Some believe it is "God" crying.... scientists will tell you that it is caused by when particles of water or ice are large enough to reach the ground. The chief difference between a cloud drop and a rain drop is size. A typical rain drop has a volume that is more than a million times that of a cloud drop. Thus it takes many cloud droplets to make up a single raindrop. Raindrops can be produced by the collision and merging of cloud droplets. Collisions take place because the terminal velocity of a water drop increases as its size increases, over the normal size range of cloud droplets and raindrops. Large droplets fall faster then collide with and merge with smaller ones. When two rain droplets merge, "coalescence" has taken place. As a result of coalescence, the large drops can grow fairly rapidly. Raindrops are also produced by the melting of ice crystals, snowflakes, and other frozen particles. When ice crystals exist in the presence of "supercooled" water droplets in subfreezing air, the crystals grow as the droplets evaporate. There is a pressure force driving the water molecules from the water to the ice, resulting in a rapid growth of ice crystals in the presence of liquid cloud droplets. As ice crystals grow, the heavier ones fall. As a result, collisions and merging occur. A snowflake can be made up of a group of crystals stuck together. When such a particle falls through a layer of air whose temperature is above freezing, the crystals melt and raindrops are produced. In mountainous areas during the winter, valley locations often experience rain while snow falls at higher elevations. I leave it to you to decide which is the more likely...

G'day TedTed, Thank you for your question. The scientific name for the formation and falling of rain is called the Bergeron process. According to Wikipedia, "The equilibrium vapor pressure over water is greater than that over ice and therefore in a cold cloud, the water will be out of vapor pressure equilibrium and will evaporate to reach equilibrium. The ice will condense this vapor and grow into a larger ice crystal.Eventually this ice crystal will grow large enough to fall." I have attached sources for your reference. Reference Wikipedia Rain http://en.wikipedia.org/wiki/Rain Wikipedia Bergeron process http://en.wikipedia.org/wiki/Bergeron_process Weather Whiz Kids http://www.weatherwizkids.com/Rain.htm Ask an Earth Scientist http://www.soest.hawaii.edu/GG/ASK/rain.html

water evaporates from the sea and rises up to the hills and when it's cold enough it will form a cloud and condensation will accure. The wind will push the cloud to different places, when it cant hold any more it lets the water out. That's what causes rain.