Standard Temperature And Pressure (STP):
To set a reference for temperature and pressure against which measurements can be made and experiments conduct, scientists have defined standard temperature and pressure (STP). This is a more or less usual state of affairs at sea level on the Earth's surface whenever the air is dry.
The standard temperature is 0°C (32°F) that is the freezing point or melting point of pure liquid water. Standard pressure is the air pressure which will support a column of mercury 0.760 m (just a slight less than 30 in) high. This is the recognizable 14.7 pounds per inch squared (lb/in2), that translates to around 1.01 x 105 newtons per meter squared (N/m2).
Air is amazingly massive. We do not think of air as having important mass, though this is as we are immersed in it. Whenever you dive only a couple of meters down in a swimming pool, you do not feel a lot of pressure and the water does not feel massive, though if you compute the huge quantity of mass above you, it may scare you out of the water! The density of dry air at STP is around 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 big bedroom, masses 82.6 kg. In Earth's gravitational field, it translates to 182 pounds, the weight of a fine-sized, full-grown man.