Which set of conditions increases atmospheric density?

• A. Pressure - Low; Temperature - High; Humidity - High
• B. Pressure - Low; Temperature - Low; Humidity - Low
• C. Pressure - High; Temperature - High; Humidity - High
• D. Pressure - High; Temperature - Low; Humidity - Low

Answer: (D)

Air density rises with higher pressure and falls with higher temperature, while humidity tends to reduce density. Higher pressure packs more air molecules into a given space, increasing density. Lower temperature slows molecular motion and lets molecules be closer together, also increasing density. Humidity lowers density because water vapor has a lower molecular weight than the main components of dry air, so moist air is lighter at the same pressure and temperature.

So the densest air occurs with high pressure, low temperature, and low humidity. That matches a scenario with high pressure, low temperature, and low humidity, producing the greatest density. If humidity were high or the temperature higher, density would be lower.