Air pressure: the standard value, the barometer and height
Gravity holds the air around Earth, and the weight of the air above a place presses on it. This is the atmospheric pressure. It acts in every direction, not only downward. Most of the air’s mass lies in the lowest 30 km (book), so the air thins as you go up: there are fewer molecules in each breath, although the air is still about 21% oxygen. The body answers by making more red blood cells. Standard atmospheric pressure is the pressure at sea level at 0 °C: 1.013 × 10⁵ N/m² = 1013 mbar = 760 mm of mercury = 1 atm. Weather maps use millibars and isobars. In a mercury barometer the mercury stands 760 mm above its level in the dish at sea level, and the column is shorter at greater heights. The book also mentions that the low pressure at great height may make nosebleeds more likely for some climbers (dry air plays a part too). Blood has four parts: plasma (the liquid with proteins, nutrients, hormones and wastes), red cells (haemoglobin carries oxygen and a little carbon dioxide), white cells (defence) and platelets (clotting).
Card over a glass, with a grown-up and over a sink. Fill a plastic glass to the brim with water, place a stiff card on top, hold it, and turn the glass upside down. Take your hand off the card and describe what happens and why the air pressure can hold it. Then try it with a half-full glass. Write what changed. Also keep a log of your weather app’s pressure reading for three days.