Posts Tagged ‘relative humidity’

Chapter 5 has some mistakes and some interesting considerations.


p. 64: molecules in motion collide all the time

Williams has a diagram of a water molecle and a discussion of ice, liquid, and gaseous activities. In the gas part, he says that these fast moving molecules are so far apart that they seldom collide. Nonsense! They collide all the time, but they mostly bounce off each other so that – as he correctly concludes — they don’t become attached very easily. And why do they bounce? Mainly because they are moving very fast, 1000 feet per second or more.

p. 66: relative humidity is the ratio of water to total volume, not water to “dry air”.

Here is a chart of the amounts of water to be found in saturated air at different temperatures. It gives the number of grams of water in one kilogram of saturated air. So, in 95° air that is saturated, there are 37.5 grams of water. In  86° air that is saturated, there are 28 grams of water in vapor form.

Note: this is saturated air, not “dry air” as he labels it. Minor, but confusing.

p. 67: the state of a molecule is its relationship with other molecules

Williams refers to “a liquid molecule” – meaning one molecule of liquid water.

[Probably you realize what he means, so just keep reading. But a molecule is just a molecule; you can’t tell if it’s liquid or solid until you look around it for some other molecules and see how they are attached, if at all.]

Then he talks about that molecule breaking its bonds, and if a lone molecule of water breaks its bonds, you might think it was breaking its internal bonds between hydrogen and oxygen. No, he’s just saying that a single molecule of water needs energy to break out of a liquid environment.

p. 67-68: a linguistic reflection

Sublimation means the change from a solid to a vapor without becoming a liquid in between. Sometimes when the air warms after an icy night, you see “sea smoke,” tendrils of fog rising from the air just slightly above the ice. This fog is actually condensed from invisible vapor that is leaping off the ice. This change from solid to gas is called sublimation. It then becomes liquid (tiny drops in the visible fog) for a short time before evaporating again.

Using the word sublimate to mean depositing vapor as ice is new to me. I don’t mean it can’t happen; it certainly can. But it’s a new use of the word sublimation to go from vapor to ice as well as from ice to vapor. It’s a pity because it obscures the metaphorical use of the word sublimation to mean the fulfillment of the human person by spiritual means, rather than by material and then social or other secular/human pursuits. So the person goes from his animal (material) nature to his angelic without the transition of social pursuits that most of us experience. Some of the saints may have lived so. But the metaphor is lost.


Page 78 – 79: Thoughts about raindrops and clouds and diversity…

If raindrops fall at the rate of 14 mph, how long will they take to fall from a cloud that is 1 mile high? Two miles high? Three and a half miles high?

If a thunderhead in the middle latitudes were 7 miles high, how long would it take for a raindrop to fall from the top to the bottom? (Not that many would do that; most get pulled up again, and those that free fall are likely to evaporate halfway down…)

Besides, a drop it might fall faster part of the time, because the drops in high clouds get larger and then fall faster; but then they break …  it does make you think differently about rain, right? It’s not instantaneous; it has a history.

If you don’t have a project, you can write a story such as: The Story of a Drop: the saga of a drop of rain from a thunderhead to my head.

p. 79: How many differences in clouds

Thomas Henderson says that the whole difficulty of studying clouds is that they’re all so different. No two alike. Shall we have a contest? Shall we offer a prize if anyone can find two clouds alike? Did your Mom ever ask you to look for two blades of grass perfectly alike?

p. 66, 81 How much water up there?

If you have ever wondered how much water is up there, this chapter gives you two chances to figure it out.

One is on page 66 where we learn how many grams of water you could expect to find in saturated air at a certain temperature. Of course saturated air doesn’t mean a cloud; it’s just before the cloud forms. But it can give you an idea.

On page 81, we find an actual amount of water in a “small thunderstorm”  — whatever that is. Presumably it means a single thunderhead as opposed to a super thunderhead which pulls others into itself.

33 million gallons. (Glug!)

Are you surprised? Of course, not all the water available to a thunderstorm falls to the ground, right? 19 million gallons condense into clouds that just blow away afterwards; 4 million gallons fall on the ground, and the rest – out of 33 million gallons — evidently remains as uncondensed vapor, ongoing humidity.

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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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