This is an important point, but as a commenter down below has noted, this explanation alone actually produces a result in the wrong direction - better visibility when it is warm!
The haze you see is scattering of light, primarily from atmospheric aerosols. Things like sea salt, desert dust, sulphate droplets from burning fossil fuels. These scatter light as haze and reduce the visibility (some examples here).
As the relative humidity (RH) increases, these aerosol particles take up water and swell, making them larger. These larger particles are even better at scattering light, making the atmosphere hazier as the relative humidity increases (even if you have the same number of aerosol particles).
You can actually see this effect near clouds (where the humidity is very high) - (Koren et al, 2007)
However, if you keep the absolute (or 'specific') humidity the same (i.e. keep the same number of water molecules), the relative humidity increases as it gets colder. This means that following the explanation above, the visibility actually gets worse in colder weather (all else being equal)!
The full reason for clearer skies when it is colder comes from a variety of effects.
Cold conditions often follows a cold front and the rain from this front 'cleans' the atmosphere of aerosols, increasing visibility.
Several processes in the Earth system, particularly trees and other vegetation, produce more aerosol as it gets warmer (reducing visibility on a warmer day).
Warm air promotes atmospheric mixing and convection, which reduces visibility a bit.
On cold days, the wind may be blowing in cleaner air from a different direction, avoiding sources of aerosol in nearby regions.
These factors may not be true in all places. Some of the biggests and most damaging smogs in London were in winter, as local sources of aerosol particles (from buring fuel for heat) increased significantly in cold conditions.
Obviously clouds have an effect too, but that would be a much longer post!
AerodynamicMarrow t1_j2oz73b wrote
Reply to comment by uber_snotling in Does cold temperature make vistas more 'crisp' looking? by colorado_hick
This is an important point, but as a commenter down below has noted, this explanation alone actually produces a result in the wrong direction - better visibility when it is warm!
The haze you see is scattering of light, primarily from atmospheric aerosols. Things like sea salt, desert dust, sulphate droplets from burning fossil fuels. These scatter light as haze and reduce the visibility (some examples here).
As the relative humidity (RH) increases, these aerosol particles take up water and swell, making them larger. These larger particles are even better at scattering light, making the atmosphere hazier as the relative humidity increases (even if you have the same number of aerosol particles).
You can actually see this effect near clouds (where the humidity is very high) - (Koren et al, 2007)
However, if you keep the absolute (or 'specific') humidity the same (i.e. keep the same number of water molecules), the relative humidity increases as it gets colder. This means that following the explanation above, the visibility actually gets worse in colder weather (all else being equal)!
The full reason for clearer skies when it is colder comes from a variety of effects.
These factors may not be true in all places. Some of the biggests and most damaging smogs in London were in winter, as local sources of aerosol particles (from buring fuel for heat) increased significantly in cold conditions.
Obviously clouds have an effect too, but that would be a much longer post!