or, equivalently:

This definition of relative humidity tells us that RH could change in two ways:

1. if the actual amount of water vapor in the air changes; and
2. if the maximum capacity of the air to hold water vapor changes.

Since the maximum capacity of air to hold water vapor changes when the temperature changes, it follows that relative humidity depends both on the amount of water vapor in the air and on the temperature of the air.

       (4) If unsaturated air were to cool, its maximum capacity to hold water vapor would decrease but the actual amount of water vapor in the air wouldn't change. However, if the cooling continued, the capacity of the air to hold water vapor would continue decreasing until it equaled the amount of vapor actually in the air, and the air would then (by definition) be saturated. The temperature at which this would happen is called the dew-point temperature.

(Note that at a particular pressure, the dew-point temperature--the temperature at which a parcel of air would become saturated if it were cooled--depends only on how much water vapor is actually in the air. The more water vapor there is in the air, the higher the dew-point temperature is.)

If saturated air cools further--below its dew point--then the air cannot hold as much vapor as it actually contains, and the excess vapor must condense to become liquid water.

However, water vapor requires a solid surface onto which to start condensing. In the atmosphere, microscopic, solid particles of dust, salt, ash, pollen, etc. suspended in the air provide solid surfaces onto which water vapor can start condensing. We call these tiny, solid particles condensation nuclei. Each becomes the center of a tiny droplet of liquid water. The aggregation of many, many of these tiny liquid water droplets suspended in the air constitutes a cloud.

(Note that if these liquid cloud droplets freeze, they become ice crystals, so a cloud may consist of an aggregate of enormous numbers of tiny ice crystals as well as liquid water droplets).

       (5) There are two important mechanisms (and several less important ones) by which air can cool below its dew point and thereby cause water vapor to condense in it to form a cloud:

1. One mechanism is conduction of heat into a colder underlying surface. For example, on a clear night, the ground may emit (longwave infrared) radiation much faster than it absorbs radiation emitted downward by the atmosphere and may therefore cool rapidly. If the ground temperature drops below the temperature of the air, then heat will conduct out of a layer of air next to the ground, cooling it. If it cools below its dew point and water vapor condenses, the result is a ground-hugging cloud that we call radiation fog. This is the kind of fog that can occupy the Central Valley of California and smaller inland valleys for days at a time during the winter, for example.

   Alternatively, air that moves from one place to another may move across a surface colder than itself, and begin losing heat by conduction into that colder underlying surface. For example, air moving eastward from the Pacific Ocean moves across a strip of water off of the California coast perhaps 50 to 100 miles wide that is considerably colder than the water of the open ocean (due to upwelling along the coast). The air in contact with the colder coastal water promptly loses heat by conduction and may cool below its dew point, causing water vapor to condense in it and producing another surface-hugging cloud that we call advection fog. This is the kind of fog that blankets San Francisco much of the summer, for example.

2. A more important mechanism by which air can cool below its dew point is called adiabatic cooling. When the pressure exerted on a parcel of air decreases (as happens, for example, when air rises), the parcel will expand. To expand, a parcel must push surrounding air out of the way. To push surrounding air out of the way requires energy, and the only form of energy available to the parcel is its own heat. Hence, when rising air expands as the pressure on it drops, it uses up heat energy to push away surrounding air to make room for its own expansion, and so its temperature drops. The parcel does not exchange heat with its environment by any mechanism during this process, even though the parcel cools.

   Rising, unsaturated air that cools adiabatically like this will cool by about 5.5 degrees F for every 1000 ft. that it rises, or about 10 degrees C for every kilometer it rises. Adiabatic cooling in rising air is the primary mechanism by which most clouds form.

   The converse of adiabatic cooling is adiabatic warming. When air sinks, the pressure on it increases and compresses it. The energy used by surrounding air to compress a sinking parcel is converted to heat in the parcel, and the parcel's temperature rises (by 5.5 degrees F for every 1000 ft. it sinks, or 10 degrees C for each kilometer it sinks). Of course, air that warms becomes capable of holding more water vapor, so clouds cannot form in sinking air, and any liquid water present in the air (as cloud droplets) would begin to evaporate. Hence, clouds are suppressed in sinking air.

   The phenomenon of adiabatic warming and cooling is the reason why meteorologists are so interested in where to expect air to rise or sink. Clouds (and possible precipitation) tend to form in rising air; clouds (and hence precipitation) are suppressed in sinking air.