Passage Summary: We can tell how high volcanic ash is by looking at lightning, which is helpful because radar isn't always around to do the job.

Conclusion: Lightning discharge data is sometimes the only dependable way to determine how high a volcanic ash cloud is.

Reasoning: Lightning only occurs above 5km and correlates with height, and while radar works, it isn't available globally.

Analysis: The argument assumes that there aren't other methods besides radar and lightning to measure these altitudes. If there were a third, widely available method, lightning wouldn't necessarily be the 'only' reliable indicator left when radar fails. To find the necessary assumption, look for an answer that rules out other potential measurement tools or technologies.