Phenomena in the atmosphere
The amount of water vapour in air is described by humidity. Absolute humidity ρ = m/V is the mass of water vapour in 1 m³ of air (g/m³ or kg/m³). Relative humidity φ = ρ/ρ₀·100 % is the ratio of the absolute humidity to the saturated vapour density ρ₀ at that temperature; it can also be written with pressures: φ = p/p₀·100 %. The saturated vapour density depends on temperature (g/m³): 9.4 at 10 °C; 12.8 at 15 °C; 17.3 at 20 °C; 23.0 at 25 °C; 30.3 at 30 °C. The temperature to which air must be cooled for its vapour to become saturated is the dew point; below it vapour condenses to form dew, fog and clouds, and frost if the temperature is below 0 °C. Rain, snow or hail falls from clouds. Relative humidity is measured with a psychrometer and a hygrometer; a comfortable humidity is roughly 40–60 %.
Observation: dry the outside of a glass of cold water (with or without ice) and set it down in a warm room. What appears on its surface? Why? What happens when the glass surface is below the dew point? Use cold water only.