•     First, lets take a look at the size of a typical cloud droplet in comparison to a typical rain droplet:

BUT WAIT!  Even cloud droplets need to start somewhere right?
 

It all begins with water vapor.  As you may already know, moisture is all around us in the atmosphere in the form of water vapor.  You cannot see water vapor, it is the gaseous form of water and is one of the most variable gases in the atmosphere.  In Tropical areas it can account for up to 4% of the atmosphere (by volume) and in much drier areas, such as deserts, it can account for much less than 1% of the atmosphere.

Water vapor requires special conditions in order to form a cloud droplet of liquid water:
 
 

•  Supersaturation is one condition, but is not an easy condition to meet.  Supersaturation is defined in Meteorology as:

"The condition existing in a given portion of the atmosphere (or other space) when the relative humidity is greater than 100%, that is to say, when it contains more water vapor than is needed to produce saturation with respect to a plane surface of pure water or pure ice".
 
 
 

• The predominant, and more effective way to form a cloud droplet involves a tiny particle or aerosol, known as a condensation nucleus.      Condensation nuclei can be made of natural materials such as dust or volcanic ash, or can be made of man-made pollutants such as smoke particles.

• There are as many as 1000 condensation nuclei in one cubic centimeter of air!

 

Diagram made based on Lutgens 1992.

• These cloud condensation nuclei (CCN) are very important and very effective, because they serve as surfaces on which water vapor can condense.

As air from the surface of the earth rises, it cools through a process called Adiabatic expansion.  Adiabatic expansion occurs when a parcel or large volume of air is heated at the earth's surface so that it becomes lighter and less dense than the surrounding air.  Since the air is now less dense than its surroundings, it is able to rise up, as long as surrounding air is cooler and more dense, to heights of several thousand to tens of thousands of feet above the earth's surface.  Air near the surface is more dense than air higher up because gravity holds most of the molecules and particles of air in the atmosphere down near the surface.  [This also explains why the atmospheric pressure is greater near the surface; because there is a more air above and yes, air does have a mass and therefore weight to contribute to pressure!]  Now as this air rises up into the atmosphere where the pressure is less and the air is less dense, there are much fewer particles of air moving around and bouncing off from each other.  [When particles of air move around and bounce off each other, they are said to exchange kinetic energy, or "energy of motion".  This exchange of kinetic energy is what releases heat and causes the sensible temperature (that you can feel) of the air to be warmer.  Since the effect of gravity is less at higher altitudes, the air is able to expand and move more freely where there are not as many particles to collide with.  The air then cools because there are not as many collisions and exchanges of kinetic energy.  If the air is not yet at 100% relative humidity, it will cool at the dry adiabatic lapse rate of 18 degrees F per 3280 feet.  If the air is saturated (at 100% relative humidity) it will cool at the moist adiabatic lapse rate of around 9 degrees F per 3280 feet.  When the air cools, its ability to hold moisture decreases and the water vapor is condensed out.  That is to say warm air holds more water vapor than cold air.
 
 

Picture a hot "muggy" summer day and a cold, snowy winter day:  each one of these days may have a relative humidity of close to 100% but because the summer day is warmer, the air contains much more moisture and you can feel the wetness in the air.  A snowy winter day, on the other hand feels drier because the cold air holds much less moisture.  100% Relative Humidity simply means that the air is holding its maximum amount of moisture for a particular temperature.

Therefore as the air becomes colder, water vapor must condense into liquid water because the air cannot hold as much water vapor as it could just moments before when it was at a higher temperature.  The water vapor molecules condense on the surface of the CCN until the air is in equilibrium and there are no more water vapor molecules available for condensation without making the air surrounding the drop too dry.  Before you know it, a cloud droplet has formed.  In fact, many billions of cloud droplets can form in a matter of minutes causing a large cloud to form.