Why do clouds dissipate when mixing with dry air?

Question

So if dry air is just low humidity, meaning it has the capacity to hold more water vapour, it should make sense that the water droplets from the clouds will just evaporate and fill that dry air with the evaporated water vapour. Right?

However, why does the dry air cause the water droplets to evaporate in the first place, what provides the water molecules with enough energy to become a gas (water vapour)? Is there an intuitive way to understand this concept without any fancy equations?

From what I have read this process is called entrainment.

In advance, thank you for your answers, I have been trying understand cloud formation and this part really got to me, can't wrap my head around the reason why clouds would dissipate (or partially dissipate) when mixing with dryer air.

Answer 1

You may be assuming cloud droplets are in a "rest" state or some finite ordered state. Cloud droplets are actually growing/shrinking continuously as each molecule of water reacts to its environment and either condenses, evaporates, or remains in solution. Atmospheric pressure aloft is also low enough that the "boiling point" of water decreases, so it can exist as water vapor at lower temperatures than you might think.

There is plenty of "energy" in the atmosphere to allow microdroplets to evaporate: mechanical, heat, and radiant energy. In general you should think of suspended water droplets more like a chaotic system and less like a collection of droplets. Cloud droplets are never isolated or steady-state. Change in a cloud is being driven by the gradient of saturation, temperature, pressure, and the constant movement of the air. The droplets are also constantly impacting each other, which causes them to grow or break apart. As drier air becomes part of that environment, the water molecules at the edges of the droplets have less of a chance of remaining in solution.

Answer 2

Clouds form in areas where the relative humidity exceeds 100%. Add enough dry air to a cloud and the relative humidity drops below 100%, making the cloud dissipate. It's that simple.

Or almost that simple. Entrainment occurs because clouds are large. The dry air isn't instantaneously injected throughout the cloud. It instead erodes the cloud from the edges. The process of evaporation decreases the temperature thereby increasing the density. This in turn makes the cooled air mass sink, which allows more dry air to enter the cloud. The relative humidity decreases even more as the sinking cooled air warms.

Or perhaps not that simple at all. Dry air can at times create clouds, very nasty clouds, called supercells. In the North American Great Plains, a large mass of very dry air can be flowing eastward from the Rocky Mountains and meet with a large mass of very moist air flowing to the northwest from the Gulf of Mexico. Because dry air is more dense than than moist air, the dry air at the boundary between the two air masses can slip underneath the moist air mass. This buoys the moist air upward, thereby creating clouds. This phenomenon of a dew point difference between the dry and moist air is known as the Dry Line. If conditions are just right (or just wrong), those clouds can become very large and very tall. A Supercell can result in large hail, and at times, tornadoes.

Answer 3

Clouds are large masses of tiny liquid water droplets suspended in water vapor-saturated atmosphere (100% relative humidity). At a constant altitude and temperature, this system is in equilibrium: there's always some exchange of water molecules from the surface of these droplets into the surrounding saturated atmosphere and vice versa. Water droplets will not grow in size and neither will the size of the cloud; kinetic energy that exists at any gas/liquid interface controls this system and is fed by the sun (thermal energy) and wind (motion energy).

Once the liquid water droplets find themselves in an atmosphere that is now, say, 20% relative humidity, the water molecules that spall off the surfaces of the droplets are no longer replaced by nearby water molecules and the droplets become smaller until they disappear entirely. This happens from the cloud edges and progresses inward until the cloud disappears.

Answer 4

(English is not my first language, so be nice if the words are wrong)

Basically it's kinetic energy of the water molecules (above 0 K, absolute zero); that energy isn't evenly distributed, so a few molecules have higher energy than others.

Some have enough energy to "jump off", becoming an isolated "gas molecule", leaving the others behind with somewhat less kinetic energy (preservation of energy).

So that's how sweating works: You dry and cool down at the same time.

The mechanism also allows you to boil water; if it weren't like that then your water would be all 99.9°C hot, then suddenly all gone as vapor when it reached 100°C.

When vapor rises to higher atmosphere, the air is colder (typically) and the air can hold less water, so it's condensing (fog, clouds). If the process continues, drops of water may fall down. That does not mean they actually hit the ground as they may evaporate again.

Answer 5

The energy for the the evaporation of cloud droplet generally comes from the thermal energy present in the droplets themselves The reason some clouds dissipate only partially is several-fold
1: the thermal energy becomes depleted and is insufficient to produce further evaporation.
2: the cooler water droplet absorbs heat from the air, causing it to contract which changes the air flow.
3: the amount of water Vapor in the air increases to a point where water is replaced from water Vapor at a speed that balances out evaporation.