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The tread pattern of an F1 tyre and which particular pattern to use at any given time is dictated solely by the weather conditions.

Slick tyres are the ones that F1 drivers use most of the time - i.e. tyres suitable for dry conditions. A normal slick tyre actually has no tread pattern at all! This is because in dry conditions the aim is to get the maximum grip from the tyre and this is achieved by having as much rubber in contact with the racetrack as possible.

In fact there is a tread pattern of sorts on modern F1 tyres that was introduced as a safety measure from the 1998 season onwards. Four longitudinal grooves, each of 2.5mm depth, are cut into the tyres to reduce the area in contact with the road. This measure was introduced by the governing body of F1, the FIA, in order to artificially reduce the ever-increasing speeds of the cars to a safe level. Before 1998, dry weather tyres truly were "slicks" and had no tread pattern - simply a plain rubber surface.

Damon Hill's 1996 slick tyre
Michael Schumacher's 2002 slick tyre

The presence of water on the track surface changes the nature of interaction between tyre and tarmac considerably. The long rubber molecules of the tyre can no longer interact directly with the track surface to provide grip due to a thin film of water being present between the tyre and track. In order to remove this film of water, grooves have to be cut into the surface of the tyre which allows the water to escape and the interactions between the track and tyre are re-established. The particular pattern of grooves that the manufacturers apply to their wet tyres varies but they are all designed to remove as much water as possible from the racetrack, to allow the tyre to grip.

Even Schumacher can get caught out in the rain when grip levels are reduced (Brazil 2003)

Intermediate tyres are exactly what their name suggests - a compromise between a full wet and a fully slick tyre. They have slightly shallower grooves that are cut in a different pattern and remove a fair amount of water from the racetrack surface while also providing a decent level of grip once the track has dried. These are the tyres that are used most often when weather conditions are bad. Only in absolutely torrential conditions where there is continuous rain are the fully wet tyres needed.