The Weather

[Pappa]

Coinkadink. I just happen to be reading this right now.

It's totally related

It totally is! It's part of what makes weather forecasting so interesting.

I'll have to pick that one up.

I'd recommend it too. Amazing book. Though I've not read it for a few years.

[epepke]

Hiya. I did some work with Peter Ray, who's pretty big in lightning.

[FBM]

That's very interesting, FBM. I had no idea that thunder and lightning were so rare in Korea.

I don't know of any specific meteorological reason for it, so it might simply be due to your local geography.

If you live in Seoul, storms you get there would likely be due to orographic lift. Moist winds blow onshore and up the side of the mountains, and they dump out that moisture as rain as they rise. Conditions weren't necessarily good for storms to begin with (the orographic lift forces the issue), so the necessary processes to produce the charge differentiation for lightning to occur couldn't get going until the storms are further up the mountain. This would likely be the case for any locations on the west side of the country (west of the mountains).

If you're further south or east, let's say in Pusan, that lies in a river valley. Winds blowing from the west would cross the mountains to the west of the city and dump most of their rain (and lose a lot of energy). The storms would be suppressed as the air flowed down the eastern side of the mountains (sinking air is very stable and resists convection), but could start to build again as they approached Pusan, but would likely not build to the right intensity to produce lightning.

That's off the top of my head, anyway. I'll look into it more, and see if I can come up with anything else.

I'm about an hour south of Seoul and, yes, I'm west of the major mountain chain, though we have some low mountains all around. This may help: http://maps.google.co.kr/maps?hl=ko&new ... CB4Q8gEwAA

[ScottyMet]

Hiya. I did some work with Peter Ray, who's pretty big in lightning.

Very cool. I'd love to do the kind of work he does.

I'm about an hour south of Seoul and, yes, I'm west of the major mountain chain, though we have some low mountains all around. This may help: http://maps.google.co.kr/maps?hl=ko&new ... CB4Q8gEwAA

Ah, I see. You're tucked away in a little valley there, aren't you? It would definitely depend on where the storms were approaching from, in that case. You have the lee-side mountain flow from both the southwest and west. Any storms coming from the west would be severely diminished, and the larger collection of mountains to the southwest would pretty much guarantee that you wouldn't be seeing much from that direction. Looks like the only time you'd really get anything is when they approach from the NW.

[Random Mutant]

Fantastic stuff, thanks Scottymet!

I am an ex-glider pilot who often attracts strange looks as I get enthusiastic about what's going on in the sky. One thing has always eluded me however- tephigrams. I know a bit about adiabatic lapse rates, the ICAO standard atmosphere, and a few years of chemistry and physics at university still leaves me scratching my head when I see one.

I first came across them in Meteorology For Glider Pilots, by Wallington and would love to understand them further.

My favourite met site is www.metvuw.com, which is NZ-based but has some good forecasts for much of the world, a great photo of the day feature, and of course tephigrams.

Can you shed some light on this area, with the first question, how do you pronounce it?!? Tea FYE grams? TEF ee grams?

Thanks in advance!

[FBM]

Ah, I see. You're tucked away in a little valley there, aren't you? It would definitely depend on where the storms were approaching from, in that case. You have the lee-side mountain flow from both the southwest and west. Any storms coming from the west would be severely diminished, and the larger collection of mountains to the southwest would pretty much guarantee that you wouldn't be seeing much from that direction. Looks like the only time you'd really get anything is when they approach from the NW.

Ah-ha. Thanks for that. Of course, most typhoons approach from the S and SE, with rare exceptions, and they're pretty well dampened down by the time they get here, too. Ah, well, at least we don't get tornados.

[ScottyMet]

Fantastic stuff, thanks Scottymet!

I am an ex-glider pilot who often attracts strange looks as I get enthusiastic about what's going on in the sky. One thing has always eluded me however- tephigrams. I know a bit about adiabatic lapse rates, the ICAO standard atmosphere, and a few years of chemistry and physics at university still leaves me scratching my head when I see one.

I first came across them in Meteorology For Glider Pilots, by Wallington and would love to understand them further.

My favourite met site is http://www.metvuw.com, which is NZ-based but has some good forecasts for much of the world, a great photo of the day feature, and of course tephigrams.

Can you shed some light on this area, with the first question, how do you pronounce it?!? Tea FYE grams? TEF ee grams?

Thanks in advance!

Ah, Tephigrams... *contented sigh*

I miss seeing tephigrams. They use Skew-T's here in the US. I much prefer the tephigram. It is, indeed, a T-PHI graph, but everyone pronounces them TEF ee gram.

A tephigram is used to plot several pieces of information at the same time, which combined, tell you more about the weather than each piece of information being viewed on its own.

It is a plot of T-Temperature vs PHI-Potential Temperature.



For the graph itself, the straight black lines that slant diagonally to the right are lines of constant temperature, in degrees Celsius. The straight black lines that slant to the left represent the potential temperature (PHI), in degrees Kelvin. This can also be used to track the Dry Adiabatic Lapse Rate. The curved black lines that slant to the left represent the Moist Adiabatic Lapse Rate. The dashed lines that slant to the right are mixing ratio (grams of water per kilogram of air), and the lines that arch across the other ones, from left to right, are lines of constant pressure, in millibars, which are used as a measure of height. Meters or feet could be used, but pressure is more useful to a meteorologist. Along the right hand side are wind barbs, given at various pressure levels (in millibars). For winds, 1 short barb is 5 kts (nautical miles per hour), 1 long barb is 10 kts. So, just above the 600mb line, the wind barb is very clear there. With 4 long barbs and a short barb, that is a speed of 45 kts. A flag denotes 50 kts, and you add on more barbs from there to denote higher speeds. If you look closely, you can match up the lines for each pressure level with where the potential temperature lines on the graph cross the curved pressure lines.

A few definitions
Adiabatic: no heat is added or taken away during the process.

Dry Adiabatic Lapse Rate: the rate at which the temperature of a dry parcel of air cools as it rises or warms as it descends. This rate is roughly 1 degree C per 100 meters.

Moist Adiabatic Lapse Rate: the rate at which the temperature of a saturated parcel of air (100% relative humidity) cools as it rises or warms as it descends. This rate is about 0.5 degrees C per 100 meters.

Potential Temperature - the temperature a dry parcel of air (at any given temperature and pressure) would have if it were expanded (rises) or compressed (descends) adiabatically to a standard pressure, usually taken as 1000mb. Since a dry parcel of air rises or falls at the Dry Adiabatic Lapse Rate, the lines of potential temperature on the graph can be used to track a parcel of air along as it rises dry adiabatically.

Dew Point Temperature - the temperature a parcel of air will be at if you lower the temperature of it enough so that it reaches moisture saturation (100% relative humidity).

What you're seeing plotted on the graph, in red and blue, are the temperature (red) and dew point temperature (blue). These values are recorded by an instrument known as a radiosonde, which is attached to a weather balloon and released to rise up into the atmosphere. The farther those two lines are apart, the drier it is, and the closer they are together, the more humid it is. Typically, if they are within 3 degrees of each other, you will expect to be seeing clouds at that level. Thus, based on the graph above, there are likely some clouds near the surface, and possibly some cirrus clouds around 350-325mb, and 300-250mb (remember, the numbers go down as you go higher up), because the blue and red lines are very close to each other at those points.

You can also plot other information along with T and Td, which can tell you a lot more information, such as how much energy there is for potential thunderstorms (CAPE - Convective Available Potential Energy), if you will have hail, and how much, and how big, if you have temperature inversions (which ties into my previous posts), if there is a warm layer of air aloft, which may cause conditions such as freezing rain.

If we look at a different plot from the same location, 12 hours later...



You can see that it is going to be cloudy from near the surface all the way up to just above 700mb, since the red and blue lines are close together. There is also probably a thunderstorm happening at this point too, or there is at least the potential for it. It is hard to see on this smaller image, but if you download the larger pdf from their website (http://www.metvuw.com/upperair/201004220000.93844.pdf)... you start at the base of the red line, and trace another line, diagonally up to the left, making the line parallel to the potential temperature lines. When you reach the first dashed line, stop. Then, from there, plot another line starting at the point at which you just stopped, this time following (as close as you can) parallel to the curved black lines that angle to the left (the Moist Adiabats). When that moist adiabatic line crosses over the red temperature line, anything between the temperature line and the line you just drew is CAPE. Eventually, if you keep following a moist adiabatic line, you will cross back over the red temperature line. The area you just enclosed is the total amount of CAPE. The more CAPE there is, the more potential for thunderstorms, and the stronger the storms will be if they actually occur. There can be conditions that prevent these from occurring, of course, so just because you have some CAPE doesn't automatically mean you'll have thunderstorms. In this case, the CAPE area will be between around 900mb and about 675mb. It's not a lot, but it could be enough.

As you can see from the radar from that morning (9am), there was some precipitation along the coast, and the midday radar shows that there is still a little bit around.

[Random Mutant]

Ah, Tephigrams... *contented sigh*

Contented sigh indeed. This is the best explanation I've seen... what others I could find were way too technical for a non-meteorologist. I'm sure I have a few more questions about tephigrams, but at this stage I'll just digest all the goodness above.

I'm going to roll over and purr now.

Thanks!!

[ScottyMet]

No problem RM. I'm happy to be of service.

The nice thing about all this is that it really gets me to exercise my knowledge. It's easy to forget some of this stuff, so I'm glad that I remembered so much about tephigrams, considering how long it's been since I worked with them. I had a look up a few things to refresh my memory, but I actually remembered quite a bit.

[ScottyMet]

Oh, I've read through this again, and there are a few parts where I should have added more information... I can't edit the original post, so I'll quote it here and make my corrections.

For the graph itself, the straight black lines that slant diagonally to the right are lines of constant temperature, in degrees Celsius. The straight black lines that slant to the left represent the potential temperature (PHI), in degrees Kelvin. This can also be used to track the Dry Adiabatic Lapse Rate. The curved black lines that slant to the left represent the Moist Adiabatic Lapse Rate. The dashed lines that slant to the right are mixing ratio (grams of water per kilogram of air), and the lines that arch across the other ones, from left to right, are lines of constant pressure, in millibars, which are used as a measure of height. Meters or feet could be used, but pressure is more useful to a meteorologist. Along the right hand side are wind barbs, given at various pressure levels (in millibars). For winds, 1 short barb is 5 kts (nautical miles per hour), 1 long barb is 10 kts. So, just above the 600mb line, the wind barb is very clear there. With 4 long barbs and a short barb, that is a speed of 45 kts. A flag denotes 50 kts, and you add on more barbs from there to denote higher speeds. If you look closely, you can match up the lines for each pressure level with where the potential temperature lines on the graph cross the curved pressure lines.

I should add that the wind barb also shows direction. Think of the wind barb as an arrow with fletching. The arrow is pointing in the direction the wind is flowing, and is denoted in text by the direction it is flowing from. Thus, that wind barb just above 600mb is pointing towards the east-northeast (about 80 degrees or so), but we would call that a west-southwest wind (about 260 degrees), since that is the direction the wind is flowing from. This convention is simply because knowing where the wind is flowing FROM is far more useful information than where it is flowing TO, from a location station perspective (you are more interested in what is coming towards you than what is moving away from you).

A few definitions
Adiabatic: no heat is added or taken away during the process.

Any change in temperature is ONLY due to expansion (as the parcel rises) or compression (as the parcel descends).