Fog: How & Why Does It Form & Disappear?

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Discussion Overview

The discussion revolves around the formation and dissipation of fog, particularly focusing on its occurrence just before sunrise and the conditions that lead to its presence. Participants explore the physical processes involved, including temperature changes and humidity levels, as well as the differences between fog and clouds.

Discussion Character

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

Main Points Raised

  • One participant notes that fog forms when water vapor condenses into droplets due to cooler air temperatures at night, which cannot hold as much moisture.
  • Another participant questions why fog typically appears just before sunrise rather than during the middle of the night, indicating a need for further explanation.
  • Some suggest that local conditions, such as ground cooling and moisture from the ocean or plants, may contribute to fog formation.
  • A participant mentions that evaporation occurs with the rising sun, leading to mist as water transitions back to vapor.
  • There is a query about the differences between fog and clouds, with one participant stating that the primary distinction is altitude, while both consist of similar components.
  • One participant shares an observation about fog and clouds in mountainous regions, highlighting the dynamic nature of clouds compared to fog.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the specific timing of fog formation, particularly its occurrence just before sunrise. There are multiple viewpoints on the contributing factors to fog, and the discussion remains unresolved regarding the precise reasons for its timing.

Contextual Notes

Some assumptions about local climate conditions and the specific mechanisms of temperature and humidity interactions are not fully explored, leaving room for further investigation.

daniel_i_l
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Where I live, sometimes in the morning there's heavy fog just as the sun is coming up but after the sun somes up it all goes away. Why does it get foggy right before the sun comes up and why does the fog disappear shortly after?
Thanks.
 
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Water vapur is held in the air. Water vapour is invisible but when it condenses into water droplets like clouds or 'steam' from a kettle you can easily see it.
The amount of water vapour that air can hold depends on the temperature.
At night as the air is cool, all the water vapour cannot be held n the air and it condenses out into fog/clouds. As the air heats up these droplets evaporate back into vapur and are absorbed by the air - as vapour they are invisible.

The same amount off water (roughly) is present it's just that as widely spread individual water vapour molecules as a gas you can't see it, condensed into liquid water droplets in a fog it reflects and scatters light so you can.
 
Thanks for that explanation.
But how does that explain the fact that the fog starts just before (+-1 hour) the sun rises and not in the middle of the night?
Thanks.
 
Depending where you are, it can be the ground cooling down to draw wet air in from the ocean or mountains, or locallly it can be water from ground plants/dew.

You do get fog in the middle of the night - you just don't tend to notice it (unless driving!)
 
It's generally cooler at night than in the morning, so the liquid water settles to the ground as dew. With the sun up, evaporation starts. A lot of the water changes back and forth between the liquid and vapour states as it ascends, which is why you see mist rising from the ground in the morning.
 
daniel_i_l said:
Thanks for that explanation.
But how does that explain the fact that the fog starts just before (+-1 hour) the sun rises and not in the middle of the night?
Thanks.
It is coldest outside just before sunrise.
 
How does fog differ from a cloud?

Can a cloud exist above a substantial fog bank?
 
The only difference is in height above the ground. Both are composed of water droplets, water vapour, and sometimes ice crystals suspended in air. A lot of clouds are far more dynamic than fog simply because of the air currents at higher altitudes. Clouds will generally be colder as well, also due to altitude.

edit: I don't know where you guys live. What's really cool here is to see a couple of mountains in the Rockies engulfed in clouds, then visit one of the towns up there and find it foggy from their perspective.
 
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