Ahead of today's opening practice session the air temperature is 19 degrees C, while the track temperature is 27 degrees. It is currently overcast, to put it mildly, but dry, though we are expecting rain in FP2 and plenty more over the course of the weekend.

A number of changes have been made since last year, most notably the resurfacing of the entire track, including the pitlane. Also, the pitlane exit has been realigned further to the left to provide a run-off area around the outside of T2, while the pitlane entrance has been realigned to the left in order to allow the pit wall start to be moved further away from the track.

There are two DRS zones. The first has a detection point at the apex of T2, with activation 20m after T3, while the second has its activation point 30m after T13, with an activation point 60m after T15.

While Marussia and Caterham are still missing, a number of reserve drivers are on duty, Daniel Juncadella at Force India, Max Verstappen at Toro Rosso and (Sauber bound in 2015) Felipe Nasr at Williams. They replace Perez, Vergne and Bottas respectively.

Tyre choices are medium (prime) and soft (option), though Pirelli originally nominated the hards and medium which were the choices in 2012 and 2013.

The lights go green and Maldonado leads the way, followed by Grosjean, Juncadella, Button and Kvyat.

As in Austin, teams will be trying various components for 2015 today.

An early problem for Button who stops at the pitlane entrance and has to be pushed to safety by the marshals before his own crew come to rescue him. The Briton's car is fairly 'clean', unlike teammate Magnussen's which is covered in various sensors.

Maldonado, who stays out for a second lap, complains that his DRS is not working.

In no time at all, probably with an eye on the weather, everyone has been out.

"Everything just cut off," says Button, "suddenly everything switched off, steering, hydraulics, everything."

Ten minutes into the session, Magnussen is the first out for a second run... complete with sensors. A lap later he's back in his garage.

Verstappen makes the first mistake of the day, running too deep into T1 and touching the damp artificial grass with a wheel. He goes on to post the first time of the day (1:19.999) while Sutil posts 20.332. The German subsequently runs wide in T6.

As more drivers take to the track the times begin to tumble, Alonso goes quickest (16.921), ahead of Raikkonen, Kvyat, Verstappen, Sutil and Rosberg.

Going quickest in all three sectors, Rosberg raises the benchmark with a 15.450 only to be eclipsed by (guess who?) who posts 15.337. Elsewhere, Juncadella spins at T6 following a close encounter with Raikkonen in the pitlane.

Nasr, who goes third with a 16.047, complains that there is no grip.

Rosberg re-takes the top spot with a 15.002 as more and more drivers complain of a lack of grip.

Twenty-five minutes into the session, all but Button, Gutierrez and Magnussen have posted times.

As Rosberg complains of too much understeer, teammate Hamilton posts 14.172. The Briton runs wide heading into T12 kicking up a cloud of dirt.

Again Hamilton improves, the world championship leader banging in a 13.468, 0.533s clear of his teammate.

The opening half-hour comes to an end, at which stage the 'free' tyres have to be given back.

Hamilton is quickest, ahead of Rosberg, Kvyat, Alonso, Vettel, Ricciardo, Verstappen, Grosjean, Raikkonen and Nasr.

In the McLaren garage warning lights are flashing and safety barriers are in place as mechanics work on an ERS issue on Button's car.

As most drivers take a quick break, Magnussen is the only driver on track. "You've got a clear track," the Dane is told, "we're going to be doing five timed laps." His first timed lap (15.721) sees him go 11th, the McLaren sending up a shower of sparks as it bottoms on the pit straight.