Temperature inversion: is a reversal of the normal behaviour of temperature in the troposphere (the region of the atmosphere nearest the Earth’s surface), in which a layer of cool air at the surface is overlain by a layer of warmer air. (Under normal conditions air temperature usually decreases with height.) – Encyclopædia Britannica
Temperature Inversions play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a cap on the upward movement of air from the layers below. As a result, convection produced by the heating of air from below is limited to levels below the inversion. Diffusion of dust, smoke, and other air pollutants is likewise limited. In regions where a pronounced low-level inversion is present, convective clouds cannot grow high enough to produce showers and, at the same time, visibility may be greatly reduced below the inversion, even in the absence of clouds, by the accumulation of dust and smoke particles. Because air near the base of an inversion tends to be cool, fog is frequently present there.
Temperature Inversions also affect daytime variations in air temperature. The principal heating of air during the day is produced by its contact with a land surface
that has been heated by the Sun’s radiation. Heat from the ground is communicated to the air by conduction and convection. Since an inversion will usually control the upper level to which heat is carried by convection, only a shallow layer of air will be heated if the inversion is low and large, and the rise in temperature will be great.
There are a number of kinds of Temperature Inversions: ground, turbulence, subsidence, frontal and vegetative:
- A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.
- A turbulence inversion often forms when quiescent air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer. The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.
- A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion. Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.
- A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.
- Vegetative inversions are the effect of canopy and the temperature above/underneath them. Vegetative inversions can occur in the early morning with warmer air moving over the canopy layer or later, on a hot day when transpiration from dense canopy cover can cause a cool layer over the canopy, acting the same as an inversion layer.
The key for pesticide users is to understand the potential impact of temperature inversions on our spraying activities.
Q. What is it about Temperature Inversions that can cause problems for spraying?
A. Spray drift can be concentrated due to the lack of wind movement/turbulence at ground level and directional movement due to slope can move spray drift long distances in unpredictable directions. The localised inversion pattern may bear little relationship to broader weather conditions. Sometimes fine sprays may not be able to penetrate the inversion layer and may be transported away from the target.
Q. Are there clues to help identify temperature inversions?
A. A surface temperature inversion is likely to be present if:
- wind speed is constantly less than 11km/h in the evening and overnight;
- cool, off-slope breezes develop during the evening or overnight;
- distant sounds become clearer and easier to hear;
- and aromas become more distinct during the evening than during the day.
Also there are clues that a surface temperature inversion is unlikely:
- Applicators should always expect that a surface temperature inversion is most
likely to have formed at sunset and will persist for some time after sunrise. However, a surface temperature inversion is unlikely if one or more of the following has occurred:
- continuous overcast weather,
- with low and heavy cloud; continuous rain;
- wind speed remains above 11km/h for the whole time between sunset and sunrise;
- and after a clear night, cumulus clouds begin to form.
Bill Gordon – www.ispray.com.au
Graeme Tepper, ‘Weather essentials for pesticide application’, GRDC www.grdc.com.au/GRDC-BookletWeatherEssentials
Or have a listen to this: https://soundcloud.com/whetterj/canolab_inversions