There are many challenges when it comes to pesticide application. Weather conditions can pose one of the
greatest obstacles, according to Aaron Wilson, OSU Climate Specialist.
Even when conditions seem perfect (light winds), the temperature profile near the surface of the Earth
can lead to unwanted drift. This is called a temperature inversion, a common occurrence in Ohio during
the growing season. Understanding what temperature inversions are, how they form, and signs of their
presence should help applicators judge the appropriate time to apply.
With clear skies and little wind, incoming solar radiation heats the soil surface. The heated air
expands, becomes less dense, and begins to rise. This creates a circulation pattern that grows as the
heating continues at the surface. By late afternoon with clear skies, the incoming solar radiation
decreases and the soil surface looses energy and begins to cool. Then the air near the surface will be
colder and denser that the air above it. This is the beginning of a temperature inversion.
Air within the inversion is stable; thus, it resists vertical motion. Airflow (and potential pesticide
droplets) can move horizontally within this inversion layer.
As the surface cools, overlying air also cools by conduction. Condensation occurs and fog appears when
the air temperature cools to the dew point temperature. Dew will appear before fog. Therefore, the
presence of fog nearly always indicates that an inversion condition existed prior to fog formation and
that it has intensified.