b'PSYCHROMETRICSFIGURE 3B: CONDITION OF SUPPORTING PLANKING UNDER THE COPPER DOMEFIGURE 3A: CROSS-SECTION OF A FIVE-STOREY ROTUNDA EXHIBITING SYMPTOMS OF MOISTURE DISTRESScondenses to liquid at a predictable tempera-ture, which he calls the dew-point tempera-turealways at the same temperature for a given humidity level. Whats more, holding air temperature constant and increasing the mois-ture in the air also finds a unique dew-point temperature for a given air temperature. Joining the dots defines a saturation curve on a graph of air temperature vs. humidity ratio (Figure 1).Now we can define a parameter called relative humidity (RH), which is the absolute humidity expressed as a percentage of the maximum amount of humidity that can exist as water vapour at that temperature (i.e., the percentage of the saturation value, which is thereby defined as 100 per cent RH at that tempera-ture). When these values are defined for each air temperature, the dots can be connected to define lines of constant relative humidity, which follow the shape of the saturation curve (Figure 2). If we repeat this exercise often enough, we can develop a psychrometric chart. ASHRAE is a bit fussy about copyright, so we leave the interested reader to review the psychrometric chart online, but it looks a bit like Figure 2.We are familiar with RH on a personal level, as it is generally what our skin receptors pick up when sensing the comfort conditions in a space. We can sense air temperature in a general way, but we are particularly sensitive to dew-point temperature. A room feels clammy if the humidity level approaches 100 per cent RH, no matter what the air temperature might SPRING/SUMMER 2020 15'