Sonntag 1990 dew point calculator from temperature and humidity

This calculator converts air temperature and relative humidity into a dew-point temperature using the Sonntag 1990 saturation vapor pressure coefficients for liquid water. It also reports saturation vapor pressure and actual vapor pressure, making the result useful for weather observations, environmental monitoring, building controls, storage decisions, and laboratory records. The calculation is deterministic, runs without network access, and rejects humidity values outside the physical zero-to-one-hundred-percent interval instead of quietly producing a misleading result.

What the Sonntag dew point result means

Dew point is the temperature to which air would need to cool, at unchanged water-vapor content and pressure, before becoming saturated with respect to liquid water. A dew point close to the current air temperature indicates humid air and a relatively small cooling margin before condensation can begin. A much lower dew point indicates drier air.

This calculator first evaluates the Sonntag 1990 saturation vapor pressure expression at the supplied air temperature. It multiplies that pressure by relative humidity divided by one hundred to obtain actual vapor pressure, then numerically finds the temperature whose Sonntag saturation pressure matches that value. The result describes a thermodynamic threshold, not a promise that visible droplets will form immediately.

Surface temperature, ventilation, local pressure differences, thermal inertia, contaminants, and nucleation conditions can all affect when and where condensation actually appears. Use the value as a clear quantitative boundary and combine it with measurements from the surface or process you are evaluating.

How to provide temperature and relative humidity

Relative humidity must be greater than zero and no greater than one hundred. Values below zero or above one hundred are rejected because they fall outside the declared physical interval. Exactly zero percent is also rejected because it corresponds to zero vapor pressure and therefore has no finite dew-point temperature; reporting an arbitrary large negative number would disguise that mathematical limit as a measurement.

A value of one hundred percent produces a dew point equal to the supplied air temperature, apart from the displayed rounding. Use humidity and temperature observations that refer to the same air sample and time. Combining an indoor temperature with an outdoor humidity reading, or readings recorded far apart, produces a precise calculation for a state that may never have existed.

Sensor quality matters as well: humidity uncertainty becomes especially influential in dry conditions. The output pressures are expressed in hectopascals, while dew point remains in degrees Celsius.

Using the calculation in monitoring and controls

When the surface approaches the dew point, the available condensation margin shrinks; when it falls below the dew point, condensation risk becomes significant. Building operators can apply that comparison to glazing, chilled-water pipes, ventilation ducts, basement walls, or cooling coils. Storage teams can use it when checking whether moving material between spaces may expose it to condensation.

Weather stations and environmental loggers can record the computed value beside the original temperature and humidity so later reviewers can reproduce the calculation. Keep the original observations rather than storing only dew point, because recalculation may be needed after sensor calibration or a method change. For safety-critical control, add sensor validation, reasonable alarm hysteresis, equipment-specific limits, and an independent fail-safe.

This capability calculates a psychrometric indicator; it does not model heat transfer, forecast weather, calibrate instruments, or decide whether equipment should be energized.