Acrylics are often the fresh air medium I take back with me into the studio. Vibrant, light weight and easy to work off of a small palette, I find that sometimes just one excellent brush works for the whole image.
Staying quick can affect the energy of the painting.
The painting below is 12" x 12", painted in the studio. A favorite view of mine, it is the northern end of Nantasket Beach in Hull, seen from R Street.