Acrylic paints dry extremely fast, and that can be a severe handicap to those who plan to use acrylic paints in a climate such as mine, here in Arizona, where the humidity is low, and the temperatures are usually high.
There are several kinds of palettes for acrylics which help to keep them wet with water. Usually they have some sort of spongy material that you saturate with water, and this keeps the acrylic paint moist.
Also, I would paint with a brush in one hand, and a spray bottle of water in the other hand, to keep both my canvas, and my palette wet, continuously.
Acrylic paint not only dries fast, but it also dries darker than when applied, wet. This makes it extremely difficult to mix a color with fresh paint to match a previously dried color that may exist on the canvas. This means that if you wish to touch up a small area of a sky after your initial sky has dried, you may need to re-paint the whole sky. There are also ways to "feather" the fresh, touch-up into the existing sky, but it is extremely difficult to do, and make it blend with the existing, dried sky.
For some excellent techniques, you might try to watch Jerry Yarnell's TV program. He paints with acrylics, and offers many helpful hints on his show, while he is painting.