Dew Point Chart
Dew point is the temperature where air gives up its moisture and water starts to condense. Cool a surface below it and that surface turns wet. In a greenhouse the surface is usually the crop, and free water on leaves or fruit is where Botrytis and mildew get their start.
The chart reads off the dew point for any greenhouse climate. Air temperature runs down the left, humidity across the top. If your coldest surface sits below the value in the cell, expect condensation.
Dew point chart in °C
| °C \ RH | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% |
|---|---|---|---|---|---|---|---|---|---|---|
| 16 | 5.6 | 7.0 | 8.2 | 9.4 | 10.5 | 11.6 | 12.6 | 13.5 | 14.4 | 15.2 |
| 18 | 7.4 | 8.8 | 10.1 | 11.3 | 12.4 | 13.5 | 14.5 | 15.4 | 16.3 | 17.2 |
| 20 | 9.3 | 10.7 | 12.0 | 13.2 | 14.4 | 15.4 | 16.4 | 17.4 | 18.3 | 19.2 |
| 22 | 11.1 | 12.5 | 13.9 | 15.1 | 16.3 | 17.4 | 18.4 | 19.4 | 20.3 | 21.2 |
| 24 | 12.9 | 14.4 | 15.8 | 17.0 | 18.2 | 19.3 | 20.3 | 21.3 | 22.3 | 23.1 |
| 26 | 14.8 | 16.3 | 17.6 | 18.9 | 20.1 | 21.2 | 22.3 | 23.3 | 24.2 | 25.1 |
| 28 | 16.6 | 18.1 | 19.5 | 20.8 | 22.0 | 23.2 | 24.2 | 25.2 | 26.2 | 27.1 |
| 30 | 18.4 | 20.0 | 21.4 | 22.7 | 23.9 | 25.1 | 26.2 | 27.2 | 28.2 | 29.1 |
| 32 | 20.3 | 21.8 | 23.3 | 24.6 | 25.8 | 27.0 | 28.1 | 29.2 | 30.2 | 31.1 |
| 34 | 22.1 | 23.7 | 25.1 | 26.5 | 27.8 | 28.9 | 30.1 | 31.1 | 32.1 | 33.1 |
Dew point in °C. Any surface colder than the value shown will collect condensation.
Dew point chart in °F
| °F \ RH | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% |
|---|---|---|---|---|---|---|---|---|---|---|
| 60 | 41.3 | 43.8 | 46.1 | 48.2 | 50.2 | 52.1 | 53.8 | 55.5 | 57.1 | 58.6 |
| 64 | 45.0 | 47.5 | 49.8 | 52.0 | 54.0 | 55.9 | 57.7 | 59.4 | 61.0 | 62.5 |
| 68 | 48.7 | 51.2 | 53.6 | 55.8 | 57.9 | 59.8 | 61.6 | 63.3 | 65.0 | 66.5 |
| 72 | 52.3 | 55.0 | 57.4 | 59.6 | 61.7 | 63.6 | 65.5 | 67.2 | 68.9 | 70.5 |
| 76 | 56.0 | 58.7 | 61.1 | 63.4 | 65.5 | 67.5 | 69.4 | 71.2 | 72.9 | 74.5 |
| 80 | 59.7 | 62.4 | 64.9 | 67.2 | 69.3 | 71.4 | 73.3 | 75.1 | 76.8 | 78.4 |
| 84 | 63.4 | 66.1 | 68.6 | 71.0 | 73.2 | 75.2 | 77.2 | 79.0 | 80.7 | 82.4 |
| 88 | 67.0 | 69.8 | 72.4 | 74.8 | 77.0 | 79.1 | 81.1 | 82.9 | 84.7 | 86.4 |
| 92 | 70.7 | 73.5 | 76.1 | 78.5 | 80.8 | 82.9 | 84.9 | 86.8 | 88.6 | 90.4 |
The same chart in °F.
Reading the chart
Relative humidity shifts all day as the house warms and cools. Dew point hardly moves, which is why it beats humidity as a condensation warning. A greenhouse at 90% before sunrise and 70% at noon can hold the same dew point through both.
Keep surfaces warmer than that number and you stay dry. The ones that fail first are the cold spots: gable ends, the screen, north glass, and the crop head late at night after it has lost heat to a clear sky.
The leaf runs colder than the air
A transpiring leaf sits a degree or two under the surrounding air, so it can reach dew point while the air still reads comfortable. That is the condensation a climate computer misses, because it watches the air, not the plant. A weather app gives you the dew point outside; what counts is the air right at the crop, which is why growers log temperature and humidity in the canopy with the Pixel or Air+. Leaf temperature is the last piece, and the Stomata Camera reads it straight off the crop, so you catch a leaf sliding toward dew point before the water shows up.
Lowering the dew point
You only lower dew point by pulling moisture out of the air. Vent to trade humid inside air for drier outside air, or warm the pipe and crack a vent so the moisture has an exit. Heating on its own changes nothing: it drops the relative humidity and the air feels drier, but a cold surface still condenses the moment it dips under dew point. A bit of air movement and a dry buffer around the crop carries the plants through the risky hours around dawn.
Dew point and VPD
Dew point and VPD are the same humid air seen from two sides. Dew point flags when water condenses; VPD says how hard the crop can push transpiration. Steer the wet end on dew point to keep disease out, and the working range on VPD. For that side, use the VPD calculator or read VPD in the greenhouse.