Atmosphere is usually hazier near the horizon on a normal weather day. Color would be less noticeable near the horizon and cooler than anywhere else in the painting. The color would depend on what your scene has in it. We will look at some examples later.
Color is painted a little differently with every medium you use. An example of this would be an oil painting vs watercolor painting. When you paint an oil or acrylic painting and it dries your able to paint a lighter coat of paint over it to change the color. By using white paint mixed with a cool color your able to tone down the color already on the canvas. In watercolor you must plan for the white or light areas of the painting. Your always able to paint darker shades over the paint but to lighten it is almost always impossible without ruining the concept. It must be planned before-hand as to how your going to handle the values which recede from the horizon line.
Let's now assume we are going to paint in watercolor and we must plan out a landscape with mountains in the far distance, pine trees and water in the middle ground and water and rocks in the foreground. You would now plan your scene with pencil sketches. Thinking distance you would arrange size, value and color pertaining to what your plans are. Atmosphere, as I said before on a normal day, is always hazier and cooler on the horizon. So since blue is a cool color, it would be bluer and lighter at the horizon. Anything in the distance would also be smaller in size. As we precede forward things become gradually larger, brighter and sharper until we reach the foreground where the color should be the most distinct and richest in color saturation and objects the sharpest. In other wards the most emphasized. When painting the mountains in the background each level of mountains would be a different value. Then the pine trees would have levels of size and color. An example would be the green of the trees, which should be grayed to different levels with blue or violet. Green is a cool color but blue is always the cooler of the two. The water also would have changes, bluer in the background and greener with reflection colors as you come closer to the foreground.
Atmosphere can also be referred to as the mood of the day. It can be heightened by Mist or Fog. It changes when a day is "Gloomy" which would be dark-dismal and could be void of light altogether. On a "Rainy day" where atmosphere perspective not only effects the horizon but the whole painting. Its evident on the short distances or could have light effects on foreground roads, puddles and sidewalks with reflections from patches of sunlight. Even a "Snowy day" which could be dark and dismal or sunny. Atmosphere affects everything.
Painting on a dismal day would have very little change in value or color, no shadows would be present, as in a sunny or bright day.
Atmosphere plays a large part of our painting. If not approached correctly from the beginning of the painting it will have a flat two dimensional feeling. Surreal or unrealistic.