Greenhouse humidity records often contain temperature and relative humidity but omit the two quantities needed for a repeatable moisture calculation: humidity ratio and dew point. This guide shows how to convert a measured temperature, RH, and absolute pressure into those values, how to check the units, and how to interpret the result in a greenhouse input record. The examples are hypothetical worked calculations. They are not field measurements, crop setpoints, condensation guarantees, or equipment capacity claims.
What this calculation answers
The calculation answers a narrow question: given air temperature (T), relative humidity (RH), and absolute pressure (p), what are the corresponding vapour pressure, humidity ratio, and dew-point temperature? Those outputs make a sensor record easier to compare across operating periods and provide consistent inputs for a later moisture balance.
It does not answer how much water the crop releases, how much outdoor air enters, whether a surface will condense, or how much moisture-removal capacity a machine will deliver. Keep those tasks separate from this conversion. The broader greenhouse dehumidification project design data guide covers project inputs and load-accounting context, while this page focuses on the psychrometric conversion itself.
Record the inputs with their conditions
Use measurements from the same location and operating period. Record whether pressure is measured or assumed, whether temperature is in °C or °F, and whether RH is reported as a percentage or a fraction. Absolute pressure is required; a weather-station pressure corrected to sea level is not automatically the pressure inside a greenhouse.
| Input | Required form | Check before calculating |
|---|---|---|
| Air temperature, T | °C for the equations below | Convert °F first; identify the sensor location and time |
| Relative humidity, RH | Percentage with 0 < RH ≤ 100 for this dew-point path | Do not enter 0.60 as 60% unless the formula is changed accordingly |
| Total absolute pressure, p | kPa in this guide | Use the pressure at the measurement location or label an assumed value |
| Sensor context | Location, timestamp, averaging period | Avoid combining readings from different zones or periods |
Temperature and RH should be paired because RH changes when temperature changes even if the actual vapour content stays similar. For sensor placement and calibration context, see the greenhouse humidity sensor selection and calibration guide. The ASHRAE Handbook—Fundamentals provides the moist-air property framework used here.
Use these equations consistently
For an above-freezing educational calculation, estimate saturation vapour pressure with the NOAA/NWS relationship:
es = 0.6112 × exp(17.67T / (T + 243.5)) kPa
Then calculate actual vapour pressure:
e = (RH / 100) × es
The humidity ratio on a dry-air mass basis is:
W = 0.621945 × e / (p − e) kg water/kg dry air
Multiply W by 1,000 to express it as grams of water per kilogram of dry air. The dew point from the same vapour-pressure approximation is:
Td = 243.5 × ln(e / 0.6112) / (17.67 − ln(e / 0.6112)) °C
The NOAA/NWS vapour-pressure and dew-point worksheet presents the saturation-pressure and dew-point relationships. Keep every pressure term in the same unit: if es and e are in kPa, p must also be in kPa. The factor 0.621945 represents the water-vapour-to-dry-air molecular-mass ratio used in the ideal-mixture humidity-ratio relationship.
Worked example 1: 25 °C, 60% RH, 101.325 kPa
Assume a greenhouse sensor record contains (T=25) °C, (RH=60%), and (p=101.325) kPa. These are illustrative inputs, not a site record.
- Calculate saturation vapour pressure: es = 0.6112 × exp(17.67 × 25 / (25 + 243.5)) = 3.167429 kPa.
- Calculate actual vapour pressure: e = 0.60 × 3.167429 = 1.900458 kPa.
- Calculate humidity ratio: W = 0.621945 × 1.900458 / (101.325 − 1.900458) = 0.01188821 kg/kg dry air.
- Convert the ratio if useful: W = 11.888 g/kg dry air.
- Calculate dew point: Td = 16.7054 °C, or about 16.7 °C.
The resulting record can therefore state: 25 °C, 60% RH, 101.325 kPa → 1.900458 kPa vapour pressure, 0.011888 kg/kg dry air, and 16.7 °C dew point. If a documented greenhouse surface is below approximately 16.7 °C under comparable conditions, condensation risk requires a separate surface-temperature and heat-transfer assessment. Dew point alone does not prove that condensation will occur.
Worked example 2: 18 °C, 80% RH, 99.0 kPa
The second example shows why pressure and temperature should be retained with the result. Assume (T=18) °C, (RH=80%), and measured absolute pressure (p=99.0) kPa.
- es = 0.6112 × exp(17.67 × 18 / (18 + 243.5)) = 2.062584 kPa.
- e = 0.80 × 2.062584 = 1.650067 kPa.
- W = 0.621945 × 1.650067 / (99.0 − 1.650067) = 0.01054188 kg/kg dry air.
- W = 10.542 g/kg dry air after conversion to grams per kilogram.
- Td = 14.5010 °C, or about 14.5 °C.
Although the second case has a higher RH, its calculated humidity ratio is lower than the first case because the air is cooler. This is why RH values from different temperatures should not be compared as if they were direct measures of water mass. Store (T), RH, (p), W, and Td together with the timestamp and location.
Handle erroneous or incomplete inputs
Stop the calculation and correct the record when an input fails one of these checks:
| Problem | Why it matters | Correct handling |
|---|---|---|
| RH entered as 0.60 while the formula also divides by 100 | Vapour pressure e becomes 100 times too small; W and Td respond through their respective equations | Use RH = 60 for the percentage formula, or remove /100 when using a fraction |
| Pressure is gauge pressure or sea-level-adjusted pressure | It is not the local absolute air pressure required by the equation | Convert or replace it with absolute pressure and document the source |
| p ≤ e | The humidity-ratio denominator is zero or negative | Treat the input as invalid and inspect units or sensor data |
| T is in °F but inserted into a °C equation | Saturation pressure and dew point become invalid | Convert T to °C before applying the equations |
| RH ≤ 0 or RH > 100 | At RH = 0 the logarithmic dew-point expression is undefined; values outside 0–100% are outside this stated method | Flag the reading for sensor, transcription, or condensation review |
| Values combine different timestamps or zones | The output no longer describes one air state | Re-pair the readings before calculating |
The equations above are a bounded above-freezing approximation. They should not be used to claim frost point, freezing-surface behaviour, or a universal greenhouse control threshold. For a production record, preserve raw readings, calibration status, pressure basis, calculation version, and rounding policy. If uncertainty matters to a decision, report the measurement conditions and uncertainty basis; NIST's Technical Note 1297 explains how standard uncertainty components are combined and reported.
Interpret the outputs without overextending them
Humidity ratio W is useful when a later calculation needs a mass difference between two air states. For example, a difference in W can be multiplied by a stated dry-air mass flow to estimate the moisture transported with that flow, provided the flow basis and operating assumptions are independently defined. Do not turn one sensor snapshot into a daily water load or equipment rating.
Dew point Td is useful for comparing an air state with a known surface temperature. It is not a crop target, VPD, or proof of condensation. If the purpose is plant response, continue to the humidity and plant-growth explanation. If the purpose is zone distribution, review the uneven crop humidity zones guide. The agriculture applications overview provides the wider application route.
For a project record, a concise result line might read: Timestamp; zone; T; RH; absolute p; es; e; W; Td; sensor ID; calibration status; averaging period; assumptions. That line is a reproducible input record. It is not a complete load calculation, a dehumidifier selection, a field acceptance result, or a performance promise.
References and scope
The conversion method uses the NOAA/NWS vapour-pressure relationship and the ASHRAE moist-air property framework linked above. NIST TN 1297 is included for uncertainty-reporting context. All numerical examples on this page are recomputed hypothetical examples. The page intentionally leaves crop water balance, ventilation exchange, VPD interpretation, airflow distribution, equipment selection, and commissioning to the relevant calculation and application guides.
