Key Difference: American Meteorological Society glossary defines a ‘cloud’ as "A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface"; while fog is defined as "water droplets suspended in the atmosphere in the vicinity of the earth's surface that affect visibility.”
‘Fog’ and ‘Clouds’ are two of many weather phenomena that happen on Earth, that form under the right conditions. These two are similar, though have slight differences in the sense of formation and location. Let’s look at each term separately.
The American Meteorological Society glossary defines a ‘cloud’ as "A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface". The suspended particles in the atmosphere are also known as aerosols. Two processes, possible acting together, could produce the right conditions for the formation of a cloud. These processes could cause the air to become saturated; cooling the air or adding water vapor to the air. Clouds are formed because of condensation of water vapor present in air. Condensation is the process in which water vapor is converted into liquid water. There are many types of clouds such as cirrus, stratus, cumulus, and nimbus to name a few.
Clouds play an important part in the water cycle on the planet, as it provides us water in the form of precipitation. Water evaporates from the ground because of heat, which then condenses to form clouds that provide water back on earth in form of rain, snow, hail, etc. Though clouds are not always visible, it does not mean that water is not present in the atmosphere; at any given time, water in the form of water vapor and small droplets are present in the atmosphere though they cannot be seen. When these particles mix with dust, salt, smoke and other chemicals or gases, they grow in size and form into clouds. Water droplets in the air can vary from 10 micros to 5mm in size.
According to the international cloud classification system, clouds can be classified into cumulus, stratus or cirrus forms. Prefixes are used in connection with clouds: strato- for low clouds with limited convection that form mostly in layers, nimbo- for thick layered clouds that can produce moderate to heavy precipitation, alto- for middle clouds, and cirro- for high clouds.
Condensation is not only limited to appearing high up in the atmosphere, it can also happen on ground level, though this is not known as cloud, more as ‘Fog’. The American Meteorological Society glossary defines fog as "water droplets suspended in the atmosphere in the vicinity of the earth's surface that affect visibility". Have you noticed that fogs generally form in areas where humidity meets with cooler temperatures? Well, that how they develop. Air that is filled with moisture, or is humid, comes in contact with cooler temperatures on the ground, which then condenses and forms into fog.
Another manner in which fog can form is when warm air moves over a cooler surface creates a foggy atmosphere that affects visibility. Different types of fog include Advection fog, Radiation fog, Steam fog, Precipitation fog, Upslope fog and Valley fog. Advection fog forms when hot air moves over a cooler surface, while radiation fog forms on clear, cool nights. Steam fog forms over bodies of water often in cooler seasons such as autumn. Precipitation fog forms when rain or snow falls, while upslope fog develop on large mountains and hills and valley fog forms in mountain valleys during winter and can be more than 1,500 feet thick.
In a nutshell, fog and clouds are generally the same thing; they form from the same process and under the same conditions. The only difference between a fog and a cloud is the location, whereas fog can form on ground level, clouds form in the atmosphere.