Ferrari joined the Barcelona Formula 1 test on Tuesday morning as the start of the second day was interrupted early by a brief red flag caused by Red Bull and then rain began to set in.
Teams can drive on three of the five days of the test this week with seven already getting under way on Monday.
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Ferrari and McLaren deliberately held back until Tuesday with the Italian team hitting the track as soon as the pitlane opened at 9am.
Red Bull joined Ferrari soon after with Max Verstappen getting his first taste of the RB22, but he caused a red flag on his outlap.
It is understood he briefly went off into the gravel at Turn 5 and was able to continue, but it prompted a pause in the session of just over five minutes.
Ferrari resumed its running as soon as the test restarted and Verstappen was soon back out too.
But around an hour and a half into the session, what had started as light rain set in fully, with the sky around the track looking ominously murky.
Only a small number of teams are set to be on track on Tuesday. As of the morning, McLaren had not yet committed to making its debut.
It was always open to potentially only starting on Wednesday having prioritised developing its car more. The first time the MCL40 runs here will be its first ever shakedown as it was still in Austria last week running advanced simulations with engineering specialist AVL.
Most teams that took part on Monday are not driving on Tuesday. Red Bull is the only team to appear so far but Mercedes, Audi, Alpine, Cadillac and Haas are all understood to be skipping today.
This is due to a mix of reasons including the weather - with the rain forecast in advance - and also a chance to resolve problems as the likes of Audi and Cadillac had technical issues on Monday.
Mercedes trackside engineering director Andrew Shovlin explained: “I'm sure every team is keeping a bit of an eye on the weather.
“There's a few that haven't run yet, so no doubt they'll be keen to get going as soon as they've got their cars ready.
“But we're just having to look at the forecast, look at the tyres that we've got available, and make sure we pick the right days.
“Because if we get a very wet day, we'll be running out of intermediate tyres as will some others. But equally, a bit of running in the wet would be quite useful.”
