In the petroleum industry, cloud point refers to the temperature below which paraffin wax in diesel or biowax in biodiesels forms a cloudy appearance.
[1] The wax also accumulates on cold surfaces (producing, for example, pipeline or heat exchanger fouling) and forms an emulsion or sol with water.
Therefore, cloud point indicates the tendency of the oil to plug filters or small orifices at cold operating temperatures.
Olive oil begins to solidify (via liquid-solid phase separation) at around 4 °C, whereas winter temperatures in temperate countries can often be colder than 0 °C.
The cloud point of a nonionic surfactant or glycol solution is the temperature at which the mixture starts to phase-separate, and two phases appear, thus becoming cloudy.
The wax crystals typically first form at the lower circumferential wall with the appearance of a whitish or milky cloud.
An array of optical detectors continuously monitor the sample for the first appearance of a cloud of wax crystals.