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Air / Know the conditions

Put your room’s
humidity in context.

Combine a thermometer and hygrometer reading to understand air vapour-pressure deficit. Keep observations instead of chasing a universal target.

01

At leaf height

Place the sensor near the plant and away from direct heat or spray. Supported temperature range: 0–50°C (32–122°F).

Air vapour-pressure deficit

Record the conditions ↗

The same humidity can feel different.

Warm air has a higher saturation vapour pressure. The FAO-56 equation is es = 0.6108 × exp(17.27T / (T + 237.3)), with T in °C and pressure in kPa. Actual vapour pressure ea = es × RH / 100. Air VPD = es − ea.

At 20°C and 50% RH, es ≈ 2.338 kPa and air VPD ≈ 1.169 kPa. At 100% RH, air VPD is zero.

Method: FAO Irrigation and Drainage Paper 56, equations 11 and 17 · checked 4 Oct 2026

A reading, not a diagnosis.

This is air VPD. It does not measure leaf temperature, leaf-to-air VPD, transpiration, mould risk, or a plant’s preferred range. Instrument error and changing conditions matter.

There is no universal “perfect humidity” setting for all houseplants. Look up your species and observe the plant over time before changing its environment.

Read sourced care notes ↗