STANDARD TEMPERATURE AND PRESSURE (STP)

To set a reference for temperature and pressure against which measurements can be made and experiments conducted, scientists have defined standard temperature and pressure (STP). This is a more or less typical state of affairs at sea level on the Earth’s surface when the air is dry.
The standard temperature is 0°C (32°F), which is the freezing point or melting point of pure liquid water. Standard pressure is the air pressure that will support a column of mercury 0.760 m (just a little less than 30 in) high. This is the proverbial 14.7 pounds per inch squared (lb/in²), which translates to approximately 1.01 × 10⁵ newtons per meter squared (N/m²).
Air is surprisingly massive. We don’t think of air as having significant mass, but this is because we’re immersed in it. When you dive only a couple of meters down in a swimming pool, you don’t feel a lot of pressure and the water does not feel massive, but if you calculate the huge amount of mass above you, it might scare you out of the water! The density of dry air at STP is approximately 1.29 kg/m³. A parcel of air measuring 4.00 m high by 4.00 m deep by 4.00 m wide, the size of a large bedroom, masses 82.6 kg. In Earth’s gravitational field, that translates to 182 pounds, the weight of a good-sized, full-grown man.