Why does some rainfall leave a coloured dust on my car?
Through the action of widespread and vigorous duststorms over places such as the Sahara, huge quantities of very fine desert sand can be carried to high enough levels (around 12000 ft/4000 m), where it can be dispersed for considerable distances downwind of the source. On many occasions, such dust is so diffused vertically and horizontally that there is little or no effect observed at ground level.
However, sometimes the dust remains in sufficiently high concentrations, and can become involved with a medium level weather system, which results in the dust being transported towards such places as France, Britain and Ireland. If rain falls, the dust falls as well. This is primarily due to washing out of the dust by large raindrops. This leaves a dusty residue on car windscreens, rain gauges etc., with the most common colour being similar to old mortar: i.e. light beige, but deeper brown, orange and red hues have been observed. The effect is usually noted after light, showery rainfall, often involving medium level instability - heavier rainfall tends to wash the evidence away. A warm, southerly (Tropical continental) low-level airstream, together with a strong southerly middle level flow (circa 700 mbar), originating from the North African area are the conditions required for such events in the northwest of Europe.
Such reports are always of interest...some guidelines are contained in this section (Notes on observing.)