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How can it possibly be raining outside

  1. Sep 11, 2011 #1
    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?
  2. jcsd
  3. Sep 11, 2011 #2


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    Staff: Mentor

    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.
  4. Sep 11, 2011 #3
    +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.
  5. Sep 11, 2011 #4
    Thanks guys.
  6. Sep 13, 2011 #5
    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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