Jet Fibres are long and narrow stripes of high, cold clouds, frequently found in satellite images. From the beginnings of satellite image interpretation, these clouds were considered to be connected to the jet stream and were used for the detection of the jet axis.

• They appear white (or very light grey) in both IR and WV images, with pronounced fibrous structure. In WV images they are almost always accompanied by black stripes on the cyclonic (left) side. In VIS images Jet Fibres are nearly transparent with colouring varying from light grey to grey. Although their colouring may be very similar to that of the surrounding clouds, their structure is very different, making them clearly recognisable. When a fibre appears above a lower-level cloud feature, the cloud texture is different, which makes the fibre distinct from the surroundings. VIS images sometimes reveal that the fibre casts a shadow on the cloud below or the Earth surface.
• Jet Fibres can appear wherever there are jets, but they are more easily recognisable above the sea then above the land because of the greater contrast they make with the sea surface.
• They are narrow with an order of magnitude of several tens of kilometres (usually a under hundred) but may be very elongated, with an order of magnitude from several hundreds up to couple of thousand kilometres.
• In most cases Jet Fibres persist for 8 to 12 hours, but in some cases they dissipate, the new ones developing with the whole process lasting for 24 hours or even longer.

Although they can be detached from the synoptic scale systems, they are most often found over a low-amplitude upper-level ridge (above the Warm Front Shield (see Warm Front Shield )) or to the east of a high-amplitude trough behind a Cold Front (see Cold Front ). The images above show a typical example of the Jet Fibre formed on the poleward side of a Warm Front cloud shield. In the IR image, the fibre is white in appearance meaning it consists of cold clouds. In the VIS image the texture of the fibre differs from the surrounding frontal cloud band by being darker and transparent. In the WV image the fibre is accompanied by the Dark Stripe on cyclonic side. This case shows an example of a well developed Jet Fibre to the rear of a Cold Front. It appears whiter than the rest of the frontal cloud band in both IR and WV image. In the WV image it is accompanied by a pronounced Dark Stripe on the cyclonic side. In the VIS image the fibre is darker than the frontal cloud band and its texture is different.

There are also cases (images beneath) when Fibres appear at an angle to the Cold Front, or even perpendicular to it. That is the case when the Cold Front is a Kata type (see Cold Front ). To the rear of the frontal cloud band it can be seen from WV imagery that a jet axis lies approximately perpendicular to the frontal cloud band and jet fibres are located on the anticyclonic side of the WV black stripe. In the VIS image, the structure of Jet Fibres is different from the surrounding cloudiness, and narrow light grey stripe perpendicular to the frontal cloudiness can be recognised. In many of these cases there is an area of increased cloudiness within the frontal cloud band of the Cold Front (see Front Intensification by Jet Crossing ).

Appearance in the Meteosat 8 HRVIS and RGB composite images

High resolution of Meteosat 8 HRVIS images can be highly beneficial for the recognition of fine-scale cloud features. It can be especially useful in this particular Conceptual model, for the distinction between a small-scale Jet Fibre and its parental Frontal system.
In the following image fine structure of elongated and narrow grey high clouds over West Mediterranean extending from the Iberian peninsula to the Genoa bay, represent Jet Fibres, and they can be easily differentiated from the lighter broader frontal cloud band over Corsica, Sardinia and mid Italy. Combinations and differences of satellite channels enable a deeper insight into the synoptic situation, structure of cloudiness and development over some area.

Airmass RGB is an RGB composite based upon data from infrared and water vapour channels and can thus be used day and night. It is designed and tuned to monitor the evolution of cyclones, in particular rapid cyclogenesis, PV (potential vorticity) anomalies and jet streaks which enable us to notice jet fibres easily.
The Airmass RGB is combined of the WV6.2 - WV7.3 Brightness Temperature Difference (BTD) on red colour, the IR9.7 - IR10.8 BTD on green and the WV6.2 channel on blue. All three features are strongly related to air mass characteristics in cloud-free areas and to the height of the clouds in cloudy areas.
High clouds appear in white colour, mid-level clouds in light ochre and cloud-free areas in dark green (warm air mass with high tropopause) or blue (cold air mass with low tropopause). A particular feature of this RGB is that dry descending stratospheric air is marked by a reddish colour (link to the Airmass RGB key).

In the image above white narrow clouds over the Atlantic (west of Pyrenees and west of British islands, both attached to the warm front shield) represent Jet Fibres.

The images beneath show the appearance of Jet Fibres in some other RGB composites, with different channel combinations.

First of all, RGB combination of visual and infrared channels is shown (VIS 0.6, NIR 1.6 and IR10.8 inverted). Yellow colour represents low clouds or fog, bluish colour displays high cold transparent clouds while magenta colour displays ice cloud, possibly connected to convection (link to the 139 RGB key). Narrow bluish stripes over the Atlantic in the middle of the image and more to the north (hardly noticeable) are jet fibres connected with the jet streak.

This RGB nicely shows this particular cloud feature, but it is available only during daytime. For that fact, insight into some RGB available 24 hours is necessary.

The next one is so-called Dust RGB. This is a combination of IR channels and differences as follows: IR12.0-IR10.8, IR10.8-IR8.7, IR10.8. It helps to detect dust (evolution of dust storms over the desert), thin clouds and contrails. In this RGB combination thin high level ice clouds, such as Jet Fibres, appear black and can be distinguished from thick clouds which appear red (link to the Dust RGB key).

Very similar to Dust RGB, combining the same infrared channels but with different thresholds the newly constructed 24 hours Microphysics RGB is shown. It is designed and tuned to monitor the evolution of fog and low stratus on one hand, and dust and volcanic ash, on the other hand. Secondary applications of this RGB are the detection of fires and low-level moisture boundaries. However, this RGB makes thick high level ice clouds nice and distinctive in its very dark red, almost black colour (link to the Microphysics RGB key). The above image shows a nice case of a Jet Fibre perpendicular to the Cold Front approaching Portugal from the west. The fibre was originally connected to the Warm front shield far away over the Atlantic. Since it has been transported ahead of the primal cloud system it can be easily recognized by its elongated structure, as well as the dark red colour separating it from the purplish environment of the sea below, or the orange to red colouring of the frontal cloudiness.