Posts Tagged ‘fog’

Why does humid air rise?

How can the addition of water vapor make it lighter? When I take a dishrag and wet it, it gets heavier; how can I take a parcel of air, add water, and thereby make it lighter? This is all nonsense!

(I suppose this lesson should have come before the one about phase change, but better late than never.)

Yes, if you add liquid water to a parcel of air it will be heavier, but humidity is not about liquid water; it is about water vapor. We confuse the two because a fog is obviously heavy and since the droplets in fog are so small as to be invisible, we think of them as vapor. This is an understandable mistake; we can’t see fog droplets and we understand that the air is saturated in a fog, so we think that the fog is the vapor. Besides, in poetry, fog is called vapor.

But for scientific purposes, (and in sci-speak) vapor has a specific meaning; and fog is not vapor.

Fog and cloud droplets, for all that they are very small, are enormous compared to a molecule of water – and true water vapor means water floating in molecular form. Fog forms when saturated air cools and some of the water condenses. The remaining air is still saturated, and would be so even if you took away all the water droplets. The saturation is about vapor, not about droplets, however tiny.

How big is a water molecule?

Can a water molecule really be much smaller than a cloud droplet?

If the water droplets of an evaporating fog are a few 1/100ths of a millimeter wide, they are still a good five orders of magnitude larger than a molecule of water which is a few Ångstroms wide. Five orders of magnitude… That means like the difference between a softball and a city; the difference between a single grain of dandelion pollen and your kitchen table; between a beetle and a freight train, between a kitten and a thunderhead. The difference is simply enormous.

Look closely at the steam coming out of your teapot. Notice that right by the spout, the steam is invisible. An inch or two away, it is white. Right out of the spout, it is vapor; a short distance away, it has condensed into tiny droplets.


Now that we have an idea of what we mean by humid air – air full of water vapor – we can say something else which we learned from the research of a fellow named Avogadro and those who followed his lead; it is this:

Every parcel of gas, every volume of gas, holds the exact same number of molecules, no matter what kind of molecules they are. Well, that’s not quite right: of course a gas that is hotter expands and has fewer molecules in a specific volume; or a gas can have a wrapping around it such as a balloon and be squashed so more molecules fit into a specific volume. But if we have an imaginary box of gas of any precise size, then every other box of that size that has the same temperature and the same pressure has the same number of molecules, and it doesn’t matter whether the molecules are water, oxygen, nitrogen, carbon dioxide, or even much larger molecules such as vanillin or cinnamaldehyde. It will always be the same total number of molecules if the box size, the temperature, and the pressure are the same.

We don’t imagine this would be true, because we think of atoms and molecules as balls of different sizes, and we would certainly not get as many beach balls as we would marbles into any box. But the point about gases is that they are balls in motion, all banging against each other and bouncing away, and – this is the crucial point – the little ones move faster. Of course they move faster. Wouldn’t you move faster if a bear bumped into you as opposed to a rabbit or a fly? Because the little ones move faster, they effectively take up as much room as the big ones that move more slowly, and the sum of it is that a given volume of gas always has the same number of molecules. Some may be little speeding molecules that would condense into something quite small; others may be big galumphing ones that would condense into something moderately large. But if you count them, the number is the same.

And if you have 22.4 liters of cold gas down by the sea, you have precisely 6.02 x 1023 molecules in your box. That’s a famous number, called Avogadro’s number, and you can ask your chemistry teacher why they chose 22.4 liters instead of something more obvious. (There is always an interesting reason for such things.)

But always having the same number of molecules means that some gases are heavier than others. The heavy gases don’t take up any more room; they are just heavier. It follows that they are more gravity-challenged; they fall while others rise.

Saturated air rises:

So let us get back to the air with its water vapor. When water vapor gets mixed into the air, the oxygen and nitrogen have to move over to make room because only 6,02 x 1023 bits can fit into the box; ultimately, when you are out-of-doors, the gases move up, because all the space around is already filled with oxygen, nitrogen, argon, and the other components of air, and “up” is the only place where there is more room. But the water molecule is actually lighter than the oxygen molecule and also lighter than the nitrogen molecule.

Some airy weights:

Basically, you get the weight of a molecule by counting the protons and neutrons in its atoms:

That makes O2 have a weight of 16 + 16 = 32

It makes N2 have a weight of 14 + 14 = 28

Carbon dioxide (CO2) has a weight of 12 + 16 + 16 = 44

Argon is an atom that has a weight close to 40

Finally, water H2O has have a weight of 1 + 1 + 16 = 18

See how light the water is? It is amazing that it doesn’t float away altogether and become lost in space.

Anyway, you can see that a box of gas that is part water vapor will be lighter than one that is only oxygen and nitrogen. So it will rise.

In conclusion

When people talk about dry air soaking up water like a sponge, then, it’s not really like a sponge that gets heavier as it soaks up water. It’s more like potting soil that gets lighter (per cubic foot) as you add Styrofoam (or whatever that white stuff is).  So saturated air – air holding all the water vapor it can – naturally rises.

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How big is a raindrop?

Is this a reasonable question? Can’t drops come in all sizes?

Raindrops can come in many sizes, but nothing comes in “all” sizes. Basically, the size range for falling-out-of-a-cloud raindrops is between 5 millimeters (mm) and 1/2 mm. Five millimeters is about a quarter of an inch; 1/2 mm, or 0.5 millimeter, is 1/25th of an inch.

Raindrops don’t get much larger than a quarter inch because when they get to falling 20 miles an hour (which is how fast they go at this size) the air resistance starts breaking them up. Low clouds might release larger drops for a short time — we’ve all felt the big splat of first drops or drops right after thunder. But basically, this is as big as they can be because the aerodynamics of their trip through the air won’t let them get larger.

Raindrops also have a lower end of size and don’t get much smaller than a half a millimeter because they don’t fall very well below that size. This is drizzle-size drops, and they only travel 4 miles an hour, maybe a little more. At that rate, falling from a cloud that’s just a mile up, they’d take something like fifteen minutes to fall. That’s a long time, and if the air is the least bit dry, they just won’t make it at all; they’ll evaporate first.

So raindrops have a size. If we divide everything in the universe into piles, according to its size, and if every pile has things that are ten times as tall, wide, and long as the next, (or one tenth as tall, wide, and long) we have about 42 piles all together.

Meteorology magnitudes

We might ask ourselves how many of these piles contain objects relevant to the study of weather. Let’s take a look:

Near the middle is our starting pile, things that are close to one meter tall, wide or long, like your own body. We call that the zero [0] pile.

  1. Your umbrella is in this pile,
  2. Along with your raincoat
  3. And the weather balloons sent up by the National Weather Service to spy on the atmosphere’s secrets.
  4. Puddles are in this size range.

One tenth of a meter is about 4 inches, and that’s about the size of a softball. We call this the [-1] pile, because it’s going down in size.

  1. This is the size of the largest hailstones on record.
  2. It is also the width of a train track, and you may be interested to know that my husband’s dad, a pilot, said that he could feel the bumps in the air when he flew over train tracks. They are wide enough to cause updrafts on a hot day.
  3. It is also the size of a junco, which is a harbinger of winter. (More on that another day.)

One hundredth of a meter, one centimeter, is the next pile size downwards. We call it the [-2] pile.

  1. It is a more general size range for hail. Hail is often 2 or 3 centimeters.
  2. The largest raindrops are at the low end of this size range, because 5 millimeters is 1/2 centimeter.

One thousandth of a meter, one millimeter, is the [-3] pile.

  1. Here is the main collection of raindrops.
  2. And here are the drizzle drops. They are less than a millimeter, but not 10 times less; about half a millimeter.
  3. Large, dendritic (branched) snowflakes are also in this size range.

One tenth of a millimeter is the next size range. [-4]

  1. Newborn snowflakes, just getting big enough to fall, are in this size range. Medium-sized flakes belong to either [-3] or [-4]
  2. Heavy fog droplets are in this size range.

I suppose you can guess what you’ll find in the [-5] size range.

  1. Light, evaporating fog,
  2. Droples in a white cloud such as a cumulus cloud, not a gray nimbus all full of rain, not an angel-hair cloud made of snow, but a fluffy white one.

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Two definitions:

Relative humidity is the amount of water vapor the air is holding compared to how much vapor the air could hold before some of it just has to become water droplets.

I remember my Aunt Mary once shaking her head over a weather report that said that the humidity was something – say 80% — while it was raining.

“Surely,” she exclaimed, “if it is raining, the relative humidity is 100%.”.

Well, it must have been 100% somewhere – that’s why the rain began. But it might not be that high on the ground. When just one drop falls, (see Sept 25) you know the relative humidity isn’t 100%. But even if millions of drops fall, the air still may not be saturated where they land.

Dew point is the temperature at which the amount of vapor now in the air will be all it can hold — 100% relative humidity. That’s the temperature at which it’s going to make dew. So the dew point is the temperature at which the existing air is going to turn some of its vapor into water.

You need to understand relative humidity to understand dew point. And you need to understand dew point to be able to make a certain kind of forecast which is important to pilots and interesting to other people, namely this:

You know it’s normally going to get colder at night. How much colder? Well, a cold front might make it a lot colder, of course. But if there’s no cold front, how much colder can it get?

And the answer is that it won’t get much colder than the dew point because – why?

Because when water vapor turns to water droplets, the air warms. (Remember: at the rate of 500 calories per gram of water.) So the very coldness that makes the dew generates an event that warms the air. Of course the night continues and the air gets colder, but then more dew forms and the air warms more. And the night continues… In the end, the temperature doesn’t generally fall below the afternoon’s dew point, just because of the warming effect of making the dew.


If the late afternoon temperature is already close to the dew point, then the vapor in the air won’t be content to make a few drops of dew. It’s going to make an entire fog. Fog is not water vapor; it’s not water in gas form; it’s water droplets, though very small ones. Obviously pilots have to understand this and plan on an instrument landing, but if you have to be somewhere early in the morning, give yourself extra time when the dew point is close to the afternoon temp because you might be facing fog.


All these facts about atmospheric heat being generated by condensation and again by freezing point to a technique for keeping your garden going during the first frosts:

Water it.

My friend Edie used to turn on an overnight misting hose during the first frosts that threatened her garden. All that extra water gave the cold air such a vast new cooling task that it couldn’t complete the crucial phase change that harms plants. She often held her garden for several weeks by watering just a few critical nights.

Orange orchards in Florida can sometimes be protected from freezing by watering them.

Here is the story of protecting a peach orchard in Iran by watering it. You can easily find other such stories.

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