How can it possibly be raining outside

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Discussion Overview

The discussion revolves around the phenomenon of precipitation occurring when a humidity meter reads 57%. Participants explore the relationship between relative humidity, condensation, and precipitation, questioning the accuracy of measurements and common understandings of these concepts.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions their understanding of humidity and precipitation, suggesting that either their humidity meter or their knowledge may be incorrect.
  • Another participant proposes that condensation can occur at higher altitudes where temperatures are lower, allowing precipitation to fall without having time to vaporize.
  • A third participant clarifies that condensation in clouds occurs at lower temperatures, where relative humidity can reach 100% or more, contrasting with surface humidity readings.
  • A later reply emphasizes that precipitation can occur even when relative humidity at the surface is below 100%, citing the presence of hygroscopic particles and noting that average relative humidity during rainstorms is often around 85%.
  • It is mentioned that the common notion that condensation only occurs at 100% relative humidity is overly simplistic and often incorrect.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between humidity readings and precipitation, indicating that multiple competing perspectives exist regarding the mechanisms of condensation and the accuracy of humidity measurements.

Contextual Notes

Some assumptions about the behavior of humidity and precipitation are not fully explored, and there is a lack of consensus on the reliability of humidity meters and the conditions under which precipitation occurs.

mrspeedybob
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when the humidity meter on my back porch reads 57%? May be a silly question but I don't know. I thought when the humidity exceeded 100% the excess moisture condensed out of the air and that's what we call precipitation. Obviously either my meter or my understanding is faulty?
 
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mrspeedybob said:
Obviously either my meter or my understanding is faulty?

Or both. Condensation may happen high up, where the temperature is much lower. When water falls down it may have no time to get warm enough and to vaporize before hitting the ground.
 
+1 Condensation happens in the clouds at lower temperatures and there the relative humidity is 100% (or more). If your humidity meter is reading 100%, it would most likely be foggy.
 
Thanks guys.
 
mrspeedybob said:
when the humidity meter on my back porch reads 57%? May be a silly question but I don't know. I thought when the humidity exceeded 100% the excess moisture condensed out of the air and that's what we call precipitation. Obviously either my meter or my understanding is faulty?

If you are getting precipitation when your hygrometer only reads 57%, then your hygrometer either needs re-calibration or replacement.

However, the vast majority of precipitation events occur when the RH in the surface layer (lowest 100 m) is less than 100%. This is because of the abundance of hygroscopic particles in this lower layer. The average RH during rainstorms is usually around 85%. As Borek pointed out, most precipitation is initiated a higher elevations where the RH may be significantly different from that at the surface.

The notion that condensation always occurs when the RH reaches 100% (and not until) is widely taught in primary schools, but is simplistic and often just dead wrong.
 

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