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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.

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  • $\begingroup$ Drops of water evaporate when in dry air, right? Clouds are (lots of) tiny drops of water, so dry air makes the cloud evaporate. (Heck, the word vapor is built into "evaporate".) $\endgroup$
    – RonJohn
    Jul 2 at 16:24
  • $\begingroup$ I am confused to why dry air (which I don't know the temperature of or does it even matter) cause evaporation on the clouds, I know how evaporation works, just wanted to know the reason of it. It doesn't (to me) make intuitive sense. $\endgroup$
    – Miah Tayen
    Jul 2 at 17:09
  • $\begingroup$ Matter, energy, pressure, etc all want to "equalize". For example, put a cold ice cube into a bowl of hot soup; what happens? Heat from the soup transfers into the water-which-is-an-ice-cube. The effect is that the hot soup cools down, and the ice warms to liquid. $\endgroup$
    – RonJohn
    Jul 2 at 17:14
  • $\begingroup$ Likewise, a balloon full of air. It's got lots of air molecules all bumping into one another (aka, it's high pressure). Open the hole, and the air rushes out; thus, the pressure in the balloon equalizes with the pressure outside. $\endgroup$
    – RonJohn
    Jul 2 at 17:17
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    $\begingroup$ en.wikipedia.org/wiki/Le_Chatelier%27s_principle "When any system at equilibrium [in our case, a cloud] for a long period of time is subjected to a change in concentration, temperature, volume, or pressure, [in our case, the addition of dry air] (1) the system changes to a new equilibrium [the air now is moister than what was the dry air, and dryer than what was the cloud], and (2) this change partly counteracts the applied change." $\endgroup$
    – RonJohn
    Jul 2 at 17:24
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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.

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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.

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  • $\begingroup$ So, to check my understanding, the low atmospheric pressure makes water droplets within the clouds easy to evaporate than on sea level, makes sense, Thanks. This is what I interpreted from your answer: Is it correct of me to think that the dry air, or any environmental things (sorry for using things) that interacts with the clouds are at a higher temperature than the clouds itself to evaporite it (not instantaneously). $\endgroup$
    – Miah Tayen
    Jul 2 at 17:03
  • $\begingroup$ Dry air can often be colder... especially if it's descending air. I think that unless you specify the type of cloud you are concerned with, and the meteorological conditions, you can't broad-brush a simple answer for all clouds that dissipate. $\endgroup$
    – f.thorpe
    Jul 2 at 19:01
  • $\begingroup$ Alright, thanks for the insight. In the case of cumulus clouds, what would happen if the dryer colder air is mixed with it. I am in the mind set that because it is cold air interacting with what I assume is warm clouds (compared to the dry air), condensation will occur, but it evaporites, am I still missing something?? Is there a pressure difference that is invoked by the dry air that causes evaporation? $\endgroup$
    – Miah Tayen
    Jul 2 at 19:48
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    $\begingroup$ I would ask "how do cumulus clouds dissipate" on stack exchange and generate more discussion. $\endgroup$
    – f.thorpe
    Jul 3 at 3:53
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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.

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  • $\begingroup$ Thanks, the dry air makes more room for extra water vapour. Does this also imply a pressure difference that is invoked by the dry air that causes the water droplets to convert to water vapour? $\endgroup$
    – Miah Tayen
    Jul 2 at 17:15
  • $\begingroup$ @DavidHammen - In the last paragraph can I make an edit and link to this wiki - en.wikipedia.org/wiki/Dry_line ? $\endgroup$
    – gansub
    Jul 4 at 7:17
  • $\begingroup$ @gansub Go ahead. You can also link to en.wikipedia.org/wiki/Supercell . A dry line is exactly what I was thinking of in that last paragraph. $\endgroup$ Jul 4 at 7:28
  • $\begingroup$ Thanks for the edit, @gansub. $\endgroup$ Jul 4 at 7:44
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(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.

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  • $\begingroup$ In your first line did you mean 0 K or 0 degrees C? $\endgroup$
    – Fred
    Jul 2 at 14:11
  • $\begingroup$ Absolute zero (no kinetic energy) is 0°K AFAIK. 0°C is freezing point of water. Amazingly ice can be colder than 0°C. $\endgroup$
    – U. Windl
    Jul 5 at 7:34
  • $\begingroup$ Thanks for clarifying that. Given that all the other temperatures in the question were in degrees Celsius I thought I'd ask to be sure. $\endgroup$
    – Fred
    Jul 5 at 9:59

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