[thedoc (OP)]

Chris Staines asked the Naked Scientists:
   
My wife and I were arguing about air clarity. We live in Cape Town, and sometimes the view across False Bay the mountains on the other side is so clear you can pick out every detail. My wife's reasoning is that the air is full of moisture, thus magnifying the detail in the distance. My reasoning is that the air is devoid of moisture, as otherwise this would cause a slight haze and the detail would be lost. My wife further thinks that the great air clarity is a portent of rain to come  - whereas I see no relation at all. Could you clear this up for us. Does great air clarity mean more or less moisture in the air?

What do you think?

[Colin2B]

Chris Staines asked the Naked Scientists:
   
My wife and I were arguing about air clarity. ....... Does great air clarity mean more or less moisture in the air?

What do you think?

Haze can be due to moisture, smoke or dust in the air, so air is clearer without these.
Clearer air doesn't have anything to do with rain to come. Look instead for high wispy clouds (jet stream cirrus) turning to lower strata clouds as a depression approaches. You can also look for cross wind effect.  Stand with your back directly into the wind, look up at the clouds, if they are moving from your right side (Southern Hemisphere) then the weather will normally deteriorate. If they move towards or away from you it will probably stay the same. From left should get better.
This is based on Buy Ballot's Law - if you are standing back to the wind your right arm (Southern H) will point towards low presure which is usually a depression.
The only thing I can think of that would have an effect would be a local condition say heat onto ground, hot air dragging moisture up into the air. In this case you would see Cumulus clouds in the sky, possibly becoming rain or thunder clouds, but the moisture going up should cause haze.
How about keeping a log and letting us know if you see a pattern?
A suggestion however, don't say anything to your wife. Do you really want to win this one?  []

[evan_au]

When the relative humidity reaches 100%, you have fog or rain, and this impairs visibility severely.

Provided the humidity is less than 90%, the main cause of poor visibility is probably dust or pollution. Capetown is on the west coast of Africa, so wind blowing in from the sea will carry less dust than wind blowing from the west or northwest.

If you live close to sea level, a strong surf or strong wind can cause a haze from salt spray.

[yor_on]

Could it be some sort of mirage effect? " Mirages are optical phenomena in which light rays are bent due to thermal variations in the refraction index of air, producing displaced or heavily distorted images of distant objects"  I'm not sure if you can use droplets aka atmospheric humidity for it though? Although, maybe you can in a way, as for example  " The superior mirage forms when cold air lies beneath relatively warmer air, a conditions known to meteorologists as a temperature inversion. In this condition, light rays refract, or bend, toward the colder (and denser) air -- that is, downward. This bending causes the image of the object to appear to us to be above its actual position because our brains assume the light rays have taken a straight path from the object to our eyes. The rate of increase of temperature with height (the temperature gradient) affects how the light rays travel from the object to our eyes and thus how we see the resulting image pattern. The superior mirage may cause the image (or parts thereof) of an object to appear"  http://www.islandnet.com/~see/weather/elements/supmrge.htm

And there are https://en.wikipedia.org/wiki/Fata_Morgana_%28mirage%29, as well as this wiki on superior and inferior mirage's https://en.wikipedia.org/wiki/Mirage#Inferior_mirage

[Pecos_Bill]

After a rain in California's central valley, you can see the mountains clearly...just as you could have done for centuries before 1850.

This is entirely unrelated to the relative humidity that day.

The rain has washed all the caca out of the air which had been, until today, steadily decreasing all your pulmonary functions. That caca is now in the watershed traveling down to the sea. Take a deep breath and enjoy it during the brief interlude before the caca level inevitably rises again.

Breathe deep while you sleep.