How To Read a Skew-T Graph

What is a Skew-T and Why should I care?

What if there were a way, in one image, to find out…

  • Top of lift
  • Thermal strength
  • Wind speed
  • Wind direction
  • If it will be cloudy or sunny
  • Types of clouds in the sky (i.e., stratus, cumulus, cirrus, cumulonimbus, or none)
  • Cloud elevations (i.e., “cloudbase”)
  • Air temps at all flyable altitudes

Sounds like a mystical, mythical image, right? Yeah. It does. But it’s not. It’s the skew-t, and if you’re a paraglider pilot (or a hang glider pilot, of course) it behooves you to become adept at interpreting one. In this document I’m going to help you do that.

Here’s what one looks like:1basic

 At first glance, it looks pretty daunting, with all those lines criss-crossing everywhere.

However, right away we can get rid of most of it…2important_part2

Why is that?

Well, notice the scale along the right-hand side. It’s the altitude, in kilometers.  As paraglider pilots, we don’t really have any interest in going higher than ~5.5K, MSL (roughly 18,000 ft., which is the bottom of the Class A airspace, which we’re not allowed to go into) So, we can ignore everything above that height (Well, that’s not strictly true, but to keep this document simple, we’ll go with it. When you’ve come to understand the Skew-T and how the white and blue lines affect each other, you’ll know what I mean).

So, what about all the stuff that’s left?

Parts of a Skew-T

Measured Temperature Lines3temp_lines

The white line that I’ve circled clumsily in red is the air temperature as a function of altitude, also known as the environmental lapse rate (ELR).

The blue line that I’ve circled in green is the dewpoint temp as a function of altitude.

The air temperature line is the most important line on the Skew-T to us, though the dewpoint temp can become important, too.

We’ll go into more detail about this stuff in a bit.

Dry Adiabats4dry_adiabats2

The lines I’ve highlighted in red are called the “dry adiabats”.

These are the second-most important lines on the graph for us, as they tell us how the temperature of a theoretical parcel of air (read: THERMAL) should change as it rises. They show the dry adiabatic lapse rate.

At the most basic level, for you to know whether a day’s weather will be stable or unstable, you must compare the environmental lapse rate (ELR) with the dry adiabatic lapse rate. But first we have to talk about the other parts of the graph.

Isotherms5isotherms

This graph is called a SKEW-T for a reason: It’s skewed. Take another look at the previous image. See the dry adiabat that I’ve highlighted in yellow? It meets the temperature scale at the bottom of the image at roughly 75 degrees. At the ~13,000-foot mark, the graph seems to be telling us that this parcel of air should have a temperature of about, oh… 28 degrees.

If you’ve been paying attention in your weather studies, you’ll remember that the dry adiabatic lapse rate is -5.5 degrees for every 1,000 feet, which means that a parcel of air that starts out at a sea-level temp of 75 should, when it gets to 13 thousand feet, be at something close to 75+(-5.5*13)=3.5 degrees.

So what’s going on? Again, the answer is that the graph is skewed, so you can’t simply follow the scale up. Isotherms to the rescue!

Notice how the top of the yellow dry adiabatic line intersects with the isotherm for just below zero? That’s pretty close to my 3.5 degree estimate!

Saturation Adiabats6moist_adiabats

Also known as the “moist” adiabats, these lines represent the change in temperature of a saturated air parcel (you might call it a “cloud”) as it rises–the moist adiabatic lapse rate.

Since we don’t really want to fly in or above clouds we don’t generally care about these lines–except when they tell us that the sky will be filled with cumulonimbus clouds.

Saturation Mixing Ratio Lines7saturation_mixing

These lines represent air with an equal saturation mixing ratio. I won’t go into any detail about what that means because, for the most part, the only thing important to us here is that these lines can tell us what the cloud base will be. More on this later.

Isobars8isobars

 This one’s pretty obvious, I imagine. The isobars represent points of equal pressure. Their units are millibars. As we go up in elevation the pressure drops, which is why those numbers get smaller.

Wind9wind

This one’s pretty obvious, too. This mess shows the wind speed and direction at the various altitudes.  Great info to have so that you can know what your penetration will be, and if there are any big shear layers hiding up there.

Interpreting a Skew-T

Now we know what all the parts mean, but we still need to understand what it all means for paragliding. Let’s start with the most important thing: How to know what the thermals will be doing. Here’s where we compare the environmental and the dry adiabatic lapse rates!

Recall that a parcel of air that is warmer than its surroundings will rise, like a hot air balloon.  This air will continue to rise until the air around it is the same temperature. At that point it becomes “neutrally bouyant” and stops rising.

Okay. Armed with that reminder, let’s take another look at our graph…10therm1Notice that at ground level (which is where the white line terminates) the air temperature is ~86 degrees (remember we have to read the temp by the isotherms).

Now imagine that we’re out flying and we get really lucky. A big thermal gets to 93ish degrees before it triggers and starts rising (it’s represented by the yellow dot above).

That thermal is going to rise and cool at the dry adiabatic lapse rate, which means: parallel to the dry adiabats, like so…

11therm2There it is, at ~1700 meters, now cooled down to ~70 degrees, and still rising, hopefully carrying a happy paraglider pilot along with it.

Why is it still rising?

Because it’s still warmer than the surrounding air.

What’s the temperature of the surrounding air?

To find that, you must follow the associated isobar until it meets the ELR line.

So, our thermal above is at about 840ish millibars, and the ELR line at that pressure is showing a temp of, oh… 61ish.

Our imaginary (and very powerful) thermal is going to continue to rise, until…

12therm3…it gets to ~2400 meters MSL and has cooled to ~50 degrees, the temperature of the surrounding air.

Suddenly there’s no more lift, and we have to go look for another thermal.

Some other things to note, here:

  1. The environmental lapse rate at these altitudes is either the same as, or steeper than the dry adiabatic lapse rate. This is an indicator of a stable atmosphere. Remember this: Steep temp line = Stable atmosphere
  2. I picked 93 as the starting temp of my imaginary thermal. But on a stable day you won’t be seeing a 7-degree temperature differential between a thermal and the environment. What happens if there’s only 3ish degrees of difference? Let’s see…

13therm4Now, instead of getting to 2,400 meters, we’re lucky if we make it to 2,000, and the weak climb will probably take quite a while.

And what if it’s early in the morning and we still have an inversion layer?

14therm5Boom! We slam right into that inversion layer just like a roof is over our heads–in the above case, at 900 meters.

Determining Cloud Base (and tops)

So, how can we use the graph to figure out the cloud base?

First, there’s the hard way…

1) Starting at ground level, find the surface temperature and the surface dewpoint.

2) From the temperature, follow the imaginary line that is parallel to the nearest dry adiabat, going up. (See the orange line below)

3) From the dewpoint, follow the imaginary line that is parallel to the nearest saturation mixing line, going up. (See the red line below)

4) The point at which these two lines intersect (the purple smudge, at approximately 4.5 kilometers, MSL) is the elevation when our imaginary air parcel becomes saturated, and its moisture starts to condense out.  However, since the actual air temperature at that elevation is a lot warmer, we’re not going to see any clouds forming on this day.

15cloud_base1Now, onto the easier way:

At the top of this Skew-T graph you’ll notice there’s a bunch of numbers…

16legend

“P sub LCL” stands for the pressure at the “Lifting Condensation Level.” In this case, it’s 607 millibars, which is amazingly close to my purple dot, considering the crudeness of my technique. T sub LCL, I’m sure you can guess, is the temperature at the lifting condensation level–also reasonably close to what my purple dot shows.

But there’s yet another way to know what the cloud base will be.  To see that, though, we have to look at the Skew-T graph for a more interesting day…

17sept11Let’s zoom in on the interesting part…

18close-up

The story this graph is telling is:

1) Our imaginary thermal that’s at ~90 degrees at the surface rises (parallel to the dry adiabats), expands, and cools until about the 2 kilometer mark–or at a pressure of 788 millibars and a temp of ~56 degrees.

2) At that point it becomes saturated (see the dewpoint line, and the saturation mixing ratio line?), so instead of rising along the dry adiabats, our parcel of air starts rising along the moist adiabatic line.

3) It’s going to continue to do that as long as it’s warmer than the surrounding air, which means that the tops of the clouds on this day will be up at the 11 kilometer mark–and, as I’m sure you can appreciate, this is a good reminder of why it’s a very bad idea to be directly below a cumulonimbus cloud at cloud base.

From the Abstract to the Concrete

Let me throw some facts at you…

  • Air that is warmer than the air around it will rise, cool, and expand–until it’s the same temperature as the surrounding air
  • The sun can’t heat air directly. The sun heats the ground, which then heats the air next to it.
  • Air is a good insulator, but is constantly mixing
  • A cubic inch of air near the ground at sea level has about 14 pounds of air sitting on top of it, pushing it down
  • Air holds water, but only up to a certain amount of it–more than that and you get condensation
  • The higher the dew point temperature, the more water is in the air

By themselves, the above facts probably seem obvious, or trivial, or irrelevant. They are not. Keeping them in mind while you look at a Skew-T is crucial to being able to interpret it.

Now, go here and watch two days’ worth of hourly Skew-T graphs. Think about the above facts as you watch the story the animation is telling you…

See how the air temperature near the ground drops way down, while the air above it stays relatively warm? That’s our morning “inversion layer.” Until the air near the ground gets warmer than that air, we won’t be going anywhere.

19super_detailIn the afternoons (at least when it’s not winter time), do you see how, close to the ground, there’s a very narrow layer of air that gets very warm, and cools off very quickly as you go up in elevation? Meterologists call this the super-adiabatic layer, because it cools faster than the dry adiabatic lapse rate. In the image on the left, it’s the part of the temp line highlighted in yellow.

You might wonder, “Why doesn’t that warmer air rise, then, making the super-adiabatic layer go away?” Well, the answer is that it does! They’re called “thermals.” The bigger the super-adiabatic layer, the stronger your thermals will be. They can’t just rise whenever they want to, however, because remember that there’s all that air above them. It has to be pushed out of the way. Now, here’s the bottom line: No super-adiabatic layer, no lift. If that makes sense to you, congratulations! You get it!

Next, pay attention to the dew point line.  Is it far away from the temp line, or close to it?  If they are touching, you can be sure there’s a stratus layer of clouds at that elevation. If they are far apart, clear skies.  Close together? You’ll have intermittent clouds.

An Awesome Day

If everything I talked about above makes sense to you, then you’ll understand why this shows an awesome day…

20awesomeday

Where to Find Skew-T Graphs

The U of A has models as Flash animations that update hourly. These are awesome! (I apologize if you’re reading this and you’re not in Arizona. Hopefully you’ll find a resource as helpful as the UofA’s weather studies department).

 Note that they rotate through these over time, so if you don’t find the day you’re looking for, try the next one.

Zero One Two Three Four
Phoenix: WRF WRF WRF WRF WRF
Tucson: WRF WRF WRF WRF WRF
Flagstaff: WRF WRF WRF WRF WRF

 

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