One of the joys of instrument flying is being able to pop in and out of the clouds. In the real world, pilots who have only a basic license are required to stay clear of clouds. With a computer flight simulation, there are no such limitations. You may fly into whatever kind of cloud you want. The problem is that creating realistic looking clouds is a challenge.
Adding clouds to your simulation can be a challenge, especially if you are trying to create fluffy clouds that you can fly through. We will address those challenges in a separate discussion. In the meantime, there are a couple of types of clouds that you can easily add to your simulation. Both are thin cloud layers, which means that you can model them with a flat plane and double-sided transparent textures.
The first type you can easily model is a thin undercast fog layer, which you can fly through. Since this will be a moving layer, you can use an approach similar to the approach we used for terrain grids - a scrolling set of textures. Because these do not have sharp definition, you can use large grids only. To create the fog effect, you would use a fog-type transparent texture. Or you could try for a more "patchy effect.
The second type you can easily model is high cirrus clouds. Because these are so high, you don't have to use a moving plane, but can use a single stationary plane positioned above your aircraft. The challenge here will be to find some good-looking transparent textures. Fortunately, if you can find a decent photo of clouds from directly below, you can convert the photo into an alpha layer by changing the color to black and white. Ideally, the image will include fairly high resolution clouds. If you have found a picture of isolated clouds, you can easily cut and paste to add more clouds. You will want to make sure that the contrast is high enough so that the darkest part of the image is black, rather than gray. Otherwise, you will have a clearly visible square that you might have trouble blending with the default sky.
Pilots who have only a basic pilot license are required to stay clear of clouds. But, once you have an instrument rating, you fly along "highways" in the sky and are allowed to fly through clouds that stand in your way. Of course, this requires judgment as you do not want to fly into an extremely "active" cloud or, even worse, a thunderstorm.
Shortly after getting my pilot license, we took a family trip to Orlando in a Cessna Skyhawk. Flying over Florida on a warm summer afternoon, we soon found that we had to weave our way around a growing number of cumulus clouds. One time, we flew so far out of our way that we had to use binoculars to read the name of the town on a water tower.
This trip provided me with the motivation to obtain an instrument rating. The next time we flew to Orlando, I looked forward to being able to fly straight through the clouds. But I soon discovered that flying into a solid wall of clouds at 7,000 feet could be more terrifying than flying around them at 5,000 feet. You never know when that light gray cloud in which you are flying is going to start turning very dark. To be safe, I called Flight Service to get a weather briefing and a suggested path. But when I called Jacksonville (JAX) Center, they told me that path would take us straight into a thunderstorm. Although Centers are not supposed to be able to provide weather guidance, they did a great job of directing us around any rough weather.
Once I had my instrument rating, I would generally follow the published routes in the sky - unless I was allowed to proceed direct to the destination. While enroute, one of the most enjoyable parts of the trip was flying through scattered cumulus clouds. You can see these clouds from a distance and decide whether you want to fly through them or just skim past them. When you enter a cloud, you might get a little bump and there is sometimes some light turbulence inside. When you pop out the other side of the cloud, you often get the sensation that you are leaping out over a cliff. (In law school, one of the annexes had transparent floor tiles on the upper floors. Stepping out onto those for the first time took a bit of courage.) Exiting a cloud often gave the same sensation.