Gas laws
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It is because temperature ans pressure are directly proportional. So as the temperature decreases, so will the pressure. PV=nRT, and if you plug in higher or lower temperature, the pressure will increase or decrease directly.
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Because air expands when warm (hot) and contracts when cold. So if you inflate, say, a basket ball to normal pressure in a warm room at 90 degrees then take it out into the winter cold at, say, 30 degrees (F), to play basketball, the air in the ball would contract because of the cold and lose much of it's pressure, so it wouldn't bounce very well. Likewise, if you fill a tire with air to normal pressure - say, 32 PSI on a cold day then go driving on the highway, the tire would heat up from friction and if you check the pressure after several hours of driving, the air in the tire would have warmed considerably and you might find you had 45 or 50 pounds of pressure.