Veteran Advisor
Posts: 947
Registered: ‎10-18-2016

Wind Speed For Sprayer Records

Wind measurements at airports and other weather stations such as ASOS and AWOS are typically taken at a 10m reference height.  That is about 33 feet above the surface.  They are often given in knots (nautical miles per hour).  A nautical mile is 6076 feet.  A statue mile is 5280 feet.


Wind speeds closer to the surface are slower because of friction.  Here is a paper that discusses common observations.


"As can be seen on this chart wind speed of 12 knots measured at 5 meters above the ground will be 13.3 knots 10 meters above the ground and 8.2 knots 1 meter above the ground due to surface friction creating a boundary layer of air near the surface of the earth."


The airport reading at 10m will be 13.3 knots.  The wind at 1 meter (3.3 feet) above ground level will be 8.2 knots.  Converted to miles per hour these examples are 15 miles per hour at 33 feet and 9.4 mph at 3.3 feet.


No, your university extension agent is not going to know anything about this.


However, it's obvious that if the label says 10 mph, you can spray legally when the airport says 15 mph or 13 knots.  


Of course, what the airport says 25 miles away may have no relationship to what is at spray boom height on your farm - that is a different question.

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