Valtteri's day was dominated by the weather - F1 tyres aren't designed to be run in such cold conditions, which made it tricky for all teams to get the most out of the session.
Nevertheless, the Finn ploughed on and completed 94 laps, putting in a best time of 1:19.976 on the Medium tyre in the slightly warmer afternoon running - which put him second quickest.
"It was very cold on track today; I don't think I've ever driven in such cold conditions before in Formula One," he said. "Testing is always difficult in these temperatures, the tyres are just not made for these conditions.
"But we made the most out of the day. In the morning, we focused on aero work with different aero rakes on the car, running various tests. In the afternoon, I was doing long runs and we also worked on the set-up.
"It was quite tricky in these temperatures, but we managed to get some work done and found some answers to our testing items.
"The weather is obviously unfortunate, but there's nothing we can do about it and it's the same conditions for everyone. In these conditions, I would say today was a good day. I hope that the weather is going to get better soon and I'm looking forward to tomorrow and Thursday."