Afternoon Everyone,

Quick question regarding what happens to air once it has reached the dew point and cloud has formed. Bare with me, it might be a tad confusuing.

For example if air molecule 'a' is cooled adiabatically to the dew point and it becomes saturated, what happens to that air molecule once it has been stripped of its water vapour? Does it stay in the cloud, does it sink, does it continue to rise...? I'd imagine that once it has been mugged for all the vapour it is worth it would lighter and thus more prone to rising?

This is just a general curiosity!

G'day Dann,

Good question! First off, one thing you need to be clear on is water vapour exists in the packet of air as free water molecules and these are not "attached" to air molecules. So, when a parcel of air becomes saturated and cloud starts to form, air molecules are not "stripped of their water vapour", but rather the water vapour within the air parcel condenses around condensation nuclei (e.g. dust particles) to form water droplets.

As you know, when water vapour condenses it releases latent heat. This release of latent heat actually warms the surrounding parcel of air to a certain extent and eventually as the air warms and water condenses you will find the air is no longer saturated.

It is warmer (due to the release of latent heat), can therefore hold more water in vapour form and there is also less water vapour present since some water molecules have condensed out. This warmer air parcel is now less dense than the ambient air and begins to rise. Eventually it will reach a point where its steady cooling increases the density of the air parcel and it either comes to rest in ambient air of the same density (temperature) or it might cool sufficiently to come back to its new dew point for the water vapoour it contains.

In that case, the remaining water vapour will start to condense out and you'll get a new cloud layer forming. As before, the condensing water vapour will tend to release latent heat and the air parcel may well lose enough water vapor and be warmed again to the point where it is no longer saturated.

Once again the air will rise and it will either reach a point where it cools and forms another cloud layer or there may be insufficient water vapour remaining in the air in which case no further layers will form. The air parcel will continue to rise until it finds itself in an environment where the ambient air density is the same.

Please note, if condensation occurs at very high levels or at very low temperatures, you may not get normal water droplets forming but rather super-cooled water droplets or even ice crystals (such as we see in high cirrus clouds). The principle is the same though: water vapour condenses, releases latent heat and the drier air parcel continues to rise until it reaches ambient air of the same density.

By the way, this release of latent heat slows the cooling of the air as it rises which is why there are two adiabatic lapse rates:

- Dry adiabatic lapse rate for unsaturated air, and a
- Wet adiabatic lapse rate for saturated air.

You will find the wet adiabatic lapse rate is somewhat lower than the dry adiabatic lapse rate because the condensation of water vapour tends to keep the air "warmer" through this release of latent heat.

Isn't meteorology wonderful!

Cheers,

Rich

That answers my question wonderfully! Thank you! I do, however, have another question! Well, I think there are a couple of questions in this one...

Talking about ridges and troughs, are they indicated by a respective symbol on a synoptic chart? Or do you simply need to be able to identify them (which is'nt hard to do, just wondering if I should be looking out for some sort of line).

And secondly, I'm having a little trouble understanding why air suffers strong convergence when passing through a trough? It has something to do with the surface wind veering across the isobars towards the low pressure.

Also, because ridges and troughs occur with such great spacing of the isobars, does this mean that (aside from any other associated weather) there will be relatively low winds?

Cheers! =]

Hi Dan,

Glad the explanation helped. By the way, the idea of air parcels being able to hold water is not technically correct and it would probably make a meteorologist cringe. However, this simple model can help students to understand the behaviour or water molecules with changes in temperature as long as you don't take it too seriously.

In reality, water molecules are continually switching from gas to liquid (condensation), liquid form to gas (evaporation) and even to and from the sold state if it gets cold enough. As the temperature drops, the rate of condensation is greater than the rate of evaporation and you'll end up with a net condensation effect. And, of course this would happen even if there was no air present but we luckily don't have that situation outside of a laboratory!

So, the true situation is more a continual switching of phase states affected by ambient temperature, droplet size, form of the condensation nuclei etc. Bottom line though: as air packets cool, so too do the water molecules in it, net condesation will start to occur at the dewpoint and clouds will start to form.

As for your other questions:

1.) Troughs are normally marked on the synoptic chart as a dotted line. However as you said, ridges and troughs are easy to spot from the pattern of the isobars.

2.) Remember, in an area of low pressure, air is rising. This means air will flow in towards the low pressure system to replace that rising air. In a Low, this air gets affected by coriolis and spirals round into the familiar form we see in satellite photos of low pressure systems.

In a trough though, you essentially have an extended narrow region of low pressure and rising air. This time though, even though the air is still affected by the coriolis force, you are getting convergence (or inflowing) of air from both sides of the trough. This will give rise to more significant convergence and associated bad weather there. Take a careful look at fig 5.12 on page 5.7 of the textbook and you'll see what I mean. The arrows show the direction of the surface winds. At the trough line they meet pretty much head on. If you were on the ground as the trough passed over, you would notice the surface wind backing very quickly until it virtually reverses direction.

3.) The spacing of the isobars is the clue to windspeed in those areas yes. Wide isobars means a shallow pressure gradient and therefore less wind. Tighter isobars means steeper pressure gradient and therefore stronger winds.

Cheers,

Rich

G'day Rich thanks for another great reply! I think I'm starting to get my head around the whole convergence thing in a trough, though I think I still need a bit more help.

I've drawn a (rather crude) representation of what I understand is happening in a low in 'paint'. I was wondering if you could take a look at it and maybe let me know if I actually do understand what's going on! I think you'll agree that I should quit being a pilot and take up art after seeing it...

So what I've drawn is a low without a trough and a low with a trough. The arrows represent the surface air.

In the right hand picture (without the trough), from what I understand is that surface air moves into the low in all directions to replace the air that is being sucked up into the low. It's sort of like a big vacuum cleaner, that is, before the air actually reaches the low and takes on the typical clockwise rotation and gets sucked up into its heart. Is that correct?

Now, in the left hand picture (with the trough) the surface air is being sucked into the trough the same way as it did into the low without the trough. Because the surface wind is coming from both sides of the trough, the air converges at the trough line causing the air to rise (convergence). However, as it reaches the trough it veers on one side and backs on the other across the isobars to move towards the heart of the low.

Does that all make sense? Is it right? The one thing I think I may be wrong about (although there is probably more) is that the air doesn't simply move in from all directions, it actually moves in with the clockwise direction already present.

Also, I'm still unsure about why wind would almost reverse direction if you were an observer on thr ground as a trough moved overhead.

Sorry to be a hassle, for some reason this is just one of those things that's hard to get my head around!

Cheers! =]

I've got another question for you, just out of interest.

You hear about people on the ground getting struck by lightning (or shocked by the associated spike in current when lighting hits phone wires) while using their landline. Is it possible for the same thing to happen in an aircraft through the headset? If lightning were to hit an aircraft, which can damamge electrical equipment, would there be any chance of the pilot's head becoming part of the pathway via the electrical system?

I've got yet another question! I'm just inundating you with them, aren't I!

The term 'squall' is used to describe both short lived bursts of strong wind and areas of low cloud and rain in front of a cold front, is that right?

Or is it the term 'squall' for wind and the term 'squall line' for low cloud and rain?

Thanks for putting up with my constant question asking! I was the same in school.

Cheers!

Hi Dan,

Air wants to flow from areas of high pressure to areas of low pressure. Low pressure is caused by rising air so this rising air naturally needs to be replaced so there's an in-rushing of air at the surface. You need to make sure you don't mix up the following terms:

Pressure Gradient: in the context of this discussion, it's the tendency for air to flow from areas of high pressure to areas of low pressure. It will want to flow in from all directions - that's true! In fact, in areas without much coriolis effect (i.e. the equator), that's exactly what happens: air will flow straight across the isobars into the low pressure region (see below): Note here how the pressure gradient is acting in opposite directions along the trough line.

Gradient Wind: air moving due to the pressure gradient gets affected by the coriolis force which will make it flow more along the isobars. (see below) In that diagram, you can see the gradient wind direction changing as it goes through the trough line.


Surface wind: near the ground, the gradient wind is slowed by friction and the battle between the pressure gradient and the coriolis effect favours the pressure gradient force more (the reduced speed of the wind reduces the coriolis effect). Therefore the surface wind will tend to flow more across the isobars when compared to the gradient wind. (see below)
Now, if you look at the area of convergence you can see the surface wind is not moving in the same direction on each side of the trough. The surface winds on either side are flowing more towards each other (or at least at 90 degrees in my pikkie). OK, in this Low, the surface wind doesn't "reverse direction" but depending on the shape of the trough, you will get very significant changes in wind direction as the trough passes over. That's what you need to remember: significant convergence (with associated bad weather) and significant wind direction change at the trough line.

No you are not likely to get electrocuted from a lightning strike in an aircraft. Lightning is lazy: it wants to find the easiest path. It will use the airframe to continue its journey and as long as the aircraft is correctly protected, there should be little side effect of a lightning strike.

A "squall" is a wind speed increase of at least 16 kts over the mean wind speed such that the squalling wind speed is over 22 kts during the squall and the squall lasts at least 1 minute. In other words: strong winds that rise suddenly, last for several minutes and then suddenly drop away again.

A "squall line" is a line of thunderstorms with little space between individual cells.

Time for a coffee

Cheers,

Rich

Mate, you're a legend. That was just the explanation I needed! How keen would you be to do this exam for me?

Speaking of which, if you don't mind me asking of course, are you actually in Germany or was that just where you were born? The reason I ask is because I'm a Swiss citizen (which I got automatically at birth) and have family up in those parts!

Thanks heaps for your explanation!

G'day Dan,

Glad to help. I actually live in Germany at the moment. My wife is German and our kids were born here. Brisbane is my home city though and I get over to Australia a couple of times a year if I can, even if only to harrass Lee and help Bob battle the rising levels of Merlot in his wine cellar.

Switzerland sure is beautiful. We got over there several years ago. Simply stunning countryside, that's for sure. Having dual citizenship makes life a lot easier and you should hang on to it if you can. My parents are English so I scored a British passport which makes staying here a formality at the moment.

Have fun with Met!

Cheers,

Rich