Final answer:
An increase in ambient temperature would increase the density altitude at an airport, as warm air expands and becomes less dense, simulating conditions at a higher altitude.
Step-by-step explanation:
The factor that would tend to increase the density altitude at a given airport is an increase in ambient temperature. Density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the current air density. When the ambient temperature increases, air becomes less dense, and density altitude rises, effectively creating the conditions one would experience at a higher altitude. This is because air expands and becomes less dense when it's warmer. On the other hand, increasing barometric pressure would decrease density altitude because it increases air density. Similarly, decreasing relative humidity would decrease density altitude, although its effect is much less significant than that of temperature or pressure changes.