A heat advisory is issued when prolonged periods of high heat and humidity are expected to create a dangerous situation where heat illnesses are possible. These conditions can be hazardous, particularly for vulnerable populations like the elderly, children, and those with pre-existing health conditions. During a heat advisory, it's crucial to take precautions such as staying hydrated, seeking air-conditioned environments, limiting outdoor activities, and checking on neighbors and loved ones to prevent heat-related health issues.