surface inversions, atmospheric stability, and spray drift

Surface Inversions, Atmospheric Stability, and Spray Drift

Surface Temperature Inversions:

• Are very common

• Are easy to recognize

• Affect the dispersal of very small spray droplets suspended in the air

• Do not increase the amount of off-site movement

• Can increase the potential for offsite affects & the distance at which affects can be observed

Atmospheric Stability

• Inversions cause STABLE atmospheric conditions

• The concept of atmospheric stability helps understand how inversions affect drift

• Close to the ground, atmospheric stability changes regularly between STABLE, NEUTRAL, & UNSTABLE

The large scale trend in the atmosphere is that temperature decreases with height

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Where there is a decrease in temperature with height greater than the adiabatic rate the atmosphere is UNSTABLE

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When the atmosphere is UNSTABLE air parcels near the surface will rise and expand because they are warmer & less dense than the air above

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Droplets or particles suspended in the air will be dispersed/diluted

Cloud of 5-25 u oil droplets generated under unstable conditions

105 foot temperature monitoring tower

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105’ 38°F

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105’ 38°F

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A temperature inversion is an area where temperature increases with height.

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It can begin at the ground.

A temperature inversion is an area where temperature increases with height, this is area is the inversion layer

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Or it can begin above the ground.

These aloft inversions will be discussed at the end of the program

When temperature increases with height the atmosphere is STABLE.

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Vertical mixing of the air and dispersion of small droplets is suppressed.

A parcel of air near the surface is always cooler & more dense than the air above, so it can’t rise and disperse.

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If forced down by a current of air, it will immediately rise back.

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If forced up by a current of air, it will immediately sink back.

The parcel of air can’t rise and disperse, but it can be move laterally in the light variable winds typical of a surface inversion

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105’ 35°F64’ 30°F32’ 29°F16’ 28°F8’ 26°F

Temperature Profile

Surface Inversion - STABLE CONDITIONS

Wind 0.6 mph

Temperature Profile

Surface Inversion - STABLE CONDITIONS

105’ 35°F64’ 30°F32’ 29°F16’ 28°F8’ 26°F

8’ 27°F16’ 29°F32’ 31°F64’ 32°F

105’ 33°F

Surface Inversion - STABLE CONDITIONS

105’ 43°F64’ 36°F32’ 32°F16’ 30°F8’ 28°F

Temperature Profile

Surface Inversion - STABLE CONDITIONS

Cold air drainage down a slope

105’ 43°F64’ 36°F32’ 32°F16’ 30°F8’ 28°F

Temperature Profile

Surface Inversion - STABLE CONDITIONS

Cold air drainage down slope

A Surface Inversion can extend upwards 5, 50, 100, 500 ft or more.

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Height (of the inversion) doesn’t matter:

If the application is made within the inversion it, the effects will be similar.

8’ 26°F16’ 28°F32’ 29°F64’ 30°F

105’ 35°F

Surface inversion extending above the tower

Layering observed when oil droplets released at ground level or top of tower. Clouds moving in different directions.

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105’ 38°F

.5 mph wind

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Shallow surface inversion STABLE conditions up to 64’ NEUTRAL conditions at 105’

Cloud is dispersing

Surface Inversions are part of the daily cycle unless wind or cloud cover intervene

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During the night, unless clouds or wind intervene, the ground loses heat cooling the air above.

Unless wind intervenes the surface inversion will continue until the sun begins to heats the ground

During the day when cloud cover is light, the sun heats the ground warming the air above. This causes in unstable conditions unless wind intervenes.

As the sun sets the ground begins to lose more heat than it gains, cooling the air above. In the absence of heavy cloud cover and/or wind a surface inversion will begin to form.

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Inversion - Stable

Unstable Inversion - Stable

Sunrise Sunset

Daily Cycle 11/9/93Daily Cycle 11/9/93

How to recognize a surface inversion

• Sunset to just after sunrise

• Windless to low wind conditions (<2-3 mph)

• Clear to partly cloudy skies

• Ground fog (if sufficient humidity exists)

• Dust hanging over a roadway

• Smoke from a chimney forming a layer

• Dew or frost (if sufficient humidity exists)

Signs of a surface inversion in the early morning Windless or light

variable wind

Ground Fog

Frost (or dew)

Lack of heavy cloud cover

Smoke from a chimney forming a layer

Surface inversion - early morning

How to recognize the potential for a surface inversion

• Sun is getting low in the sky

• Wind is becoming light and variable

• Clear to partly cloudy skies

Surface Inversions don’t:

• Increase the amount of spray droplets moving off-site

Surface Inversions can:• Decrease the dispersion of droplets too small to

quickly settle out.• Result in a higher air concentration of these small

droplets.• Increase the potential for off-target effects.• Increase the distance at which off-target effects can

be observed.• Increase the size of the area affected.• Cause the direction of drift to be unpredictable

Reducing the effects of Surface Inversions on Spray Drift

• Minimizing production of very small drops

• Using equipment the minimizes the number of small drops suspended in the air

• Morning applications are likely to have shorter exposure to STABLE conditions than evening applications

When the atmospheric stability is NEUTRAL, there is little or change in temperature with height. Low heavy cloud cover and/or wind contribute to NEUTRAL conditions.

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Vertical mixing is not suppressed as under STABLE conditions, and the turbulence of even a 5 mph wind is effective in dispersing suspended small droplets.

105’ 32°F64’ 33°F32’ 33°F16’ 33°F8’ 32°F

Temperature Profile

NEUTRAL CONDITIONS

Wind 4.5 mph

105’ 32°F64’ 33°F32’ 33°F16’ 33°F8’ 32°F

Temperature Profile

NEUTRAL CONDITIONS

Wind 4.5 mph

Surface InversionNo wind

Neutral Conditions 4.5 mph wind

7:00 AM

8:30 AM

105’ 32°F64’ 33°F32’ 33°F16’ 33°F8’ 32°F

Temperature Profile

105’ 43°F64’ 36°F32’ 32°F16’ 30°F8’ 28°F

Temperature Profile

Neutral conditions can be the best time to spray

• Wind direction if often consistent

• Good dispersion of droplets too small to quickly settle out.

A temperature inversion is an area where temperature increases with height, this is area is the inversion layer

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Aloft Inversions

• Are not likely to effect applications to agricultural crops

• Low altitude aloft inversions can affect aerial forestry application

• Low altitude aloft inversions are often surface inversions in the process of dissipating & are therefore short lived

Surface Temperature Inversions:

• Are very common

• Are easy to recognize

• Affect the dispersal of very small spray droplets suspended in the air

• Do not increase the amount of off-site movement

• Can increase the potential for offsite affects & the distance at which affects can be observed