Question #a8da2

1 Answer
Jul 24, 2015

It isn't a mixture at all that is why there is a cloud.


Contrary to what many think the atmosphere does not readily mix. It is this fact that allows us to have weather in the first place.

The water content of an air parcel is going to depend on where the parcel has been. If the parcel moves (advection) over the Sahara then it is going to have very little water content. If it advects over an ocean or lake it is going to have a lot more water.

The water content that an air parcel can hold is dependent on temperature. Warm air can hold more water vapor. This is important because an individual parcel of air that rises in the atmosphere is going to cool, and therefore it's ability to hold water vapor will decrease. This is referred to as relative humidity or the percentage of water the air is holding compared to the maximum amount it can hold.

As the parcel of air continues to rise, at some point the temperature is going to drop to the point that it cannot even hold the water vapor that is in it. This is when it is at 100% relative humidity, and when condensation (water vapor turning into liquid water) occurs. Liquid water formed from condensation is clouds.

If mixture of air occurred, the humidity would be uniform throughout the atmosphere. We wouldn't have deserts and we wouldn't have thunderstorms (for example) because there would be equal rainfall everywhere and the atmospheric instability that results in thunderstorms would naturally stabilize itself. Also there would be no ice caps as warm air would mix with cold air making all the air the same temperature, and the average temperature of the planet is around 14 degrees C...well above freezing.

That all being said there is a small amount of mixing that occurs (I said it doesn't readily mix, not that it doesn't mix at all).

I noticed that this question is posted in chemistry so I may have misunderstood the question, if so please let me know so I can provide you with the answer you need.