There can be no rain without clouds. Rain is liquid water falling from high in the atmosphere, and the only place liquid water can exist at high altitudes is inside a cloud (1).
Precipitation Begins with Condensation
Even though they are incredibly massive, clouds are buoyant because they are filled with water vapor that is less dense than air (2). Clouds are also filled with tiny droplets of condensed liquid water, but these droplets can’t fall to the ground because they are constantly lifted by thermal updrafts. However, given enough time, these small droplets collide and coalesce, giving them the mass to fall through the updrafts (3). This is why not all clouds produce precipitation. For example, cirrus clouds never produce precipitation because they form in the highest reaches of the atmosphere and are made entirely of ice. There must be some liquid water to begin the process of coalescence.
Rain Cloud Types
The cloud types that are most likely to produce rain are cumulus and stratus, the lowest and warmest cloud types. Nimbostratus clouds are the flat, gray blanket clouds often seen on overcast days; they produce slow and steady precipitation, but seldom cause severe weather. Cumulus clouds are the fluffy, seemingly harmless clouds seen on sunny days. If enough thermal energy is present, Cumulus clouds will stack vertically into massive, anvil-shaped towers called cumulonimbus clouds. These are the clouds that produce lightening, extreme precipitation and high winds (4).
(1) “Precipitation.” NWS JetStream -. N.p., n.d. Web. 24 Jan. 2013. <http://www.srh.weather.gov/srh/jetstream/synoptic/precip.htm>.
(2) “Cloud Weight.” Cloud Weight. N.p., n.d. Web. 24 Jan. 2013. <http://www.newton.dep.anl.gov/askasci/wea00/wea00291.htm>.
(4) “National Weather Service Weather Forecast Office.” Cloud Classification. N.p., n.d. Web. 24 Jan. 2013. <http://www.crh.noaa.gov/lmk/?n=cloud_classification>.