Its the wax in the diesel fuel.
All middle distillate fuels will precipitate paraffin wax when they are cooled to a low enough temperature. Paraffin wax is a solid mixture of crystalline hydrocarbons, primarily straight chain hydrocarbons, plus some branched chain and cyclic hydrocarbons (see Chapter 4). When it is oil-free, this wax melts in the range 100°F to 180°F (40°C to 80°C). Paraffin wax occurs naturally in all crude oils; the amount depends on the specific crude oil(s) from which it was produced and on the processing used.
As fuel is cooled, it reaches a temperature at which it no longer is able to dissolve the waxy components, which then begin to precipitate out of solution. The temperature at which wax just begins to precipitate and the fuel becomes cloudy is the cloud point as measured by ASTM D 2500.
If the fuel is cooled below the cloud point, more wax precipitates. At approximately 6° to 10°F (3° to 5°C) below the cloud point (for fuels that do not contain a pour point depressant additive) the fuel becomes so thick it will no longer flow. This temperature is called the pour point or gel point as measured by ASTM D 97.
Jet-A fuel info:
When the commercial jet industry was developing in the 1950s, kerosene-type fuel was chosen as having the best combinations of properties. Wide-cut jet fuel (Jet B) still is used in some parts of Canada and Alaska because it is suited to cold climates. But kerosene-type fuels – Jet A and Jet A-1 – predominate in the rest of the world.1
Jet A is used in the United States while most of the rest of the world uses Jet A-1. The important difference between the two fuels is that Jet A-1 has a lower maximum freezing point than Jet A (Jet A: –40ºC, Jet A-1: –47ºC). The lower freezing point makes Jet A-1 more suitable for long international flights, especially on polar routes during the winter.
However, the lower freezing point comes at a price. Other variables being constant, a refinery can produce a few percent more Jet A than Jet A-1 because the higher freezing point allows the incorporation of more higher boiling components, which in turn, permits the use of a broader distillation cut. The choice of Jet A for use in the United States is driven by concerns about fuel price and availability. Many years of experience have shown that Jet A is suitable for use in the United States, especially for domestic flights.