The ex-Prez was back in the Big Apple Friday for another day of arguments and witness testimony as the Manhattan D.A.'s Office goes after him over his alleged hush money payments to Stormy Daniels ... and DT's latest gripe is all about the low temperature.

He talked to the press outside of the courtroom before heading in, and Don made it very clear he finds the space to be pretty damn cold ... which he seems to think is on purpose.

Trump made a point to note the courtroom was often very cold, and suggested it was being done deliberately to hurt him -- just like everything else in the trial, from his POV. He said it shouldn't be that hard to warm up the place ... but then sorta accepted it, saying it was fine.

So far, there doesn't appear to be much confirmation about the icebox courtroom from reporters that are sitting in and watching. One journo did note it's a little brisk, but not nearly as chilly as Trump is making it seem.

The funny thing is ... if it is, indeed, too cold for Trump ... ya gotta wonder if they're doing it to keep his ass awake. As you may have heard, there are reports Donald's dozing off during the trial ... with photos and sketches depicting him with his eyes closed at various points.

Rule of thumb ... if your body's feeling cold, it'll be forced to be alert out of instinct -- so it's certainly possible this is a tactic to keep Trump from passing out during the proceedings.

Anyway, that wasn't the only thing Trump was complaining about Friday ... he also groused that he was having to miss Melania's birthday because he had to be in court -- which seems to be grinding his gears.

Melania hasn't been present for her husband during this trial ... ditto for other family members. It's just been DT and his lawyers in there ... plus, all the Secret Service.