Outdoors, particularly when the play of diffused light and the movement of all the objects is continually felt, either through their own elasticity or because of the heat and light waves, this study is most necessary, if you would get the feeling of freedom, space, and air.
=Skies.=--In the painting of the sky there are several points to be kept in mind. The sky, even on the quietest day, is full of movement. Cloud masses change continually. If there are no clouds there is constant vibration in the blue; constant variety in the plane of color,--a throb of color sensation which is not to be expressed by a dead, flat tint.
Paint the sky loosely. Lay on the color as you will, with a broad, flat brush, or with a loose, smudgy handling; put it on with horizontal strokes, or with criss-cross touches, but never make it a lifeless tone. Have variety in it; keep a pulsation between the warm and cool color. You can work in the separate touches of half-mixed color, warm and cool, all through the sky, so that the whole tone will be flat and even, but not dense and dead. So far as the sky is concerned, the atmosphere is essential, and is to be represented not by dense color, but by free, loose, vibrating color.
=Clouds.=--If you have clouds to paint, do not draw them rigidly. Get the effect of the mass and movement, and the lightness of them. As they constantly change in form, any one form they may assume cannot be characteristic. The type form is what you must get, and the suggestion of the motion and lightness. You can suggest, too, the direction of the wind by the way they mass and sway and flow. The direction of the sun's rays, too, counts in the color of them. The outline of a cloud mass is never hard, never rigid. The pitch and luminosity and subtlety are what give you most of the effect of it.
Study the type of cloud, of course. It is a _cumulus_, _cirrus_, _stratus_, or what not. This character is important; but the character lies in the whole body of the cloud form, not in the accidental outlines or the special position of it for the moment.
=Sky Composition.=--The massing of cloud forms is a very useful factor in the composition of the landscape. The cloud bank or cloud line is capable of giving accent or balance to the picture. As it is not constant in position any more than in form, you can place it with truth to nature pretty nearly always where it will do the most good as an element in the composition. Make use of them, then, and study the forms and the possible phases of them so as to make the best use of them.