A front is created by aeration when the incoming air has properties (temperature or PMC) different from the grain. A front can be a temperature front or a moisture front though these are intertwined but often one is dominant. 'Cooling' or 're-wetting' are examples of fronts. The illustration shows a drying front.
Fronts are often thought of as bad or something to be avoided. This view came about from experiences when warm air was blown on cold grain that caused condensation (either liquid water or frost). Turning off the fan before the front moved all the way through the bin often resulted in disaster — either air blockage, mold or both.
However, fronts are necessary to bring about change in grain which is why grain benefits from aeration. Understanding fronts is crucial for proper aeration. This is true whether the objective is to dry, re-wet or maintain.
A temperature front moves much faster than a moisture front. However a temperature front can be altered at a moisture front by either evaporative cooling or the heat of condensation — this can be a change of over 40° F.
Large fronts (the size has to do with the difference between temperature or MC, not how thick the zone may be — a 2° F front is small, a 30° F front is very large) change either temperature or moisture content faster. However small fronts are necessary to achieve uniformity and precise MC control.
Multiple fronts may exist in a bin due to changing weather, erratic fan operation or differences in incoming loads. Selecting fan run times to introduce a consistent PMC will eventually even out these fronts. The kernels will tend to change to the air that flows by them. If the incoming air is consistent the bottom zone will become stable first. Gradually the bottom zone will increase in height until about the same conditions are achieved throughout a bin. One zone may be cooling while another is warming while another is drying and another is re-wetting.
The necessary step to manage fronts is to monitor the weather and introduce consistent air to achieve your goals. The grain will cooperate by equalizing to what is blown through it.
The colored layers represent fronts. There are 3 kinds of fronts of interest to store grain though they correlate with one another. These are temperature fronts, moisture content fronts (MC) and water activity (Aw) fronts.