Time 06:00 Dry 0 °C Wet 0 °C RH 0 % Dew point 0 °C
Drag to orbit, wheel or pinch to zoom. Red columns: dry bulb (left) and wet bulb (right, wet wick). The wet bulb reads lower because evaporation cools it. The cup rotor is shown at a quarter of its real speed.
Stevenson Screen Psychrometer: Wet Bulb, Dew Point, Fog and Frost
A wet thermometer cools below the air temperature until evaporation balances heat gain. That gap, the wet-bulb depression, gives humidity and dew point. Run a day of weather and watch for dew, fog and frost.
Pause Reset Screen door: open Fan: aspirated Observer table: ventilated Quality: High
Weather
Readouts
Last 24 hours
Orange: dry bulb. Green: wet bulb. Cyan: dew point. Grey band: fog (air within 0.5 K of its dew point).
Vapour pressure diagram
Curve: saturation pressure. White dot: the air (T, e). Cyan: dew point (cool at constant e until the curve). Green: wet-bulb point; the straight line from it to the air has slope A p.
Checks
Run independent checks
Compares the solver with textbook dew-point values, the empirical Stull wet-bulb formula, the standard atmosphere and an analytic fog-onset time.
The science
Saturation vapour pressure. Magnus formula over water: e_s(T) = 6.112 exp(17.62 T/(243.12 + T)) hPa (T in C). Relative humidity is RH = e/e_s(T); the dew point is the temperature at which e_s(T_d) = e, i.e. T_d = 243.12 g/(17.62 - g) with g = ln(RH/100) + 17.62 T/(243.12 + T). Below 0 C the frost point uses the ice constants 22.46 and 272.62. Magnus is accurate to a few hundredths of a degree between about -40 and 50 C.
Psychrometer equation. A wet bulb at equilibrium loses heat to evaporation as fast as it gains it from the air: e = e_s(T_w) - A p (T - T_w). The constant A depends on ventilation: about 6.6 x 10-4 per K for a fan-aspirated or whirled psychrometer and nearly 8 x 10-4 in still air inside a screen (typical values). If an observer uses the ventilated table on a still-air reading, the inferred humidity is wrong by several percent: switch the fan and the table to see it. The solver finds T_w by bisection; p falls with height as p = 1013.25 (1 - 2.25577e-5 h)^5.25588 hPa, so the wet-bulb depression for the same humidity shrinks on a mountain.
Dew, fog and frost. The air holds a fixed vapour pressure; as it cools at night T reaches T_d and the excess condenses (dew, fog). The model keeps e at or below e_s(T) so condensation removes vapour, and the ground slowly re-humidifies the air (6 h time constant, illustrative). Rain relaxes e towards saturation in 10 minutes (illustrative). Frost is flagged when the dew point is below 0 C and T falls to the frost point.
Other instruments. Wind chill uses the Environment Canada formula 13.12 + 0.6215 T - 11.37 v^0.16 + 0.3965 T v^0.16 (v in km/h, valid for T up to 10 C and v above 4.8 km/h). The heat index is the Rothfusz regression, valid above about 27 C and 40 % RH. The cup anemometer turns at roughly v/(3 r); the rain gauge counts 0.2 mm tips on a 0.0314 m2 funnel.
Simplifications. A sinusoidal day, fixed mean dew point, thermometer time constants of 1-3 minutes that shrink with wind, no radiation error, no supercooling subtleties for the wick below 0 C, and a made-up weather generator: this is a teaching model, not a forecast.
Blender models, Three.js r160. Educational model.