Air pressure and Bernoulli's Law

The density of air at 20oC is 1.2 Kg/m3 . In a room with dimensions 10 m X 10m X 2m there is roughly 0.3 tons of air. But why don't we feel this weight? The weight of the air that surrounds us is constantly pushing against us. This weight of air that constantly pushes from all sides is called air pressure. Since is pushes equally from all sides we hardly notice the weight of air that surrounds us. The only time we notice it is when the pressure of air suddenly changes, as happens on a windy day.

With so much weight acting on the walls of a room you might be expected to predict that the windows would be smashed under this enormous pressure. But they don't. There is equal pressure pushing inwards as is pushing outwards and so the window remains safe.

In heavy winds a great deal of damage is caused as roofs are ripped from their supports and flung a great distance away. But what actually causes the roof to lift off the house.

It is actually the air pressure inside the house that pushes the roof off its supports. According to Bernoulli's Law fast flowing air causes low pressure. As the wind flows quickly over the roof of the house the air pressure drops. The air pressure inside the house remains constant and eventually overwhelms the low air pressure outside with enough force to push the roof off its supports.

1) During strong winds it is recommended that some of the windows of the house are kept open. Why do you think this may help to prevent the roof from blowing away in the wind?

2) Below is a table of air density at different temperatures.

 Temperature oC Density kg/m3 -173 3.56 -73 1.75 20 1.21 227 0.70

a) Calculate the weight of air in a room with dimensions 11 m X 10 m X 3 m at -73oC .

b) Why does the density of air decrease as it gets hotter? Refer to particle theory.and density