What happens to specific humidity and relative humidity when air passes through a cooling coil below its dew point?

When air passes through a cooling coil that is below its dew point, condensation occurs. This leads to a significant reduction in the amount of water vapor present in the air, which is reflected by a decrease in specific humidity. Specific humidity is defined as the mass of water vapor per unit mass of air, so when condensation takes place, the mass of water vapor in the air decreases, resulting in a drop in specific humidity.

At the same time, as the air continues to cool and the water vapor condenses, the relative humidity increases. Relative humidity is the ratio of the current moisture content of the air to the maximum amount of moisture the air can hold at a given temperature. When air cools, its capacity to hold water vapor decreases, meaning that even if the actual amount of moisture in the air decreases, the percentage of saturation rises, pushing the relative humidity higher.

Thus, while specific humidity decreases due to the loss of water vapor from condensation, relative humidity increases because the cooler air can no longer retain the same amount of moisture it could at a higher temperature. This captures the interaction between temperature, moisture content, and the resulting changes in both specific and relative humidity during the cooling process.