Humidity and the body's water balance
Humidity is the water vapour in the air. Warmer air can hold more vapour. Air is saturated when it holds the most it can at that temperature and pressure. Relative humidity = (vapour present ÷ the most the air could hold at that temperature) × 100%. A hygrometer measures humidity, and 40% to 60% is comfortable. Sweat cools us only as it evaporates, because evaporation takes heat from the skin. In humid air sweat evaporates slowly, so the body cools poorly, we feel hotter and tired, and long exposure risks heat stress or heatstroke; humid heat feels worse because sweat cannot evaporate, not because the air is hotter. In dry air sweat evaporates fast, and skin and the lining of the nose and airways dry out, which risks infection and dehydration. Drink water and ask an adult if you feel unwell. Osmoregulation keeps water and salts in balance, which is needed for normal blood pressure and for cells to work. In dry air the body loses more water through sweat and breath; receptors in the brain sense this and the kidneys reabsorb more water, giving concentrated urine. The kidneys reabsorb water; they do not make it. In humid air plants lose water more slowly by transpiration, so less water and salts are pulled up the stem.
Wet and dry cloth, with a grown-up. Wet one cloth, keep a second dry, and put each on a different forearm in a gentle breeze. Say which feels cooler and why (evaporation takes heat). Wash your hands after. Then put a damp paper towel in a sealed bag and another in an open space for a day and describe the difference. Finally read the humidity from a weather app on two days and say which day sweat would evaporate faster.