Why are "premium" laptop vendors still putting vents on the bottom of their machines? Did they never try actually putting their laptop on their laps and realise how much that design sucks?
These are not laptop computers, they are notebooks. The only laptop I've personally seen had a 80286 processor... We call them laptops, but historically that name is wrong.
While it is possible to use a notebook on your lap, you are not supposed to. It is a terrible place that is unergonomic. You are supposed to put them on a table of some sort. If you are using it for more than something quick you should have a separate keyboard and mouse (ie attached to a docking station on your desk). The portable form factor is useful for meetings, presentations, or other such - but for real work they are terrible.
Since the lap what it was designed for, and a lap is a bad idea anyway: putting vents on the bottom isn't bad.
I frequently sit on my balcony, with my MacBook Pro in my lap. I'm still alive.
I also frequently use that computer, that doesn't have vents on the bottom, for watching movies in bed. And I don't need to think about vents. My last non-Apple computer was some ThinkPad, with vents on the bottom, and I remember always chasing some book to put under the computer to make the vents free. Boy, how I hated that.
I doubt any computer you can lift will kill you if you put it on your lap. It might be uncomfortable, but it won't kill you. (I'm not about to put some of VAXen I've seen on my lap even though I can technically lift them, but...)
People do all kinds of terrible things to their body. That you do something and seem to be getting away with it doesn't mean you should. Talk to a real doctor trained in this (your regular doctors probably is not) for details.
When I'm working, I'm at my desk, with a keyboard, mouse, and monitors. But the reason why I have a light laptop instead of some beefy desktop computer is the convenience of having a computer when I'm not at my desk. At a café. Or to check emails when I'm at relatives'. Or to work on my hobby project in a car, waiting for my daughter to finish her training session. Or to be next to me, on a carpet, while I'm checking why the network under the TV is behaving strangely.
I've been living laptop life since 2002, IIRC. In all that time, the most stupid design decision I saw was on some HP EliteBook, where designers, in their infinite wisdom, put tiny legs on a laptop. Those four stupid pieces of plastic and rubber bite my legs every time when I try to use the convenience of having a mobile computer.
Fans on the bottom of the laptop case are firmly in the second place on my list. And no amount of ackchyually you're holding it wrong will change that.
How does gravity make vents at the bottom useful? A normal non-passive laptop uses forced convection with fans, the natural convection should be completely negligible in that case.
Vents in the bottom just make sense, does it have to be explained? The fans aren't going to be running full blast during idle times or when portable for example. Saves power to use gravity.
Macs don't have to worry about that since they made a huge efficiency jump with M1. Before that they overheated due to poor thermals.
Arguably it helps so the air can go through the top holes in the keyboard better, you probably want openings on top and bottom for that.
But I still think this effect is negligible compared to the fan. I don’t think you would notice a temperature difference if you rotated your laptop 90 degrees with the keyboard vertical.
While it is possible to use a notebook on your lap, you are not supposed to. It is a terrible place that is unergonomic. You are supposed to put them on a table of some sort. If you are using it for more than something quick you should have a separate keyboard and mouse (ie attached to a docking station on your desk). The portable form factor is useful for meetings, presentations, or other such - but for real work they are terrible.
Since the lap what it was designed for, and a lap is a bad idea anyway: putting vents on the bottom isn't bad.