| I need to know what family of orchids you have to be more specific but will discuss them in general terms.
In a very general sense air roots are the result of the plant not finding anything on which to grow. Since one purpose of the root is to hold the plant in place, when the plant does not find anything it will often discard the root and grow another one.
When you say thick roots I think of Phalaenopsis or Vandas.
Phals do not grow those thick air roots naturally but it is a result of growing in a pot, which is unnatural for the plant. Phals normally hang down from tree branches not up. The roots grow up to find the branch above. Air roots on phals will often shrivel and die when they are not needed. Much the same as our hair and fingernails.
Vandas are another issue. Their roots are normally air roots. In general they do not like their roots crowded. Growing vandas requires a large amount of water. Bare roots dry out fast and need constant watering. I recommend soaking vandas 5-6 hours a week in a bucket of water. Healty vanda roots will have a bright green tip. This is the growing point. When this turns black or white the root will no longer grow. It is still a healty root. The plant will start new roots.
Other familes of orchids cattleyas and oncidium alliance being the most common, will grow air roots when the roots can not find a medium in which to grow or a branch on which to cling. These roots will often shrivel and die. It will not hurt the plant and is discarded when not needed. Mounted cats and oncs will grow air roots that seek out the branch and attach on to it.
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jerry  |