Temperature and humidity have a very strong influence on both oil and paper properties, often more through dynamics than through absolute values. Rising temperature increases oil’s water solubility, so the same moisture content in ppm can correspond to very different relative saturation and dielectric risk. At the same time, higher temperature accelerates oxidation of oil and hydrolytic aging of cellulose exponentially. Ambient humidity mainly affects the system through breathing and seals: moisture ingress slowly raises paper moisture, which then governs long-term dielectric strength and mechanical life. In practice, it is the combination of temperature cycles and moisture migration that drives most insulation aging, not either factor alone.