At sea level, the weight of 1 cm2 of air is approximately 1 kilogram. This value, 1kg/ cm2 is also known as the atmospheric pressure. For this analysis, we will assume a featureless Earth without continents. This will simplify but not significantly alters your calculations.
Question : How much does the atmosphere weigh over each square meter of surface? Express this in tonnes (units of 1000 kg). Recall the conversion factors: 104 cm2 = 1m2, 1 tonne = 1000 kg = 106 g.
