ANS:C - high humidity

Desert air coolers being rendered ineffective in the rainy season is primarily due to high humidity. Desert air coolers, also known as evaporative coolers or swamp coolers, work by evaporating water into the air, which results in a cooling effect. In dry climates, such as deserts, where humidity levels are typically low, evaporative cooling is highly effective because the air can absorb more moisture. However, during the rainy season, humidity levels increase significantly due to the moisture content in the air from rainfall and higher atmospheric humidity. When the humidity levels are already high, the air has less capacity to absorb additional moisture. As a result, the effectiveness of evaporative cooling is reduced because there is less evaporation of water into the already humid air. Therefore, desert air coolers become less effective in the rainy season primarily due to the high humidity, which limits the cooling capacity of evaporative cooling systems.