When is the atmosphere more dense?

The atmosphere is denser when both temperature and humidity are low, while pressure is high due to the fundamental principles of gas behavior. Cold air is more dense than warm air because the molecules are closer together at lower temperatures, increasing the mass per unit volume. High pressure contributes to this density as it compresses the air, forcing the molecules even closer together.

Additionally, low humidity means that there is less water vapor in the air. Water vapor is less dense than the nitrogen and oxygen molecules that make up the majority of dry air. Therefore, when humidity is low, the air remains denser overall because it is composed predominantly of these heavier gases.

In summary, the combination of high pressure and low temperature leads to a denser atmosphere, while low humidity further contributes to this effect by minimizing the presence of lighter water vapor.