The three sources of light outdoors are the primary light source of the sun, the secondary light source of ambient or sky light and the tertiary source of reflected light. Painting outdoors people tend to pay attention to the primary light source in a scene and maybe some reflected light but rarely do people include all three sources. If you spend a little more time observing before painting, you will see the effects of all three giving your paintings that sense of light that we all seek as painters.
These three light sources really affect the illusion of form and that is what makes their inclusion so important to a painting. The aspects of light are its direction, its hue and its strength. Because of their relative strength compared to the sun, reflected light and ambient light have their greatest affect on shadows. Painting them correctly requires sensitive observation and an understanding of the form principle. This principle uses the abstract concept of geometric forms as the bases for any complex object. The idea is once you understand how light affects these simple geometric abstractions you can use that information to help you see the affect in the more intricate elements in nature.
The first thing to look for is the direction of the light source. Ambient light or sky light is easy since it comes from the sky. Reflected light is a little trickier so remember this idea
Any object that has light falling on it becomes a source of light itself
This will help you to first look for reflected light within shadows or other areas and determine the direction of the light. Once this is understood you can paint the objects planes that are facing the source.