I enjoy the occasional hot drink, but place it below a small fan in order to cool it to a drinkable temperature. Unfortunately, as expected, I commonly forget about my drink, and it ends up very cold.
In fact, it ends up so cold that it feels much colder than I would expect given my relatively warm room ($\sim \mathrm{25^\circ C}$). However, this could be an illusion caused by room temperature still being less than body temperature, or the mug being fairly cold.
I’m wondering if the fan is able to cool the liquid below room temperature. I’m aware that evaporation works on the warmest molecules, and leaves the remaining liquid cooler, but I’m not sure that it can ever become cooler than the surrounding air temperature?
Best Answer
Yes, as other answers have stated, the temperature could drop below room temperature through evaporative cooling. In fact it could get as cold as the wet-bulb temperature of the air in the room. If you know the temperature and humidity of the air, you can figure out the wet-bulb temperature by using a psychrometric chart:
Find your room temperature on the green "Dry Bulb Temperature" scale and the room relative humidity on the red "Relative Humidity" scale. Locate the point where they meet, and read off that position on the light blue "Wet Bulb or Saturation Temperature" scale. For example, if the room temperature is 25°C and the relative humidity is 30% (which you indicated), the lowest temperature your drink could reach by evaporative cooling would be roughly 14°C. If the air was completely dry however (relative humidity 0%), it could reach about 8°C.
This does not necessarily mean that your drink will reach that temperature. It may require a large air flow, a large evaporation surface, and considerable time to actually reach the wet bulb temperature.