Talking About The Weather, Part One

No technology today; just talking about the weather.

I love talking about the weather.

I mentioned the other day that I had just returned from my ancestral homeland on the shores of Lake Huron, the great inland sea of southwestern Ontario. We got some rip-roaring thunderstorms this year. I love thunderstorms on the lake. They are extremely dramatic, particularly at sunset.

Think about this for a minute: even small thunderstorms must move around literally millions of tonnes of air and water, often at high speed. Storms are massive concentrations of energy. Where does the energy come from? How does it get so concentrated?

Coincidentally, I had just started reading Frank Bethwaite’s book “High Performance Sailing”. Bethwaite is not just an amazing sailboat designer, but also a professional meteorologist. As someone who knows next to nothing about the weather I found it fascinating to learn just why it is that thunderstorms are so powerful.

Understanding this phenomenon required me to first be disabused of an incorrect notion that I’ve held probably since the fourth grade. You were probably taught this too, in your elementary school or high school science classes: “the reason clouds form when moist air cools is because cold air can hold less water than warm air”.

Nope. Not true. Your science teacher was wrong. That statement, though arguably not entirely false, is sufficiently misleading as to work against a correct understanding of the weather. Nitrogen and oxygen, the gasses which make up most of the atmosphere, have no ability to “hold” anything. Their molecules are way too far apart to hold onto anything. Furthermore, phenomena such as cloud formation can be explained entirely without this idea, so just based solely upon Occam's razor we should dispense with the notion that the air has something to do with it.

Ironically, the actual situation is not really very difficult to understand. First, consider the behaviour at the molecular level:

  • A molecule of liquid water will vaporize when its energy is high enough.
  • Conversely, a molecule of water vapor will condense into liquid when its energy is low enough.

Now consider the aggregate behaviour of zillions of individual molecules. We call the average energy of many molecules the “temperature”. When in gaseous form, that energy manifests itself as the molecules exerting force on their surroundings by slamming into things at high speed. We measure the number of molecules which inhabit a given region at a given temperature by measuring this force, which we call “pressure”.

Since temperature is an average, obviously some of the molecules in the sample will have less energy than average, some will have more. If the temperature is high, then a higher percentage of the liquid molecules will have enough energy to vaporize and a lower percentage of vapor molecules will have low enough energy to condense. A warm puddle of water evaporates faster than a cold puddle of water for this reason. And when you sweat, you cool down -- the warmest water molecules on your skin are evaporating and taking their heat with them, away from you.

Since pressure (at a given temperature) is proportional to how many vapor molecules there are in a given amount of space, it should be equally clear that at high pressure, there are more molecules available to condense than at low pressure. So we should expect that, all else being equal, the higher the vapor pressure, the more condensation we’ll see. “Damp air” – air that has a lot of water vapor pressure - condenses onto a cold window a lot faster than dry air.

With that simple molecular explanation, cloud formation can now be understood with no reference whatsoever to nonsense like “the water-holding capacity of the air”. Why should we mention the air at all? The air has nothing to do with it, aside from the fact that heating up the air also heats up the water vapor in it. Clouds would form equally well if our atmosphere was a mixture of helium and water vapor, or for that matter, just pure water vapor. All that matters is the temperature of the water and the vapor pressure in the region. A cloud is exactly that portion of the atmosphere where the vapor pressure is high enough and the temperature is low enough that a significant quantity of water condenses.

(To be nit-picky, yes, there are other factors. Foreign substances mixed into the liquid water, whether any of the water is in solid ice form, whether the liquid water is in round droplets or flat sheets, and so on, influence the rate of vaporization and condensation as well. My point though is simply that the "holding capacity" of the non-water part of the air is not relevant right now.)

With that in mind let’s look at the larger picture of how a cumulus cloud forms.

The sun heats the surface of a body of water. Say, my beloved Lake Huron. The temperature of the surface rises, so the rate of vaporization increases. Some warm vapor rises until it encounters a region of the atmosphere where the temperature is low enough and the vapor pressure is high enough that the water vapor turns back into liquid water. The liquid molecules combine into droplets which we can see. If the droplets get big enough then they fall back to earth in the form of rain.

That explains clouds and rain but does not explain the phenomenon I’m trying to understand here: why thunderstorms are so energetic. It seems like what we’re doing is moving the energy from the sunlight into the lake surface, then into the atmosphere via evaporation, and then that energy is transferred randomly into to the rest of the atmosphere when the cloud cools and condenses. After all, that’s what “cooling” is – moving heat energy to somewhere else. If this explanation is right then clouds should be dispersing energy, not concentrating it.  Something else is going on here. What are we missing?

Next time on FAIC: Entropy!

Comments (7)
  1. Bahbar says:

    > If the droplets get big enough then they become denser than the surrounding atmosphere and fall back to earth in the form of rain.

    I am not a meteorologist by any means, but that notion that density increases as droplets get bigger is wrong.

    As soon as the water condenses, its density is pretty much fixed. It is immediately denser than the atmosphere too. It does not get "denser" as the droplets merge. So what is making the condensed water stay in the air ? winds ?

  2. Eric Lippert says:

    OK, good point.

    Look at it this way. The amount a droplet is influenced by updrafts is proportional to both its surface area (which is being pushed by the draft) and its mass (which gives it resistance to changing direction).  But the amount it is influenced by gravity is proportional only to the mass.

    The mass is proportional to volume.

    The volume rises as the cube of the radius of the droplet. The surface area rises as the square.  So as the droplet grows, the amount of loft it can get from an updraft gets less and less, because its area is not increasing nearly as fast as its mass.

  3. BigTuna says:

    > So what is making the condensed water stay in the air ? winds ?

    If I’m remembering flight school correctly, wind is indeed the answer.  Raindrops condense as parcels of warmer surface air are lifted and cooled in the updraft of a thunderstorm.  The raindrops continue to condense and merge with each other until they become too heavy for the air column to support.  The stronger an updraft is, the larger the raindrops will be.

    Eric – please delete this post if I’m spoiling any secrets that you’re saving for followup posts.  Glad to see a fellow coder interested in meteorology!

  4. Pascal says:

    Well I think water molecules have a strong magnetic polarity, and moving them up and down creates a inducted current .

    This current strips some electrons in the air, slowly building up a charge difference.

    When the charge is high enough – BLAM, the air becomes conductor and a lightning occurs.

    So I think the clouds are turning mechanical energy (the vapour going up) into electrical energy until it becomes a storm and discharge to the ground.

    Does it make sense?

  5. Fluvial says:

    One of my best friends is getting her PhD in Atmospheric Science so I shall present this question to her and get back to you with a formal answer from a real live scientist.

    Carry on.

  6. Harold says:

    Suggestion for part n: forget about relative humidity, it’s all about the dew point.

  7. dietas says:

    Eager to read the Part 2 of this post.

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