Dew or Frost, Dew Point Difference Calculation/Prediction

Dew or frost can be predicted a few hours ahead of its actual occurrence by subtracting the temperature of the leaf sensor from the 2m dew point and watching the trendline of the plotted result progression to 0F. Once the difference line equals or exceeds 0F (is within the green zone), dew or frost, depending on the temperatures of the leaf temperature sensor (surface temp on the second graph)and absence of winds (+3 mph or more), will form on surfaces, i.e., leaves, grass, exposed automobile windshields, roofs, etc. The dew point difference graph below depicts this dew point calculation. Note: If a lawn has been recently watered, the humidity and dewpoint immediately above the grass (only a few inches) is much higher than measured by this weather station and so there maybe condensation in these ultra micro climate conditions that is not predicted by this graph.

Due to the infrared radiation on cloudless nights, heavy radiation frosting can occur even when the ambient air temperature is a few degrees above freezing. Indeed if the air contains significant water content (high humidity), the latent heat given off by the frost formation can prevent the lower few meters of boundary layer air from reaching the freezing point.

If the surface temperature (leaf sensor temp) is well above freezing, the latent heat given off by dew condensation during humid nights will constrain the boundary layer air from cooling to the dew point.

If the dew point difference curve exceeds the green 0F line by a few degrees, fog or ice fog (pogonip) may occur. These conditions are often seen above the Arrow Creek neighborhood after a rain or snow fall as the air begins to warm.

During extended precipitation events, the surface temperature (leaf sensor temp) will match the temperature of the precipitation and if there is a wet snowfall, the temperature of the surface sensor will flat line at 32F.

The dew/frost prediction only works during non-daylight hours. Solar radiation heats the leaf sensor above the 2m air temperature which then precludes its use as a dew/frost predictor. Late afternoon shade allows cooling of the sensor in the summer.

2.00m Dew Point - Leaf Sensor Temp (0.8m) Dew Point Difference --- Dew or Frost at 0F

0.8m Leaf Surface Temperature, 2m, and 8m Air Temperatures