On a chilly Friday morning, with the outside temperature at 7 °C and the heating just turned on, the air inside a living room is at 20 °C with 60 % relative humidity. The air in such conditions has a dew point of about 12.0 °C, meaning that if any surface in the room drops below this temperature, the water vapor in the air will condense into liquid. A single-glazed window, which is at approximately 10.2 °C on the inside, falls below this dew point, causing fog and water to run down the glass. This happens even though the room is warm and heated, because the window is the coldest surface in the room. The dew point is calculated using the Magnus formula, which relates relative humidity and temperature to the temperature at which condensation occurs. For example, at 20 °C and 60 % humidity, the saturated vapor pressure is about 23.4 hPa, and the air contains 14.0 hPa of water vapor, or 60 % of that maximum. This leads to a dew point of 12.0 °C. MétéoSuisse, a Swiss meteorological service, defines the dew point as the temperature to which air must be cooled for condensation to occur. Vaisala, a manufacturer of humidity sensors, notes that the dew point remains constant regardless of air temperature, making it a more stable indicator of humidity than relative humidity. A double-glazed window, by contrast, has a much higher interior surface temperature—around 17.8 °C—because it is more insulating. This temperature is well above the dew point, so no fog forms. The difference between the two window types is 7.6 °C, showing how insulation can significantly affect condensation. However, the dew point in the room remains at 12.0 °C, regardless of the window type or room temperature. This stability makes the dew point a useful measure for predicting condensation on surfaces. The temperature of a window’s interior surface is determined by factors such as the U-value, which measures how well a material conducts heat, and the surface resistance, which accounts for the insulating effect of air near the surface. For a single-glazed window, the calculation shows that the interior surface temperature is around 10.2 °C, which is below the dew point and causes condensation. A hygrometer, a device that measures humidity, can help determine the relative humidity in the room and compare it to these dew point values. For instance, at 50 % humidity, the dew point would be about 9.3 °C, and the margin between the window’s temperature and the dew point would be very small, making condensation more likely.