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When atmospheric relative humidity (RH) exceeds 100%, water must condense out of the air. We typically see this as rainfall.

In North-Eastern U.S. summers it is common for daytime air to reach above 80°F and 70% relative humidity. I.e., the daytime dew point (the temperature at which RH would exceed 100%) is commonly above 75°F. It is also common for air temperature to fall 15-20°F during the night. Assuming a dryer atmosphere doesn't move in every night, shouldn't we expect it to rain every night under these conditions?

As a resident of this region I have observed that in reality it frequently does not rain at night under these conditions. So how is the atmospheric humidity dissipated during the night if not through rain?

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  • $\begingroup$ I’ve added a line to my answer that you may find helpful $\endgroup$ – Bob D Sep 9 at 19:27
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So how is the atmospheric humidity dissipated during the night if not through rain?

It is correct that when the temperature falls to the dew point, the water vapor in the air will begin to condense and form droplets. This is commonly observed in the morning in the form of dew on grassy surfaces, thus the term dew point.

I think a meteorologist can give the best answer (and I'm not one), but I suspect that the question of whether reaching the dew point results in rain may have to do with how much water vapor in droplet form already exists in the air in the form of clouds, and how much additional condensation the clouds can accommodate before the droplets get large enough to fall as rain. For an interesting piece on how rain drops are formed and their size, check this out: https://www.usgs.gov/special-topic/water-science-school/science/raindrops-are-different-sizes?qt-science_center_objects=0#qt-science_center_objects

For example, if the preceding hot and humid day was relatively cloudless, then the condensation that occurs when the dew point is reached might be able to deposit on surfaces, as well as cause cloud formation, without necessarily resulting in actual precipitation. One example of that is the formation of fog near the surface of the ground in the morning, which is essentially a low lying cloud formation.

Hope this helps.

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  • $\begingroup$ Good points. IIRC "rain" may also require airborne particulates to form the nucleus around which water condenses before it gets heavy enough to fall as droplets. $\endgroup$ – feetwet Sep 9 at 12:49
  • $\begingroup$ @feetwet Yes, and there is a nice description of the sized of droplets in clouds and how raindrops are formed (around nuclei) their size which I will add to my answer. $\endgroup$ – Bob D Sep 9 at 12:54

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