Air temperature, pressure, altitude and humidity variations make up the ambient air density. Decreased air density will result in a decrease in drag, and increased air density will result in a rise in drag. Humidity has a counter intuitive impact. Since water vapor has a density of 0.8 grams per litre, while dry air averages about 1.225 grams per litre, higher humidity actually decreases the air density, and therefore decreases the drag.
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