The Pirelli P Zero Orange hard and P Zero White medium tyres nominated for the United States Grand Prix faced a variety of weather conditions during the two free practice sessions in Austin, ranging from fog in the morning to 25 degrees centigrade in the afternoon.

The quickest driver of the day was Red Bull's Sebastian Vettel, who set a time of 1:37.305s on the medium compound tyres halfway through FP2. His team mate Mark Webber was second fastest, ahead of the two Mercedes, while Heikki Kovalainen was quickly up to speed with both Pirelli compounds to set fifth-fastest time on his debut for Lotus.

The opening free practice session in the morning was delayed by fog and then cut short because of problems with the medical helicopter. Ferrari's Fernando Alonso was quickest on the hard tyre, but with little running in the morning, the teams had to make the most of the hour and a half of FP2. With the track still evolving, the teams concentrated on assessing race performance with different fuel loads on both compounds.

Paul Hembery: "We saw a wide variety of weather conditions today, but 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. In addition, we saw the track evolving throughout both sessions, with the times getting progressively faster. Generally, there seems to be more grip than last year, when the asphalt was brand new. 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. Wear and degradation is where we would expect it to be: 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. 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."

Check out our Friday gallery, here.