Humidity and dew point for spraying

Humidity controls how quickly spray droplets evaporate on their way to the target and after they land. Too dry and droplets shrink before arriving; too wet and the leaf may already carry water that dilutes the spray and lets it run off. Relative humidity and dew point describe the same moisture in two different ways, and both are useful.

Relative humidity

Relative humidity (RH) is how much water vapour the air holds compared with the most it could hold at that temperature, as a percentage. Because warm air can hold more water, RH falls as the day warms up even if the amount of moisture stays the same, and rises again in the evening. That is why humidity is usually highest at dawn and lowest in mid-afternoon.

Dew point

The dew point is the temperature to which the air would need to cool for water to start condensing. When the air temperature is close to the dew point, the air is near saturation and dew forms on cool surfaces such as leaves. The gap between air temperature and dew point is a quick way to judge whether leaves are, or soon will be, wet.

When humidity is too low

In dry air, water in the droplet evaporates quickly. Droplets shrink, become lighter and more prone to drift, and some never reach the target. On the leaf, the spray may dry before the product is absorbed. Low humidity usually goes with a high Delta T, which combines temperature and humidity into a single evaporation indicator. When humidity is low, common measures include a coarser droplet class, a higher water volume where the label allows, and moving the job to a cooler, more humid part of the day.

When humidity is too high

Very humid air on its own does not wash product off, but it usually comes with conditions that do cause problems:

• Dew, fog or wet leaves. Water already on the leaf dilutes the spray and can make droplets merge and run off, especially with high water volumes. Some labels advise against applying to wet foliage; others allow it. Follow the label.
• Very low Delta T. Near saturation, fine droplets evaporate slowly and stay airborne longer.
• Inversions. Humid, calm mornings are often the same mornings with a thermal inversion.

High humidity can also help: some products are absorbed better when the leaf stays moist longer. The problem is free water on the leaf and stable air, not humid air itself.

A practical humidity band

There is no single standard humidity band for spraying; recommendations vary with climate, product and droplet size. Many operators rely on Delta T as the main evaporation check, because at a given humidity the evaporation rate also depends on temperature, and use humidity and dew point to spot wet leaves and near-saturated air. Where the label sets humidity conditions, the label prevails.

Using dew point in practice

• In the evening, if the temperature is forecast to fall to the dew point overnight, expect dew and possibly fog. Plan to finish before leaves get wet.
• In the morning, wait until the dew has dried or eased enough for the label's conditions.
• A dew point close to the air temperature with little wind is a sign to check for an inversion.

In PulveCast

PulveCast checks relative humidity each hour against an adjustable band — its guidance treats 55–90 % as ideal, and most crop presets allow up to 95 % — and shows the dew point alongside it so you can see when condensation on the canopy is likely.

More guides

• Delta T for spraying: what it is and why 2 to 8 °C
• Delta T chart: how to read it
• Thermal inversion and spraying
• Spray drift: causes and how to reduce it
• Wind speed for spraying
• The best time of day to spray
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Droplet size and nozzle selection
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist