Ahead of today's opening session, the air temperature is 18.1 degrees C, while the track temperature is 24.1 degrees. It is cool and overcast. The risk of rain is 20%.
Other than the clouds that cover the Principality, there is a little added darkness following the death, earlier this week, of three-time world champion, Niki Lauda, a two-time winner here.
Understandably, the sport is paying tribute to its fallen hero, with all the cars sporting some form of acknowledgement of the Austrian legend.
While he won his titles with Ferrari and McLaren, in recent years it is with Mercedes that he has worked and it is clear that the German team is still very much in shock. While many fans want to see the Mercedes juggernaut derailed some time soon, few would begrudge the German outfit a poignant win this weekend.
That said, even without Niki looking down on them, Mercedes has to be favourite this weekend, the only likely threat looking to be from Max Verstappen. Ferrari says it learned much from the Barcelona test, but it is likely that the results won't become evident until Canada. If then.
The lights go green and one by head the drivers head out, Giovinazzi followed by Sainz, Norris, Grosjean, Kvyat and, 2018 winner, Ricciardo.
Most are on softs, the exceptions being the Toro Rosso duo who are both on mediums.
The Williams duo posts the first times of the weekend, Kubica a 22.956 and Russell a 25.394.
Bottas and Hamilton head out, followed by Vettel who is sporting a very nice Lauda tribute helmet.
Bottas posts a 17.771 with Hamilton posting a 17.787 moments later.
The Ferrari duo are welcomed at Rascasse by a stand of tifosi, all waving their prancing horse flags.
Vettel posts 18.943, with Leclerc stopping the clock at 18.143.
Vettel improves to 16.207, but again Leclerc goes quicker (15.572) while Bottas bangs in a 14.240.
We're only ten minutes into the session, but already the big guns are pushing hard.
Then again, with an eye on those dark skies, perhaps they are right to get their laps in early.
A 14.292 sees Hamilton go second, while Ricciardo goes fifth (15.619), ahead of Hulkenberg, Kubica and Russell.
Thirteen minutes and Gasly is the first of the Bulls to appear.
Vettel makes early use of the escape road at Ste Devote, bringing out the yellows for a brief while.
A 13.636 sees Hamilton go top as Bottas posts 13.706, the Ferraris both over 0.9s off the pace.
A 14.961 sees Ricciardo leapfrog Vettel for fourth, the Australian clearly thinking that he's still at the wheel of a Red Bull.
Eighteen minutes into the session, first Grosjean and then Magnussen is black-flagged. Elsewhere, Gasly does well not to hit the barriers after losing the rear end of his RB15.
Hamilton has improves to 13.624, while, before the black flag, Magnussen had gone fourth with a 14.473.
Leclerc improves to third with a 13.780, just 0.156s off the pace.
Verstappen goes with a 14.756, ahead of Hulkenberg, Gasly, Vettel and Giovinazzi.
As debate continues as to why the black flags, Haas reveals that Magnussen's radio isn't working, and the team is unable to call him in.
It appears the black flag was down to a telemetry and radio issue, Haas calling on race control to stop its two drivers as it was unable to contact them. Replay shows frantic waving of the drivers pit signs but to no avail.
After 26 minutes, Sainz is the only driver still to post a time. The McLaren clearly has a problem as the Spaniard isn't in his car.
Hamilton bangs in a 12.932 to put 0.774s between himself and his teammate.
The air turns slightly blue when Raikkonen is baulked by a slow and erratic "Force India".
Bottas has clearly had a moment, his W10 being worked on having damaged a bargeboard.
"I'm getting strange wheelspin out of the chicane," reports Ricciardo, "and it's getting worse."
Verstappen improves to second with a 13.507, the Dutchman advised that Hamilton is "seven-tenths quicker" in sector one, while the youngster is "purple in the final sector".
Albon continues to impress, going seventh with a 13.827.
"Stroll reveals that his "rears are dropping off".
As most drivers head back to the pits to hand back their extra set of tyres, Ferrari sends its two out in order that they can enjoy a clear track on fresh rubber. Bottas joins them.
In clear air, Leclerc and Vettel post identical times (13.014), to go second and third. But Bottas then improves to 12.818. Leclerc responds with a 12.868 and Vettel a 12.852.
Hamilton has joined in the fun and games, the Briton crossing the line at 12.422. Despite a poor opening sector, he goes quickest in S2 and posts a PB in S3.
Bottas responds with a 12.248 as Leclerc improves to 12.467. Moments later, having gone purple in both the opening sectors, Hamilton posts a 12.106.
At 'half-time', it's: Hamilton, Bottas, Leclerc, Vettel, Verstappen, Kvyat, Gasly, Albon, Giovinazzi and Hulkenberg.
Giovinazzi improves to fifth with a 13.437, ahead of Verstappen.
As work continues on Sainz' car, it appears the Spaniard will not be going out in this session.
A spin for Kubica at Casino, who lightly clouts the barrier but is able to continue. For a brief moment it looked as though the Pole had stalled, while his car was stranded in the middle of the track.
Leclerc complains of major graining.
Kvyat goes off at Ste Devote, having appeared to hold up Hamilton for a brief while, he pair appearing to touch at one point.
As Kvyat warns that his medium tyres are gone, Norris makes use of the escpe road at Ste Devote.
Out come the yellows when Verstappen goes off at Mirabeau, the corner where Nico Rosberg had his infamous moment during qualifying a few years back. The youngster has difficulty finding reverse in order to get back on track. "It's not going into reverse, please tell me something?" he says.
Gasly improves to sixth, as Raikkonen goes seventh, ahead of Ricciardo and Giovinazzi.
That said, a 13.227 sees Hulkenberg improve to sixth.
A 12.165 sees Verstappen go second, just 0.059s off Hamilton's best. Two PBs are followed by a purple.
With 18 minutes remaining, Grosjean heads out, Haas' various communications issues seemingly sorted. Shortly after, Magnussen follows.
Check out our Thursday gallery from Monaco, here.