The term might sound artificial or technical, but industrial snow is normal snow – that is, molecules of water vapour arranged into crystals that fall from the atmosphere to the ground. The prefix "industrial" refers only to the process by which the snow is formed. This is because in certain conditions, snowfall occurs only as a result of man-made emissions, generally from industrial facilities. 

Unlike natural snowfall events, which are associated with low-pressure weather [internal link: Low-pressure situation] and frontal systems, industrial snow occurs in wintry anticyclonic (high-pressure) situations [internal link: Anticyclone (high pressure)].

In order for snow [internal link: Snow] to fall during calm and clear weather conditions, the following basic meteorological conditions must be met:

• A low-exchange, low-wind weather situation with a pronounced inversion [internal link: Inversion] (a pool of cold air in the lowlands).
• A low inversion height, ideally at around 700 to 900 metres, or occasionally lower.
• A cold, damp and foggy layer of air just above the ground with temperatures below -5 degrees.

Water vapour input from industrial facilities

Industrial facilities such as paper mills or waste incineration plants emit large quantities of water vapour through their chimneys, often in the order of several tonnes an hour. As this humid exhaust air is generally warmer than the ambient air, it has a lower density and rises by convection within the cooler, mixed, foggy air. 

When the air that is supersaturated with water vapour reaches the inversion layer, it loses its buoyancy and spreads out in the immediate vicinity at the base of the inversion. If this process continues for several hours, a large quantity of water vapour accumulates. The additional water vapour increasingly condenses on the supercooled fog and cloud droplets. If the temperature is low enough (-7 degrees or lower), the supercooled fog droplets begin to freeze. The surplus of moisture leads to an increasing number and size of ice crystals, which subsequently fall to the ground as light snowfall. In low-wind conditions, this happens in the immediate area of the industrial facility. If there is some wind at the base of the inversion, the snow falls a few hundred metres downwind. 

As the temperatures generally don't drop below -10 degrees, this process does not result in the biggest or most beautiful snow crystals. Industrial snow is generally made up of small ice needles or platelets.

So that the fine snowflakes don't dissipate again (due to sublimation) on their way to the ground, the air must be consistently humid from the base of the inversion to ground level. In turn, this means that the base of the low stratus [internal link: Low stratus] is very low – or even at ground level – during industrial snow events.

What role do aerosols play?

In addition to water vapour, industrial facilities also emit condensation nuclei, i.e. tiny particles that float in the air (= aerosols). Water vapour can accumulate on these particles, forming a cloud droplet. Certain aerosols [internal link: Aerosols] also act as "ice nuclei", in that the supercooled droplets attach to their surface and freeze. 

The extent to which the emitted aerosols encourage the formation of industrial snow is unclear and is presumably highly dependent on the source of the emissions and the temperature within the fog (low stratus). As there are usually sufficient condensation and ice nuclei in the air already during the corresponding weather conditions (persistent, low-wind inversions), the emitted aerosols are likely to play a subordinate role in many cases.

Industrial snow and nuclear power plants?

According to the above criteria, the large cooling towers of nuclear power plants, through which huge quantities of water vapour enter the atmosphere, should also be excellent producers of "industrial snow". But this is not usually the case. 

Given the size and heat of the cloud of water vapour emitted by the plants, it rises so quickly that it can break through the inversion and therefore "escape" into the overlying drier and warmer ambient air.