

The code 'HZ' in METAR reports indicates "haze." Haze refers to the presence of particles in the air that reduce visibility but are not heavy enough to form fog. Typically, haze can be caused by smoke, dust, or other pollutants, and while it might not significantly obstruct visibility like fog does, it can create a noticeable reduction in the clarity of the atmosphere. In contrast, the other options represent specific weather phenomena with distinct visual and operational implications, such as freezing fog, which occurs when moisture freezes upon contact with surfaces, and small hail/graupel, which refers to specific types of precipitation. Understanding these codes is crucial for interpreting weather conditions accurately in aviation and meteorology, where visibility can significantly impact operational safety.