Moisture: Dew v Rainfall – Part 2

In my last blog I spoke about the difference between 2018 and 2019. I looked at the amount of hours we had been in a "dewy" situation. This is a fairly simple thing to work out and it uses the Dew point figure I've seen in many forecasts and never really understood properly.

Since I spent a bit of time understanding it I thought I'd relay that info via the blog (which will save me looking it up next time I forget what it means!)

Dew point

The dew point temperature shown in forecasts and historic records is the temperature at which the air can no longer hold on to moisture in the air. At this point the moisture will begin to fall out of the atmosphere. On turf this creates the condensation element of dew.

When the air temperature cools to the dew point temperature, or the dew point lifts to the same as the air temperature, we begin to see dew.

This is really simple but it's not always that accurate in turf. Although the equations and formulas created are very accurate and have been used for many years in meteorology, we're not great at predicting or even measuring canopy temperature of turf. The temperature can vary several degrees at ground level, and this can often be seen on frosty mornings when you have different levels of frost on different Heights of cut turf.

For the sake of simplicity when I work periods of dew out I assume that if the dew point is within 1 degree C of the dew point temperature I assume there is a dew.

Dew Point Calculator Here is a brilliant on line calculator that show the relationship between air temp, dew point and Humidity. If you assume that disease pressure is high when air temp and dew point are within 1 degree you suddenly understand how the relationship with humidity is so important.

How is any of this useful? Using dew point temps, you can look back historic weather data and look for periods of prolonged leaf moisture - which is an indicator of high disease pressure.

Mark Hunt has done great job of chasing this subject down and it's worth taking a look at his blog where he covers this subject really well. Mark Hunt weatherblog.

Missed part one? Read it here.

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