Usually you'd expect Texas to be hot and sunny, but that certainly hasn't always been the case so far for the F1 teams and drivers assembled at the Circuit of the Americas in Austin for this weekend's United States Grand Prix.

Friday ran the gamut, with a very chilly start ushering in dense fog, which then broke up to leave blue skies and sunshine but also a swirling wind that made it difficult for the teams to get their handling set-up worked out - all of which added greater stress on the Pirelli tyres to make up the grip levels.

"We saw a wide variety of weather conditions [Friday, and] we're expecting temperatures to climb over the rest of the weekend, which of course makes it more difficult for the teams when it comes to predicting tyre behaviour and therefore strategy," was the opinion of Paul Hembery, Pirelli's director of motorsport.

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"In addition, we saw the track evolving throughout both sessions, with the times getting progressively faster," he continued. "Generally, there seems to be more grip than last year, when the asphalt was brand new."

In the truncated morning session the fastest time went to Fernando Alonso, the Ferrari driver setting his fastest time of the entire day with a lap of 1:38.343s on the Pirelli P Zero Orange hard compound tyres. In the afternoon, as temperatures rose, it was Red Bull setting the pace on the P Zero White medium option compounds, with Sebastian Vettel ending the day with the top time of 1:37.305s.

"Wear and degradation is where we would expect it to be," said Hembery. "The medium tyre will be the one to qualify on, while we're seeing very little degradation on the hard tyre which is just under a second slower than the medium tyre at the moment.

"That's a big enough gap for strategy to make a difference, particularly for the drivers who start outside the top 10 and are not obliged to begin the race with the tyres they qualified on," he added.

That makes is difficult for Pirelli or any of the team strategists to know what will prove to be the race-winning strategy for Sunday.

"From what we can see so far, we could well be looking at a one-stop race for most runners, as was the case last year, although some may try two stops if they feel it offers a speed advantage," said Hembery.

"There's plenty of data to analyse tonight, before the teams concentrate on qualifying simulations during FP3 tomorrow, which is likely to be held in conditions that are more representative of the race," he added.

At least wet weather tyres should not be called into play this weekend, although local forecast did suggests that there was a lot but nonetheless appreciable 20 per cent chance of showers on Saturday that could affect qualifying.