On Mon, 18 Mar 2019 20:27:27 -0700 (PDT), JTEM is Remarkably Flexible
Oooh, I hope so.
The Science Guys
Why does ice form on the wings of airplanes?
At high altitudes, air can be extremely cold, and be well below the normal
freezing point of water, 32 degrees Fahrenheit. Air normally contains a
certain amount of water vapor. AS THE TEMPERATURE DECREASES THE AMOUNT OF
WATER VAPOR INCREASES TO THE POINT OF SATURATION.
That DOES sound counter-intuitive, but hey, THEY are the scientists... WE
are not. But wait, there's more. (see below)
Post by JTEM is Remarkably Flexible
Do this on a cold day, compare the results.
Well, duh! That's just a non-reaction because the air is colder than the
can, but that does not MEASURE relative humidity. You could have 100%
relative humidity in "cool" air, but again, HOW cool or...
HOW cold? Colder than the can? Colder than 32.000000°F? (for Kymberly)
Water vapour CAN remain in air colder than freezing. (see above)
I am NOT a scientist (go ahead, all you other NOT-a-scientist-eithers...
all in unison say, "You got that right! What about YOU?)... are you? Is it
YOUR opinion that "cooler" air can't hold as much water vapour as "warm"?
Pun intended here, but that's all relative. What, exactly do YOU mean by
You can have 100% humidity in "cooler" air (but again, just what
constitutes "cooler" air? Cooler than what? The can? DUH!).
THAT the can might not get wet if it is warmer than the air, doesn't
matter... there can STILL be water vapour in the air, and as much as
You're comparing a cold can with "cold" air... not cooler air versus
Apples and oranges.
It's about relative humidity and dew points. Some say "colder" air can't
hold as much water vapour, but WHAT temperature are we talking about? No
one has been specific yet... not in that regard, anyways.
NO ONE has given any EXACT temps. Air temperature of 33°F can hold JUST as
much water vapour as an air temp of 100°F. Are you even bothering to read
Post by JTEM is Remarkably Flexible
Warm air holds more moisture than cold air. Period.
Take it up with the people who write you're wrong.
I see both arguments on the Internet (sorry, I canceled my subscription to
Science Nerds Quarterly)... so, who IS right?
Net Condensation: Myth and Reality
Evaporation Rates, Condensation Rates, and Relative Humidity
A Recipe for Making Clouds
By the end of this section, you should be able to discuss why the idea
that warm air holds more water vapor than cold air IS A FALLACY, and
discuss how water drops grow in terms of condensation rates and
Have you ever been taught that "warm air holds more water vapor than cold
air," or perhaps heard it when reading or watching a story about weather?
If you search around on the Web, you can find plenty of sites that explain
processes like cloud formation with the idea that cold air can't hold as
much water vapor as warm air. The explanations usually go something like
this: "air cools to the point where it can't hold any more water vapor,
and liquid water drops form." But, don't believe everything you read on
the Internet! This idea is scientific garbage, and it poorly describes
what's really happening when net condensation causes liquid water droplets
Motivating Myth: Warm air holds more water vapor than cold air. Or
alternatively, cold air can hold less water vapor than warm air.
Photograph of a "No Vacancy" sign
Air isn't like a hotel that posts a "No Vacancy" sign when it's full of
Credit: No Vacancy / Taber Andrew Bain / CC BY 2.0
For starters, let's examine what accepting this myth really implies. By
accepting this myth, we're basically treating air like a sponge, and once
all the pores in the sponge get filled with water, it can't absorb any
more water, so water starts dripping from the saturated sponge. But, air
isn't like a sponge. Air is also not like a hotel, which posts a "No
Vacancy" sign when all of its rooms are filled with water vapor. If these
ideas sound a little silly, it's because they are!
What we call "air" is really mostly empty space with tiny molecules flying
around independently of each other. If we had a box filled with air, the
"air" molecules (oxygen, nitrogen, carbon dioxide, etc.) would occupy a
really tiny fraction of the space in the box, regardless of the
temperature. In other words, no matter what the temperature is, there's
always enough room for more water vapor molecules. So, the idea that
colder air doesn't have enough room to hold more water vapor molecules is
So, why is the myth that "warm air holds more water vapor than cold air"
so common? Well, it's an "easy" explanation, and sometimes folks (even
those who should know better) take unfortunate shortcuts. This particular
myth seems to explain the observation that net condensation (and the
formation of liquid water drops) more easily occurs at lower temperatures.
But, what's really going on? Let's explore.
From the recent discussion of condensation rates and evaporation rates,
you already know what's going on when liquid water drops form and grow --
net condensation is occurring because the condensation rate is greater
than the evaporation rate. But, at higher temperatures, evaporation rates
increase, and with increased evaporation rates, even higher condensation
rates are required for net condensation to occur. As you know, higher
condensation rates occur when the number of water vapor molecules
increases, so when the air is warm, the high evaporation rates give the
potential for a higher number of water molecules to remain in the vapor
state without net condensation occurring. In other words, when it's warm,
more water vapor molecules are needed in order for liquid water drops to
form and grow. When the air is cooler, evaporation rates are decreased,
meaning that fewer water vapor molecules are required for net condensation
A photograph of a cup with liquid water drops covering part of the
Why did "dew" (tiny liquid water drops) form on the bottom part of the
metal cup, but not on the top part? It's got nothing to do with some
mythical holding capacity for water vapor.
Credit: David Babb
We can use these ideas to analyze what's going on in the photograph on the
left, which shows something that you've probably observed before -- liquid
water drops forming on the outside of a glass containing a cold beverage.
This photograph shows a metal cup partially filled with cold water. The
bottom half of the cup (approximately) is coated with a layer of small
liquid water drops (often called "dew"), while the top half is not. So,
should we believe that somehow the air near the bottom half of the cup
can't "hold" any more water vapor, which caused liquid water droplets to
form on the side of the glass, while the air just above can magically
"hold" more water vapor (since no water drops had formed on the top part
of the cup)? Absolutely not!
Remember, evaporation and condensation are occurring around you all the
time, even if you can't see the results. Therefore, water molecules are
impacting (condensing) and leaving (evaporating) all over the surface of
the cup, but the rates of evaporation differ from the bottom half of the
cup to the top half. Recall that the cup is partially filled with cold
water, which has made the bottom part of the cup relatively cold, and in
turn, a thin layer of air surrounding the bottom half of the cup cools as
Near the cold bottom half of the cup, water vapor molecules move more
slowly and the rate of evaporation is reduced. When the air in contact
with the cup cools enough so that the rate of evaporation is slightly less
than the rate of condensation (net condensation occurs), liquid water
drops form and grow. Meanwhile, the top-half of the cup, and the thin
layer of air immediately surrounding it, are warmer, leading to a higher
rate of evaporation, and the rate of evaporation is greater than the rate
of condensation. In other words, any microscopic water droplets that
temporarily form on the top half of the cup evaporate almost immediately
(because net evaporation is occurring), causing the outside of the top-
half of the cup to remain dry.
So, cooling the air (decreasing its temperature) is one way to achieve net
condensation. If the air cools enough (temperature decreases enough) that
the evaporation rate becomes less than the condensation rate, net
condensation can occur and liquid water drops can form and grow. Another
way to achieve net condensation is to increase the amount of water vapor
molecules present (increase the dew point), which leads to a greater rate
of condensation. If the amount of water vapor molecules increases enough
(dew points increase enough) to make the condensation rate greater than
the evaporation rate, then net condensation can occur and liquid water
drops can form and grow.
However, in the atmosphere, the most common way for net condensation to
occur (especially for processes like cloud formation) is to cool the air.
For example, in theory, clouds form when the air cools and the temperature
drops to, and ever so slightly below, the dew point. Observations show
that the relative humidity inside clouds is usually slightly greater than
100 percent (say, 100.2 percent as a representative value), which means
the condensation rate slightly exceeds the evaporation rate. In a cloud
that forms from rapidly rising air, the rate of condensation exceeds the
rate of evaporation because the rate of cooling is faster than the rate
that water vapor is being removed from the air via condensation. In other
words, the evaporation rate decreases more quickly than the condensation
rate (which declines as liquid water drops grow and fewer water molecules
are in the vapor phase), causing the condensation rate to exceed the
evaporation rate (and resulting in a relative humidity slightly higher
than 100 percent).
The bottom line is that the growth of liquid water droplets as "dew" on
the side of your drinking cup, on blades of grass in the morning, or as
cloud droplets (just as a few examples), depends on evaporation rates and
condensation rates. Liquid water drops grow when net condensation occurs
and 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.
The real issue is that as the temperature of the air decreases, water
vapor molecules slow down and evaporation rates decrease making it
possible for condensation rates to exceed evaporation rates (if enough
cooling occurs). But, in order to achieve net condensation in the real
atmosphere, we need another ingredient. We'll explore that on the next
page, as well as discuss the overall recipe for making clouds.
"It's all about money in the end. Keeping the Gravy Train running."