Ahead of today's opening session the air temperature is 21.2 degrees C, while the track temperature is 30.8 degrees.
Though there are two days of testing next week, this opening session, in particular, is also going to be about testing as almost every single team has brought updates, some more than others.
Indeed, Ferrari has a new engine, and Renault has a pretty big upgrade for its power unit. Then again, as the Mercedes steamroller continues, both, along with Honda, will need to raise their games significantly.
It was here a couple of months ago that Ferrari laid down the gauntlet, a gauntlet that the Silver Arrows were not only able to pick up but have reworked into a winner's garland, for the Maranello outfit - and we're trying hard to avoid hyperbole here - this is a make or break weekend.
At Haas, where the team has struggle in the last three races, Romain Grosjean is running the updates while teammate Kevin Magnussen isn't in order that the American outfit can get a better idea of how they compare.
Kvyat is first out, followed by Magnussen, Sainz, Albon, Norris and Ricciardo, sporting a mixture of all three compounds.
While most complete the obligatory installation lap, Kvyat stays out, diving into the pitlane each time for a practice start.
Five minutes into the session all but the Mercedes pair have appeared. Indeed, in the garage mechanics are working on both cars, the W10s sporting a whole range of aero updates.
Gasly posts the first time of the weekend, the Frenchman, who retired last time out in Baku, posting 1:23.115.
Despite reporting a loss of power out of the final chicane, Gasly subsequently improves to 21.114.
In the Red Bull garage, Adrian Newey oversees the work on Verstappen's car.
Nineteen minutes in and Bottas is the first of the Mercedes drivers to appear, as Gasly improves yet again (21.016).
No sooner has Kvyat posted the second time of the day (22.904), than Norris posts 20.985 and Giovinazzi 24.624.
A 20.890 sees Magnussen to top... with the old aero.
An early warning for Bottas in terms of complacency, the Finn running wide and over the gravel exiting T12.
While Hulkenberg doesn't have the upgraded engine today, the German not running it until FP3, Ricciardo does, while Norris and Sainz will also run the new unit from GP3, their engines understood to have arrived overnight.
Bottas goes quickest with a 19.734, ahead of Grosjean, Leclerc, Vettel and Magnussen.
A 19.238 sees Leclerc go quickest, as Verstappen has a very close encounter with Bottas who appeared not to be paying attention. Bottas 1.0 this weekend?
"Jesus Christ," says the Dutch youngster. "Understood, Max," comes the reply, presumably from his engineer.
Vettel goes top with an 18.825 as Perez remains the only driver yet to post a time. Hamilton is currently 12th with a 21.114.
Perez posts 22.536 to go 19th, as Bottas appears to be struggling with the Mercedes.
A 19.574 sees Ricciardo go third, ahead of Bottas, Grosjean and Verstappen.
"Check onboard, it looked like the exit of Turn 15 something was flying off," warns Vettel. A subsequent replay does indeed reveal the SF90 shedding a small piece of bodywork.
At the point where a set of tyres have to be handed back to Pirelli, it's: Vettel, Leclerc, Ricciardo, Bottas, Grosjean, Verstappen, Magnussen, Norris, Hulkenberg and Stroll.
Following several minutes of silence, the sound of Russell's Mercedes power unit fills the air. The Briton subsequently heads out.
As Russell continues, he is joined on track by Norris, Grosjean and Giovinazzi.
With the upgrades Grosjean goes third (18.943), as teammate Magnussen posts 19.812. Both drivers are on the softs.
Russell complains that he's suffering with his rears in S3, indeed, the youngster delivers a raft of interesting feedback for his team.
Grosjean takes a ride through the gravel at T7, thus bringing out the yellows. News of the Frenchman's excursion is passed on to Hamilton who is about to begin his qualifying sim.
The Briton posts 18.575 to go third, but remains 0.3s off Leclerc's pace, at which point Bottas posts 17.990.
"Tyre prep's terrible," says Hamilton, "rear tyres are done."
Despite PBs in the opening two sectors, a scrappy final sector sees Vettel fail to improve.
As Bottas improves to 17.951, Vettel responds with an 18.066, having gone quickest in the opening two sectors.
Very early days, and we have yet to see them on the softs, but the Bulls are 8th (Gasly) and 12th.
A 19.180 sees Magnussen close to within 0.2s of his teammate, which is sure to give Haas a headache moving forwards.
With 16 minutes remaining, Sainz has the track to himself, but is subsequently joined by Norris and Russell.
Kvyat (26) has completed the most laps, ahead of Norris (23), Gasly and Hulkenberg, who have both completed 22.
Mirth at McLaren. "Lando, it's Tom..." "Hello Bob, I'm Norris..." responds Sainz, whose car is sporting an inordinate amount of flo-vis.
Both Alfas are surprisingly off the pace, as are the Racing Points.
Not for the first time Haas appears to have the edge in terms of the midfield, but thus was the case on previous race Fridays only for it all to fall apart on Sunday.
As Russell reports that he's "lost a mirror", mechanics continue to work on Bottas' car. Indeed, with the Finn out of the W10 there is talk of an oil leak.
Out come the yellows as Stroll going into the barriers at T9. The session is red flagged.
Replay shows the youngster dipping a wheel on the grass exiting T8, which leaves the Canadian a passenger as his car spears towards the awaiting barriers further down the line.
"Jesus Christ," exclaims the youngster, "I hit the grass...".
The session ends under the red flag.
Bottas is quickest, ahead of Vettel, Leclerc, Hamilton, Grosjean, Sainz, Magnussen, Gasly, Kvyat and Hulkenberg.
Ricciardo is eleventh, ahead of Verstappen, Stroll, Giovinazzi, Albon, Norris, Perez, Raikkonen, Kubica and Russell.
Check out our Friday gallery from Barcelona, here.