Final answer:
The density altitude is affected by both the temperature and the pressure altitude. A decrease in temperature typically lowers density altitude, while an increase in pressure altitude raises it. The combined effect on density altitude from going from 90°F at 1,250 feet to 55°F at 1,750 feet would need specific calculations to determine, accounting for the changes in air density.
Step-by-step explanation:
The effect of a temperature decrease and a pressure altitude increase on density altitude is governed by two main factors: the change in temperature and the change in pressure altitude. Density altitude is the altitude at which the density of the International Standard Atmosphere (ISA) corresponds to the air density given the current weather conditions. As temperature decreases, the air becomes denser because the molecules are closer together, and thus, density altitude decreases. However, as pressure altitude increases, the air pressure decreases and air becomes less dense, leading to an increase in density altitude.
Going from 90°F at a pressure altitude of 1,250 feet to 55°F at a pressure altitude of 1,750 feet, the overall impact on density altitude can be construed by considering these two changes. Typically, the decrease in temperature would reduce density altitude, while the increase in pressure altitude would raise density altitude. To fully understand the combined effect, a calculation based on the standard lapse rates and pressure adjustments would be necessary.
As air pressure decreases with increased altitude, and temperature changes could either increase or decrease density altitude, pilots must consider these changes for flight planning and performance calculations, as they affect the aircraft's lift and engine efficiency.