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You are watching: Which is not needed in air for water condensation to occur?

By the end of this section, you should be able to discuss why the idea that warm air holds more water vapor 보다 cold waiting is a fallacy, and discuss exactly how water drops thrive in terms of condensation rates and also evaporation rates.


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Have you ever been taught the "warm waiting holds an ext water vapor than cold air," or maybe heard it as soon as reading or the town hall a story around weather? If friend search roughly on the Web, friend can find plenty of sites that describe processes like cloud formation with the idea the cold wait can"t hold as lot water vapor as heat air. The explanations normally go something like this: "air cools to the point where it can"t host any more water vapor, and also liquid water fall form." But, don"t believe everything you check out on the Internet! This idea is clinical garbage, and also it poorly explains what"s yes, really happening once net condensation reasons liquid water droplets to form.


Motivating Myth: warmth air holds more water vapor 보다 cold air. Or alternatively, cold air deserve to hold less water vapor than heat air.


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Air isn"t prefer a hotel that write-ups a "No Vacancy" sign once it"s complete of water vapor.
Credit: No Vacancy / Taber Andrew Bain / CC by 2.0

For starters, let"s examine what accepting this myth yes, really implies. By agree this myth, we"re basically dealing with air favor a sponge, and also once all the pores in the sponge acquire filled with water, that can"t absorb any more water, therefore water starts dripping from the saturated sponge. But, air isn"t prefer a sponge. Waiting is likewise not choose a hotel, which write-ups a "No Vacancy" authorize when every one of its rooms room filled with water vapor. If these principles sound a small silly, it"s since they are!

What we contact "air" is really mainly empty space with small molecules flying roughly independently of every other. If we had actually a crate filled through air, the "air" molecule (oxygen, nitrogen, carbon dioxide, etc.) would certainly occupy a really tiny fraction of the space in the box, regardless of the temperature. In various other words, no matter what the temperature is, there"s always sufficient room for more water vapor molecules. So, the idea that cooler air doesn"t have sufficient room to hold much more water vapor molecule is nonsense!

So, why is the myth the "warm wait holds an ext water vapor 보다 cold air" therefore common? Well, it"s an "easy" explanation, and also sometimes persons (even those who should understand better) take unfortunate shortcuts. This particular myth seems to define the observation that net condensation (and the development of fluid water drops) more easily occurs at reduced temperatures. But, what"s really going on? Let"s explore.

From the recent conversation of condensation rates and also evaporation rates, you currently know what"s going on as soon as liquid water drops kind and grow -- network condensation is developing because the condensation rate is better than the evaporation rate. But, at greater temperatures, evaporation prices increase, and with enhanced evaporation rates, also higher condensation rates are compelled for network condensation come occur. As you know, greater condensation rates happen when the variety of water vapor molecules increases, so once the wait is warm, the high evaporation rates offer the potential because that a greater number of water molecules to stay in the vapor state without net condensation occurring. In various other words, when it"s warm, much more water vapor molecules are required in stimulate for liquid water drops to type and grow. Once the air is cooler, evaporation rates are decreased, definition that fewer water vapor molecule are forced for net condensation to occur.


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Why did "dew" (tiny fluid water drops) form on the bottom part of the steel cup, yet not on the top part? It"s gained nothing to do with some mythical holding volume for water vapor.

We deserve to use these principles to analysis what"s walking on in the photograph on the left, which reflects something the you"ve most likely observed prior to -- fluid water drops developing on the exterior of a glass comprise a cold beverage. This picture shows a steel cup partly filled v cold water. The bottom half of the cup (approximately) is coated v a great of little liquid water fall (often dubbed “dew”), while the top half is not. So, need to we think that somehow the air close to the bottom fifty percent of the cup can"t "hold" any an ext water vapor, which resulted in liquid water droplets to form on the side of the glass, while the wait just over can magically "hold" an ext water vapor (since no water autumn had created on the top component of the cup)? absolutely not!

Remember, evaporation and also condensation are occurring approximately you every the time, also if girlfriend can"t check out the results. Therefore, water molecules are impacting (condensing) and also leaving (evaporating) all over the surface ar of the cup, yet the rates of evaporation differ from the bottom fifty percent of the cup to the peak half. Recall that the cup is partly filled v cold water, which has made the bottom component of the cup fairly cold, and also in turn, a slim layer the air surrounding the bottom fifty percent of the cup cools as well.

Near the cold bottom fifty percent of the cup, water vapor molecule move much more slowly and also the price of evaporation is reduced. Once the air in call with the cup cools sufficient so the the price of evaporation is slightly much less than the price of condensation (net condensation occurs), fluid water drops kind and grow. Meanwhile, the top-half that the cup, and the thin layer that air automatically surrounding it, are warmer, leading to a greater rate that evaporation, and also the price of evaporation is greater than the price of condensation. In various other words, any type of microscopic water droplets that temporarily kind on the top half of the cup evaporate almost immediately (because network evaporation is occurring), resulting in the outside of the top-half that the cup to remain dry.

So, cooling the waiting (decreasing the temperature) is one means to achieve net condensation. If the wait cools sufficient (temperature reduce enough) the the evaporation rate becomes much less than the condensation rate, network condensation have the right to occur and also liquid water autumn can type and grow. Another way to attain net condensation is to rise the lot of water vapor molecules current (increase the dew point), which leads to a higher rate of condensation. If the lot of water vapor molecules increases enough (dew points increase enough) to make the condensation rate greater than the evaporation rate, then net condensation deserve to occur and liquid water drops can kind and grow.

However, in the atmosphere, the most common means for net condensation to occur (especially for processes like cloud formation) is come cool the air. For example, in theory, clouds kind when the air cools and the temperature drops to, and ever so slightly below, the dew point. Observations display that the relative humidity within clouds is normally slightly higher than 100 percent (say, 100.2 percent together a representative value), which way the condensation rate slightly over the evaporation rate. In a cloud that creates from rapidly rising air, the rate of condensation above the rate of evaporation because the rate of cooling is faster than the rate that water vapor is being eliminated from the air via condensation. In various other words, the evaporation rate decreases an ext quickly than the condensation rate (which declines as fluid water autumn grow and also fewer water molecules room in the vapor phase), resulting in the condensation price to exceed the evaporation rate (and leading to a family member humidity slightly greater than 100 percent).

The bottom line is the the expansion of fluid water droplets together "dew" top top the next of your drinking cup, on chisels of grass in the morning, or together cloud droplets (just together a few examples), relies on evaporation rates and condensation rates. Fluid water drops thrive when net condensation occurs and also not because the air just can"t "hold" any more water vapor. Remember, there"s always plenty of room in cold air for water vapor molecules.

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The real worry is that as the temperature that the wait decreases, water vapor molecules sluggish down and evaporation rates decrease do it feasible for condensation rates to exceed evaporation prices (if enough cooling occurs). But, in stimulate to attain net condensation in the genuine atmosphere, we need an additional ingredient. We"ll discover that ~ above the next page, and also discuss the as whole recipe for making clouds.