The heat index is a measurement of what hot weather feels like to the average person for various temperatures and relative humidities. Heat exhaustion and sunstroke are inclined to happen when the heat index reaches 105°F (40°C). The reason why it feels hotter to people when it is more humid is that the body cools itself by perspiring. Beads of perspiration help cool the skin as they evaporate, but when the air is too humid to allow for this evaporation, this natural cooling system no longer works well. The danger point for humans can occur when the heat index is between 90°F (32°C) and 104°F (40°C), and when the heat index reaches above these temperatures, weather conditions become very dangerous indeed. At this extreme, heat exhaustion, heat stroke, or sunstroke may occur quickly, especially during physical activity, and it is very important to stay in the shade and remain hydrated. The chart below provides the heat index for some temperatures and relative humidities.