Author's Notes: For an overview of ancient Maya gods and goddesses, go to this post here. Also, as mentioned before on other subjects within the topic of the ancient Maya, sources do not always add up; in this post I have combined things from various sources.
Labeled God B in the Schellhas classification system, Chac (or Chaac) is the Maya god most commonly known as the god of rain. He is also known by other names including Ah Tzenul, which translates as "he who gives food to others". Of the different deities, the ancient Maya depicted Chac the most often.
The appearance of Chac varies. Variable characteristics include scales, a big and curving nose, fangs or catfish whiskers. Some depictions have tears going down his face. Another version is of a blue man holding lightning or an axe. In some depictions, he is shown with symbols connected to the planet Venus or of God H.
It is currently understood that the ancient Maya thought Chac had four aspects, like the Pauahtuns (the gods who held up the sky at a cardinal point). Each aspect was connected to a cardinal direction and to a color.
Sac Xib Chac was the Chac of the north, whose color was white. The Chac of the east was Chac Xib Chac, and his color was red. Kan Xib Chac was the Chac of the south. Ek Xib Chac was the Chac of the west -- his color was black. Each of these Chacs were depicted as a man whose skin color was his designated color.
What exactly Chac controlled and how he controlled it tends to vary by source. Chac is thought to have been the god who controlled thunder, rain, lightning and wind. He was also thought to control fertility and was the patron of the number 13 -- a lucky number to the ancient Maya.
The ancient Maya thought that lightning and thunder occurred when Chac threw stone axes, which sources such as The Ancient Maya and Handbook to Life in the Ancient Maya World describe as smoky or fiery. Rain occurred when Chac poured out a gourd filled with water -- though another source states they thought that he poked his nose into clouds to cause rain to fall.
Power over rain was delegated: each of the four aspects of Chac had the power of bringing rain from their particular cardinal direction.
In relation to Chac being a god of rain, he was associated with frogs, who were his friends. Frogs were understood to croak before a storm started.
"South and Meso-American Mythology A to Z"; Ann Bingham, Jeremy Roberts; 2010
"Star Gods of the Maya: Astronomy in Art, Folklore, and Calendars"; Susan Milbrath; 2000
"Mythology: Myths, Legends and Fantasies"; Struik Publishers, Janet Parker, Alice Mills, Julie Stanton; 2007