<p>In my recently updated answer to the question: <a href="https://earthscience.stackexchange.com/questions/18344/what-the-humidity-metric-is-hiding"><em>What the humidity metric is hiding?</em></a></p>

<p>I cite a news article, originally dated 31 January 2020 : <a href="https://www.abc.net.au/news/2020-01-31/hot-humid-weather-welcome-to-the-tropics/11917070" rel="noreferrer"><em>Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass</em></a>.</p>

<p>In the news article, meteorologists are quoted:</p>

<blockquote>
  <p>Dew point is the indicator of the amount of moisture in the air ... it's the temperature that the air needs to cool to in order to form cloud.</p>
</blockquote>

<p>The only time clouds form on the Earth's surface (in the form of fog) is when the temperature of the air in contact with the surface is same as the dew point, for the prevailing weather conditions. Generally, air temperature near the Earth's surface is higher than the dew point so no fog/clouds form.</p>

<p>Clouds form at altitude because the air temperature at that altitude is the same as the  dew point temperature. Thus the reason for a gap between the surface and the base of clouds.</p>

CC BY-SA 4.0
2020-02-08T16:30:00.023Z
My intention was that the explanation should cover the phenomenon on the other planets and Titan, not just Earth. It's difficult to imagine dew forming on Venus.
Michael Walsby
CC BY-SA 4.0
2020-02-09T18:40:56.317Z
@gansub: While I can't support this with a citation, my casual observation suggests that dew (or frost) forms on surfaces because they are colder than the air.  The surfaces might also have more nucleation sites: https://en.wikipedia.org/wiki/Nucleation
jamesqf
CC BY-SA 4.0
2020-02-10T04:13:29.070Z
@gansub: While I don't actually KNOW (I'm not a meteorologist), my guess is that it's a matter of temperature and condensation nuclei.   AFAIK dew doesn't form NEAR a surface, it forms ON it. Take a cold bottle of (your beverage of choice) out of a refrigerator.  If the warm air of the room is sufficiently humid, water - that is, dew - will condense on the cold surface.  (It's even more dramatic if you use say liquid nitrogen.) So if the ground becomes colder than the air, dew can form by water condensing on surfaces.
jamesqf
CC BY-SA 4.0
2020-02-10T18:00:53.397Z
@gansub: Not just grass.  Consider how your car's windshield (and often the rest of the car) will have dew on it.  I think the point is that, as with your beverage can, the moisture in the air is condensing directly on the surfaces, rather than forming droplets in the air.
jamesqf
CC BY-SA 4.0
2020-02-11T04:05:04.920Z
@gansub: Because the air isn't cold enough for moisture to condense?  Like when you blow your hot, moist breath on say the bathroom mirror, and it condenses there to form "dew", but not in the air.  But if you go out in cold enough weather, you can see the moisture in your breath condense into a fog.
jamesqf
CC BY-SA 4.0
2020-02-15T04:42:24.937Z
[“During the night, on a calm, clear night, the ground radiates the infrared radiation to space.” In other words, it sheds the warmth it absorbed from sunshine during the day.

The ground loses energy more efficiently — and so faster — than the air. It chills the air in contact with the ground.

“That’s what happens when dew forms or frost forms. The ground is so cold that it cools the air in contact with it” to a point where it cannot hold as much moisture.](https://ottawacitizen.com/news/local-news/science-of-winter-why-early-morning-sunshine-can-make-the-air-colder)
Keith McClary
CC BY-SA 4.0
2020-02-08T12:01:56.163Z
2020-02-08T15:44:00.830Z
2020-02-08T15:44:00.830Z
Fred
<p>On Earth there is a clear space between the ground and the cloud base. This phenomenon is not confined to Earth, it's the same on Venus, Mars and Titan (moon of Saturn). </p>

<p>Is there a law of physics which inhibits the formation of cloud at ground level? </p>

<p>Living in UK I know that ground fogs occur very occasionally, generally not lasting for more than a few hours, and for all we know it might be the same on the other planets, but it is not the normal state of affairs on the bodies mentioned above.</p>

CC BY-SA 4.0
2020-02-09T04:14:57.647Z
At not answering a basic question, which BTW has some of its facts wrong: clouds DO form at ground level, where they're known as fog.  And WRT fog on other planets, it's not uncommon on Mars https://www.planetary.org/blogs/guest-blogs/bill-dunford/the-mists-of-mars.html We have maybe an hour of Titan surface observations from one point.  And there's fog on Pluto: https://www.nasa.gov/feature/pluto-wows-in-spectacular-new-backlit-panorama
jamesqf
<atmosphere><clouds><planets>
Why is there a clear space between the ground and the cloud base?
Michael Walsby
# Why is there a clear space between the ground and the cloud base?

(Asked by: username_0 on 2020-02-08)

On Earth there is a clear space between the ground and the cloud base. This phenomenon is not confined to Earth, it's the same on Venus, Mars and Titan (moon of Saturn).

Is there a law of physics which inhibits the formation of cloud at ground level?

Living in UK I know that ground fogs occur very occasionally, generally not lasting for more than a few hours, and for all we know it might be the same on the other planets, but it is not the normal state of affairs on the bodies mentioned above.

**Tags:** atmosphere, clouds, planets

> **Comments:**
> **username_1**: At not answering a basic question, which BTW has some of its facts wrong: clouds DO form at ground level, where they're known as fog.  And WRT fog on other planets, it's not uncommon on Mars https://www.planetary.org/blogs/guest-blogs/bill-dunford/the-mists-of-mars.html We have maybe an hour of Titan surface observations from one point.  And there's fog on Pluto: https://www.nasa.gov/feature/pluto-wows-in-spectacular-new-backlit-panorama  

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## 1 answers

### Answer by username_2 on 2020-02-08. Score: 9

In my recently updated answer to the question: [*What the humidity metric is hiding?*](https://earthscience.stackexchange.com/questions/18344/what-the-humidity-metric-is-hiding)

I cite a news article, originally dated 31 January 2020 : [*Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass*](https://www.abc.net.au/news/2020-01-31/hot-humid-weather-welcome-to-the-tropics/11917070).

In the news article, meteorologists are quoted:

> Dew point is the indicator of the amount of moisture in the air ... it's the temperature that the air needs to cool to in order to form cloud.

The only time clouds form on the Earth's surface (in the form of fog) is when the temperature of the air in contact with the surface is same as the dew point, for the prevailing weather conditions. Generally, air temperature near the Earth's surface is higher than the dew point so no fog/clouds form.

Clouds form at altitude because the air temperature at that altitude is the same as the dew point temperature. Thus the reason for a gap between the surface and the base of clouds.

> **Comments:**
> **username_0**: My intention was that the explanation should cover the phenomenon on the other planets and Titan, not just Earth. It's difficult to imagine dew forming on Venus.  
> **username_1**: @gansub: While I can't support this with a citation, my casual observation suggests that dew (or frost) forms on surfaces because they are colder than the air.  The surfaces might also have more nucleation sites: https://en.wikipedia.org/wiki/Nucleation  
> **username_1**: @gansub: While I don't actually KNOW (I'm not a meteorologist), my guess is that it's a matter of temperature and condensation nuclei.   AFAIK dew doesn't form NEAR a surface, it forms ON it. Take a cold bottle of (your beverage of choice) out of a refrigerator.  If the warm air of the room is sufficiently humid, water - that is, dew - will condense on the cold surface.  (It's even more dramatic if you use say liquid nitrogen.) So if the ground becomes colder than the air, dew can form by water condensing on surfaces.  
> **username_1**: @gansub: Not just grass.  Consider how your car's windshield (and often the rest of the car) will have dew on it.  I think the point is that, as with your beverage can, the moisture in the air is condensing directly on the surfaces, rather than forming droplets in the air.  
> **username_1**: @gansub: Because the air isn't cold enough for moisture to condense?  Like when you blow your hot, moist breath on say the bathroom mirror, and it condenses there to form "dew", but not in the air.  But if you go out in cold enough weather, you can see the moisture in your breath condense into a fog.  
> **username_3**: [“During the night, on a calm, clear night, the ground radiates the infrared radiation to space.” In other words, it sheds the warmth it absorbed from sunshine during the day.  The ground loses energy more efficiently — and so faster — than the air. It chills the air in contact with the ground.  “That’s what happens when dew forms or frost forms. The ground is so cold that it cools the air in contact with it” to a point where it cannot hold as much moisture.](https://ottawacitizen.com/news/local-news/science-of-winter-why-early-morning-sunshine-can-make-the-air-colder)  

---

