Haze is really frustrating. This is not just my personal opinion, but having read countless forum posts about how to reduce haze in landscape photography, it seems to be the consensus. In this context haze refers to atmospheric distortion that affects the image quality of pictures. Haze can refer to different things such as smog or fog, but here the main issue is haze created by a phenomenon known as Rayleigh scattering. Rayleigh scattering (this is a basic, uneducated explanation) occurs when light passes through the atmosphere and scatters when it interacts with particles of gas, water vapour, dust, etc. Rayleigh scattering particularly affects light with a shorter wavelength (i.e. blue light). This scattering gives the sky its blue colour as blue light passing through the atmosphere is scattered whilst other parts of the light spectrum pass with less obstruction. For a proper scientific explanation of Rayleigh scattering check the video below.
Unfortunately Rayleigh scattering doesn't just give us a nice blue sky, it also creates haze in landscape photos. Landscape subjects such as mountains, valleys, rivers, canyons, etc. are typically photographed at a distance; they are too big to do otherwise. This distance introduces scattering as the distance between camera and subject introduces more atmosphere for the light to pass through. Added to this is the changes in the quality of the atmosphere such as dust and water vapour due to weather patterns, bodies of water, time of day, etc. In my experience the most pernicious of these influences is vegetation. Vegetation can influence the water vapour content through dew, but in Australia this is particularly bad because of eucalyptus trees (the most common of trees in Australia). Eucalypts disperse eucalyptus oil into the air which further increases Rayleigh scattering. The Blue Mountains, located on the eastern seaboard, are named after their blue colour created in large part by the large volumes of eucalyptus trees.
Here is a picture I took a while ago at the Blue Mountains. Notice the blue cast, but also the strong haze effects. This picture shows off the three main problems with haze 1) a lost of detail 2) a loss of contrast and 3) a colour tint (usually blue). Haze is a common problem for landscape photographers around the world, but here in Australia we have it particularly bad due to the abundance of eucalyptus trees. There are some methods to get around haze which usually involve manipulating colours to get rid of the blue tint or increasing the contrast. These solutions are far from ideal and can create images that look unnatural or garish. Here's the last image with Photoshop's dehaze slider set to +40. Not a great result.
A more perfect solution to this problem comes with infrared photography. Referring back to Raleigh scattering the effects are more severe the shorter the wavelengths of light. Near-infrared (700nm-~1000nm) are the longest wavelengths of light that a digital camera's sensor can detect and are least afflicted by Rayleigh scattering. Getting a camera to shoot infrared is a bit complicated to explain but here it should be sufficient to demonstrate how much difference shooting in infrared makes on haze.
To compare I have two cameras, the Fuji XT-1 and the Fuji XE-1. The XT-1 is a normal visible light camera whilst the XE-1 has been converted to shoot infrared (720nm). Conveniently for this comparison both cameras use the same 16.3 megapixel X-Trans II sensor for an apples-to-apples comparison. Added to this each picture uses the same lens with the same exposure settings of the same scene (taken from a tripod and aligned properly in Photoshop). The only post processing done was to lower the highlights in the visible light pictures and doing a black & white conversion for the infrared pictures.
As you can see the haze present in the visible light pictures is effectively eliminated when the same shot is taken in infrared. This results in an astounding difference in fidelity of the images.
Cropping in on the picture taken with the 135mm lens furthers illustrates the point. The top image is clear, clean and well defined whilst the one of the bottom is rather ugly and milky.
Clearly infrared has its advantages. These pictures, of a fairly common sort of landscape subject, exhibit haze at levels that is typical for the region (trust me it doesn't get better). Taken in the visible spectrum the pictures are pretty ghastly and even with some time spent cleaning them up, they would only approach the 'ok' territory. The infrared images on the other hand show how much better the scenery looks when the haze is removed. Using infrared sacrifices colour (unless you opt for false colour infrared which is another story) and introduces some of the unique characteristics of infrared like the glowing white foliage. That is a trade off I am more than willing to make given just how bad visible light cameras perform under these conditions.
This is the reason I got into infrared photography in the first place. Australia has some great vistas that look pretty bad when photographed. The Canberra region (where I live) is overlooked by the northern stretches of the Australian Alps and these particular photos were taken at the foothills of said alps. They are beautiful to behold, but only infrared can really do them justice.