Fernando Alonso again benefited from some persistent rain to emerge on top of the timesheets and hearten thousands of Ferrari fans on the opening day of F1's first in-season test for four years.

The Spaniard vaulted to the head of the times shortly before lunch, and remained there as rain continued to fall with some ferocity throughout the afternoon session. Whenever a drying track appeared possible, the precipitation began again, putting paid to any thought of improvement during the final four hours.

The Ferrari team leader's performance was met lustily, on the team's home soil, by a crowd the Scuderia estimated at over 15,000, with fans filling the grandstands and trackside areas, and spilling into the paddock, despite the less than favourable weather.

Alonso ran the whole day for Ferrari - with Felipe Massa due his turn later in the week - and concentrated on the development of new components for the F2012 and increasing the understanding of the behaviour of the tyres. Unfortunately, the track conditions, with only a small window of dry time in the late morning, required a drastic revision of the programme.

The afternoon was particularly limited, when the only work carried out was on a handful of practice starts. At the end of the day, a meagre total of 46 laps had been completed by the double world champion, but his quickest time of 1min 22.444secs remained the fastest outright.

"It's a shame about the rain, which complicated the running of this first day of testing - and a shame for the spectators, who deserved to see more of a show," Alonso commented, "We had a pretty busy work programme, but we only managed to get through a part of it. Therefore, it's impossible to give a serious evaluation, especially as we did so little dry running.

"I can only say that, for me, it was a pleasure to be driving at this track again. The first and only other time was back in 2001, when I was testing for Minardi. It's a spectacular track and it would be nice to be able to race on it!"

If anything, Mark Webber and Red Bull Racing completed even less running than Alonso, but the Australian found enough of a window between showers to vault up into second spot, albeit fully 1.2secs shy of the outright pace.

Initial forecasts had suggested there would be a 30 per cent chance of rain at the Tuscan track, but damp conditions in the early morning also led the Milton Keynes-based team to run a re-worked plan, which included some running on Pirelli's intermediate tyres. RBR used those outings to test a number of aerodynamic and mechanical development parts, which had been brought to Italy for the test. A brief lull in the wet weather later in the morning gave Webber a couple of dry tyre runs and the team maximised the opportunity before the heaviest rain of the day arrived to record a lap of 1min 23:648secs.

"Obviously, the weather wasn't great today, so we didn't get to do much running in the dry - really, we only got one decent run on the slicks," the Australian sighed, "Then it rained after lunch and, with limited sets of wets available, you can't do that much. We're hoping for better over the next couple of days.

"It's an important test. We have a lot of information to gather and, while today didn't quite work out as expected, I'm optimistic that over the course of the next few days we'll get the information we need."

Webber also shared Alonso's enthusiasm for the Mugello circuit.

"I have to say this is a fantastic track," he grinned, "It's very challenging for the driver and very quick. I don't know if we'll ever race here in the future, you never know, but to see the crowd here today was very special. A lot of people came to watch and that's great."

The Red Bull livery also filled third place, as Jean-Eric Vergne gave Toro Rosso an encouraging start but, with more rain in the afternoon, the Italian team aborted the session early, preferring to prepare for what is predicted to be a dry day on Wednesday.

"The car felt fine for the few dry laps I completed but, from a technical point of view, there is not much to say, because, although we have a full programme to work through, the rain meant we will have to wait until tomorrow to start assessing some of the new elements we have brought here," noted Vergne, who will take the wheel again at the start of day two.

The Frenchman was also among those who brought out the red flags during the day.

"At one point this morning, I went off the track," he confessed, "I was pushing a bit too hard, spun and got stuck, as the rear wheels were in the gravel. Fortunately, there was no damage to the car."

Jerome d'Ambrosio's first outing in the Lotus E20 resulted in the Belgian annexing fourth spot on the timesheets, building on the impressive 2-3 result taken by Kimi Raikkonen and Romain Grosjean at the recent Bahrain Grand Prix.

Nico Rosberg claimed fifth for Mercedes, lapping just a tenth slower than d'Ambrosio, while Sauber's Kamui Kobayashi was sixth fastest, over half a second further back. The Japanese started out with measurements related to a new aero package the C31 is trialling this week, including a new front wing, rear body work, diffuser and numerous other parts. While the morning's track conditions required full wet tyres, base line running on intermediate tyres and even for a comparison of the new body work on a set of dry weather tyres were possible later on - before the rain returned with a vengeance.

When race control had to red flag the session due to a heavy thunderstorm shortly after 3pm, Sauber joined Toro Rosso in electing not to venture out, preferring instead to begin crunching the date acquired ahead of day two.

McLaren was the first team in the order not to be running a driver with at least one grand prix under his belt, and fielded both Oliver Turvey and Gary Paffett during the day. Turvey began the week's programme with a number of short, data-collection, runs on slicks, but the heavy rain at lunchtime meant that the scheduled driver change to Paffett was less productive, as the wet weather, coupled with circuit closures due to the low visibility, rendered meaningful running pointless. As a result, the Woking team also opted to end the day early and start preparations for Wednesday.

Jules Bianchi was likewise on hand for Force India, but was able to complete just 19 laps behind the wheel of the VJM05.

"This morning, we had to wait a long time for the track to dry," he confirmed, "We did some runs on the intermediates just to check the car's systems and, when the conditions improved, I went back out on the dry tyres [medium] to get confident with the car and to feel the track conditions. There was a chance to do some aero checks and we got some good aero data, but it was difficult to test all the developments so that will need to be done later ."

Behind the Frenchman, Caterham rookie Rodolfo Gonzalez filled ninth spot, while Charles Pic rounded out the top ten for Marussia. The race rookie's objective for day one was an evaluation of the latest aerodynamic upgrades, set for introduction in Spain next weekend, as well as a tyre programme designed to facilitate a deeper understanding of the Pirelli rubber. As the track started to dry, Pic conducted much of his running on the intermediate tyre before switching to the hard slick compound later in the session. The weather remained changeable, however, and the absence of consistent conditions interrupted progress with the team's intended programme. When the rain intensified after lunch, the team elected to conclude the day's running early in favour of making some planned changes to the car.

"It has been difficult to find any kind of rhythm as the weather was changing all the time," the driver reported, "All running is good for us because there is always more we can learn about the car, but it will be good to spend tomorrow morning focusing more on the tyre work in the drier conditions. The aero developments look good, but there is plenty more to explore in the next two days to help us prepare for Barcelona."

Ahead of Paffett, who completed hardly any running of note, Valtteri Bottas rounded out the list of 13 runners who saw some action during the day.