Sentence:  Since the tropopauses of these planets are very cold, there is effectively a <WSD>"cold trap</WSD>", which in the absence of any other sources should keep the stratospheric water abundance very low, as discussed in Section 4.3.3. 
Meaning of the word marked with <WSD> tags:  An atmospheric layer that is significantly colder than the adjacent layers both deeper and higher. It keeps ascending gases that have high melting points from escaping the atmosphere by freezing them to a solid which then drops back to the planet surface.