How do temperature and elevation effect air pressure?

Why is it on cold days or nights my air gun doesn't seem as powerful?
 * When the temperature drops, the air density (volume of air moleules) increases. This means that if you pump your air gun to 60psi (which is usually very powerful), on a very cold day, this really means that the projectile will come out at about 20psi because the freezing cold air on the inside isn't phased by the cold air on the outside. Both the atmospheres of your air gun tank and the air outside are similar. This is known as Temperature-Density reduction. As the temperature rises you get more purchase on your compressed air.
 * Similarly, if you live on a mountain or high altitude area above sea level, your air guns function will also decline due to the atompsheric pressure at your height. (See here for a table of temperature-density readings)