Re: Why low-level clouds seperate from mid-level clouds at 2,000 m ?

I suppose that the real reason for deciding on those specific levels would 
be because that is when changes in cloud structure begin to occur.  The 
higher a cloud, the more likely it is composed of ice crystals rather than 
liquid water.  Also, different types of clouds signify different 
atmospheric processes (frontal passage, convection, etc) and should be 
treated differently.

This chart was taken from a NASA site:

Intro to Clouds



This demonstrates that clouds can be classified not only by their height, 
but also to their "optical thickness."  Generally, the optical thickness 
can be though of as how much sunlight a cloud reflects (e.g., a severe 
thunderstorm cloud would be considered optically thick while cirrus clouds 
would be considered optically thin.)

Another good site to visit for information on the types of clouds can be 
found at:  
Clouds

Hope this helps!