Naked Science Forum
Life Sciences => The Environment => Topic started by: MarianaM on 08/10/2019 13:54:55
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Barbara is wondering...
Why do clouds stay high above the earth with flat bottoms as though they are sitting on something when their substance is heavier than all other gasses in our atmosphere?
Can you help?
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Water in its gas state ( steam) is actually less dense than air. When humidity goes up, this displaces some of the nitrogen and oxygen and thus as humidity of air goes up, its density goes down. Thus, humid, water laden air ( such as what clouds are formed from), would be expected to float above a less humid lower air layer.
When you think of water being "heavier" than air, you are comparing one thing in its liquid state to another in its gas state.
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Is a cloud gaseous water or condensed water?
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Is a cloud gaseous water or condensed water?
yes
A cloud is not an object in the traditional sense. It is a region of the atmosphere in which conditions are right for water to be in equilibrium between gas and liquid (and solid). The gas is lighter than air, and the liquid is heavier (but small droplets can remain suspended as an aerosol).
The boundary of the cloud is not a true edge: water is going in and out of the cloud all the time. But only within the region of the cloud is the aerosol stable. Any water outside of the cloud is either in the gaseous state, or is precipitations (solid or liquid).
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A cloud is a dynamic system. Atmospheric temperature decreases with altitude up to 70,000 ft or so. Flat-bottomed cumulus clouds are formed when convectively rising damp air reaches the dewpoint temperature, which is quite sharply defined. If you fly through the lower reaches of an active Cu, you first encounter sinking air around the edge, then rising air in the middle of the base. As the water condenses, it releases its latent heat of condensation, which increases the rate of rise, so the interior of the cloud consists of a very vigorous vortex, which can extend to the freezing altitude, at which point a further release occurs and you can get the characteristic anvil shape of a cumulonimbus.
The water droplets and ice particles are tiny, so although they are around 3000 times the density of dry air, they fall very slowly (large surface to volume ratio => high viscous drag) and may be held up by the circulating convection current until they coalesce into raindrops or snow, or disperse by evaporation.
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Is a cloud gaseous water or condensed water?
yes
The boundary of the cloud is not a true edge: water is going in and out of the cloud all the time. But only within the region of the cloud is the aerosol stable. Any water outside of the cloud is either in the gaseous state, or is precipitations (solid or liquid).
A good example of this is the lenticular cloud that can form on the leeward side of a mountain. The wind pushes humid air up the windward slope which condenses on the leeward side. The cloud formed remains in position with respect to the mountain, but the moisture forming it is constantly being removed by the wind, as the same wind brings in new moisture.
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Barbara is wondering...
Why do clouds stay high above the earth with flat bottoms as though they are sitting on something when their substance is heavier than all other gasses in our atmosphere?
Can you help?
As an interesting addition, rain forms from particles colliding into large droplets and falls, yet a good ammount of rain never reaches the ground as it is vapourised on the decent by heat and wind (energy) even though it is falling into areas of higher pressure (pressure lowers the dewpoint thus making air less capable of carrying water vapour )