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Understanding
Thermal Inversions
What is a thermal inversion and
why does it affect a metam-sodium application?
During typical daytime conditions, the
air temperature decreases with height above the ground. This
occurs because the ground serves as the primary source of
heating the air. During nighttime periods with clear skies
and light wind speeds, the cooling of the ground can lead to
the temperature near the (cooled) surface being colder than
the air above it. This is referred to as a ground-based
thermal inversion – inverted in the sense that the colder
(heavier) air is on the bottom rather than above the surface
layer during daytime conditions. The inversion condition
also is referred to as a stable condition because when with
heavy air on the bottom and warm on the top there is minimal
vertical movement, and thereby limited atmospheric
turbulence to dilute off-gassing from a fumigation
application.
When a metam-sodium is applied during a
thermal inversion, there is much less dilution of air
emissions as the wind moves the air pollutants downwind.
Especially for chemigation or shank injection applications,
this situation increases the potential for odors being
detected downwind of the field.
It is critical that all metam-sodium
applications by chemigation or shank injection be avoided
during periods of ground-based thermal inversion as
described above.
How do you know when a nighttime
thermal inversion exists?
Although there are more scientific ways
to identify thermal inversion conditions, the simplest way
to avoid the potential for this condition is to not apply by
chemigation or shank injection at night. The preferred time
to apply in terms of odor management is to wait for one hour
after sunrise and complete the application by mid-day, with
the seal being fully in place by mid-afternoon. Sometimes,
it may be necessary to begin the application a few hours
before sunrise to complete the application prior to
exceeding the 90 F air temperature constraint. The goal,
however, is to minimize application prior to sunrise.
Application of metam-sodium by
chemigation or shank injection is not recommended during
nighttime periods.
Should you be concerned about
daytime thermal inversions?
A question that has been asked . . . "If
there is fog during the daytime do we need to be concerned
about a thermal inversion?" The answer is generally it would
be expected to be a reasonable time to conduct an
application, as long as visibility was not restricted to the
point of interfering with conducting a sound application.
For example, the Western portion of California experiences a
relatively high frequency of days with dense fog. This
condition is due to the influence of upswelling cool water
along the coast. Again, waiting for one hour after sunrise
(and later if needed to maintain adequate visibility during
the application) would be recommended.
Applying metam-sodium during daytime
periods with fog generally is acceptable. Increasing the
frequency of odor monitoring, and being prepared to rapidly
mitigate increasing odors, would be recommended as an
additional precaution.
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