14 replies to this topic

[earlyriser]

I've noticed the sky often appears darker than normal after a storm system has passed through. I used to attribute this to the air having been cleansed of aerosols by the rain. But another thought occurred to me this morning as I stepped out onto my wet patio. Wet pavement is typically darker than dry pavement. Could the darker skies after rain be due to less light being reflected up from the ground? Snow cover certainly increases sky glow, so maybe wet surfaces after rain have the opposite effect. 

Edited by earlyriser, 19 January 2019 - 05:27 AM.

[Asbytec]

No idea. I do think frontal systems and rain tend to clear the air of aerosols. 

[jaraxx]

No reason the ground couldn't get less reflective and the air cleared of aerosols, pollen, dust, what have you at the same time. Darker ground from being wet would be a pretty short term effect in a lot of circumstances.

[Araguaia]

It gets darker and more transparent here after a hard rain in the rainy season, and the nearest pavement is 11 km away.

[earlyriser]

No reason the ground couldn't get less reflective and the air cleared of aerosols, pollen, dust, what have you at the same time. Darker ground from being wet would be a pretty short term effect in a lot of circumstances.

My guess is that both effects contribute to darker skies in urban areas. After an evening rain in Ohio, the dew point and temperature usually stay about equal until the sun comes up, so surfaces normally stay wet until morning. Could be different in a dryer climate. 

[Neptune]

We live in the Phoenix Valley and it's definitely darker after a rain.  There is usually a dome of smog that hangs in the air. After a strong wind, it blows it out pretty good and our skies are much more transparent and darker. This only lasts a short while as the smog dome inevitably returns.

[csrlice12]

Every drop of rain is nature's lens to the universe....and nature is jealous and hates astronomers.

[Astroman007]

An interesting thread with some interesting thoughts and observations.

 

I can't say that I have seen the same, though. But perhaps I was simply not making the connection.

[earlyriser]

An interesting thread with some interesting thoughts and observations.

 

I can't say that I have seen the same, though. But perhaps I was simply not making the connection.

It's certainly not an observation based on scientific data or anything like that. Just something to consider. I can say with reasonable certainty that snow cover significantly brightens the sky in my area. So, it seems the opposite should be true.

[airbleeder]

    I do think the atmosphere is sometimes more transparent after a good rain, but I also think that after seeing a couple nights of city and suburban light reflecting off the bottom of the clouds makes the sky seem darker too. It's like walking out to a sunny day after a week or weeks of rain. It's often painful to me.

[vdog]

All I know is that in a light-polluted area, those post-rain skies are like an oasis in the desert to a man dying of thirst.  I don't stop to analyze; I break out the scope and dive in!

[Asbytec]

It's conceivable the lack of aerosols also reduces light pollution scattered back to Earth. 

[Maurolico]

One must consider also how in daylight time it is easier to see in the distance when we are inside a rainy day than when we are inside a foggy day. The fog, or clouds, are materials so-called "optically thick", in which the light interact many time with the droplets before escaping in every directions (scattering), no matter from where the direct light come in. The size of the rain droplets is bigger than those of the fog/cloud so its extinction coefficient is lesser (inverse dipendence on the droplets' radius). The same happen in nightime during or after a rainfall because it is a propriety of the material (rain/fog/clouds) and of their geometric cross-sections per se, independent of lights of any sort.

Edited by Maurolico, 20 January 2019 - 06:58 AM.

[Maurolico]

It's certainly not an observation based on scientific data or anything like that. Just something to consider. I can say with reasonable certainty that snow cover significantly brightens the sky in my area. So, it seems the opposite should be true.

Snow has the higher surface albedo scattering. Even without city lights pollution it is able to diffuse Milky Way, afterglow and zodiacal light. So your observation is founded; but it is on a different side about the plain reasons for which the sky is more transparent after a rainfall (coagulation and destruction of tiny droplets) and more terse (putting down of the aerosol pollutants).

Edited by Maurolico, 20 January 2019 - 09:00 AM.

[Maurolico]

The mean surface albedo for asphalt is 0.04, for water ocean is 0.06. The surface albedo of the fresh snow is 0.8; you can see how in case of a large shallow flood formed on asphalt all around the observer after a huge rainfall, the diffusion of whichever light should be way lesser than in a large field covered with fresh snow.