[thor66]

you don't need a class - just start with it set to "P" then branch out to A or S

for clouds tho, you will have an issue with Dynamic Range (DR) - bright to dark in the same scene

you either get a full frame (FF) of some brand or you exposure stack if you get m43 sensors based bodies - and MOST LIKELY you'll need to exposure stack on a full frame body - the DR is just so great

Here's how it works out - the smaller the sensor the smaller the lenses can be - matters the most with big telephoto for wildlife or sports
- but noise goes up (see the big print comment above)
- and DR goes down - maybe a full stop from FF to m43; when DR in the scene exceeds what the sensor can handle you get brownout highs - and there is just no info for any software to recover in post-processing

Here's the sensor size list:
Medium Format - Hasselblad, Mamiya - I suggest you don't get this; they are big, heavy and expensive but the big sensor gives you the best DR; Hassy XCD is worth a look if you want to spend the $$

FF - Nikon, Canon, Sony, etc. - Nikon gives the best access to many, many old lenses; & gets the most out of their sensors; Canon is preferred by many other people; Sony is the tech leader

APS-C - Nikon calls this DX; I say skip this sensor size

m43 - see above post on the 2 main co.s using this size - the critical thing for your cloud photos is you can exposure stack right in the camera, so no need to buy a post-processing software program

1" sensors - Nikon One system, which they abandoned, screwing over people who bought in - dunno if anyone else makes them

2/3 or 3/4 inch sensors - the (former) realm of compact cameras like Canon S95, etc. - nor dead from cell phones, RIP

even tinier sensors - these are used in smart phones- Apple etc. have tons of processing to make the images from these tiny systems look good; often better than 'real' cameras

So, you want a camera that is worth carrying around besides a smartphone.

That means m43 or FF system, or maybe medium format.

the above is your bottom line

unless... unless you are a crazy person and buy a b view camera and scan the negs on a drum scanner - this has the same advantages as buying a Ferrari: you have found a hole to dump stacks of cash into and it is esthetic to boot

so focus on the FF or m43 systems and then follow Scott Douglas's advice and others who said ergonomics rules