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How Do They Form?
There are two weather systems that can form stratus clouds. Weak vertical air currents can lift layers of air into the sky to an altitude where it forms condensation. This is the same composition as fog, just at higher altitude. The other way is when the air temperature at a certain altitude allows water vapor to condense into a liquid. When fog at ground level is burned off by the sun this can also form stratus clouds.
Weather Systems
Stratus clouds bring with them rather indifferent weather. The temperature is lower as the sun is hidden behind a layer of cloud, but it is also unlikely any rain will fall. This is because there is little vertical motion in the cloud. Instead, it stays as a solid thin layer of water vapor. Any precipitation from stratus clouds will be very light misty rain.
Cirrostratus
Cirrostratus clouds are commonly at a much higher altitude than other types of stratus clouds, cumulating at around 18,000 feet. They are very thin and are made up of ice crystals allowing the sun to be seen through them. The thicker the cirrostratus clouds, the greater the chance of rain or snow in the next few days.
Altostratus
A more common stratus cloud formation is altostratus. These form between 6,000 and 20,000 feet and are commonly followed by extended periods of rain. These clouds tend to be very widespread, covering hundreds of square kilometers. Not enough sun can penetrate the cloud layer for shadows to form on the ground.
Nimbostratus
Nimbostratus clouds are the most likely to create precipitation, forming below 6,000 feet and colored dark grey. The rain from these clouds is not going to be heavy, but will likely last for a long period of time with constant light showers. These are normally formed by large incoming quantities of warm air colliding with colder air.
Stratocumulus
Stratocumulus is a cloud formation which falls in between thin extended cloud cover and separated lumpy cumulus clouds. The sky will be covered with a pretty constant layer of cloud at below 6,000 feet, but there will be areas where blue sky can be seen above. The edges of these patches will have the traditional cotton wool appearance of clouds. These form when air rising from the ground meets a descending air system allowing for a combination of water vapor based clouds and cumulus clouds.