What atmospheric condition will result in an increase in air density?

Increased air density is a result of increased pressure. According to the ideal gas law, air density is directly proportional to pressure. When the pressure of the atmosphere increases, it compresses the air molecules together, resulting in a higher density. This is why during conditions such as a weather front that brings high pressure, you may experience a denser atmosphere.

Changes in temperature also affect air density, but decreasing the temperature typically leads to an increase in air density as it lowers the energy of the air molecules, causing them to pack closer together. However, with the given options, increasing pressure is the more direct and influential factor for increasing air density.

Conversely, increased altitude decreases air pressure, thereby reducing air density. Increased humidity, while it may seem counterintuitive, leads to decreased air density because water vapor is less dense than the nitrogen and oxygen it displaces. Thus, while both temperature and pressure can affect air density, increased pressure stands out as the definitive cause among the choices presented.