Why does a cooling tower emit “white smoke”?

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The cooling towers we see always emitting \"white smoke,\" as shown in Figure 1; in fact, it is not smoke but merely hot air. A cooling tower transfers waste heat generated in industrial production processes to the atmosphere through heat and mass exchange with air. In wet cooling towers, the industrial circulating cooling water comes into direct contact with air, and part of the hot water evaporates, carrying its latent heat into the air. The temperature of the air rises and humidity increases, reaching 90%–100%. Figure 1: “White smoke” appears at the outlet of the cooling tower before disappearing. Why does air passing through the cooling tower exhibit a phenomenon similar to white smoke at its outlet? This is related to the properties of air, which is composed of dry air and water vapor. The constant components of dry air are nitrogen, oxygen, and noble gases; these components remain almost unchanged as a result of the various processes in nature compensating for each other. However, the amount of water vapor in the air can vary greatly, and the properties of the air also change depending on the water vapor content. There is a limit to the amount of water vapor in the air; once this limit is reached, the air can no longer hold any more water vapor, and at that point the air becomes saturated air. The maximum level of water vapor in the air increases as the temperature rises. The amount of water vapor present in the air is expressed as relative humidity, which is the ratio of the amount of water vapor in the air at a certain temperature to the amount of water vapor in saturated air at that same temperature; the relative humidity of saturated air is 100%. When the temperature of saturated air drops, the amount of water vapor that the air can hold decreases, causing some of the water vapor to turn into liquid form. This liquid water exists in extremely small particles, giving the appearance of \"white smoke\". Fog in nature is formed in just the same way. The humid and hot air at the exit of the cooling tower is saturated or nearly saturated; after leaving the cooling tower and mixing with the cooler air around it, part of the water vapor in this humid and hot air turns into liquid, resulting in \"white smoke\". The process of this change can be seen very clearly in Figure 2, where the curve represents the saturated air curve. The air at the exit of the cooling tower is labeled A; it is saturated with a temperature of 30 degrees. The air outside the tower is labeled C, with a temperature of 2.5 degrees and is unsaturated. The humid and hot air exiting the tower mixes with the cold air outside the tower, eventually reaching point C. During this mixing process, the air condition changes from state A to state B, becoming supersaturated; it is at this stage that \"white smoke\" appears. Figure 2: Humidity diagram of moist air. Note: This article is from Zhao Shunan’s Science Blog