Monday, July 22, 2013

How To Read A Skew-T Diagram

(Note: This article is Part 2 of a series on interpreting Skew-T/Log-P Diagrams. If you have not already read Part 1, you can do so here.)

Now that we have a basic understanding of what we’re looking at, let’s move out of the theoretical and into some actual weather data so that we can put our Skew-T knowledge to use.

Almost every sounding plotted on a Skew-T, regardless of the source, utilizes a basic color scheme of red for temperature and green for dew point temperature. (A few older websites may still plot soundings in black and white, in which case typically temperature is represented by a solid black line and dew point by a dashed black line.)

Sometimes Wet Bulb Temperature may also be plotted, usually (but not always) in solid blue.

Below is a fake sounding that I drew to illustrate what a Skew-T looks like with temperature and dew point plotted on it. We call each line a “curve.”

The red line, which we call the “temperature curve,” shows us what the temperature at every level of the atmosphere is. The green line, the “dew point curve,” likewise tells us what the dew point is at every level of the atmosphere.

At the very top, where the temperature stops decreasing with height and becomes isothermal is the beginning of the stratosphere. The spot where the troposphere ends and the stratosphere begins – signified by the very point where the temperature curve forms a corner – is called the Tropopause.

Judging by what I have plotted, we can assume that this is a sounding from somewhere in the summer, or somewhere tropical. As you can see, the temperature at the surface is about 27 C (~81 F), and the dew point there is about 20 C (68 F).

(As a quick side note, the “surface” is not always the bottom of the chart. If you are looking at a sounding from Denver, where the surface is over 5000 ft above sea level, the surface pressure will be around 850 mb, and the temperature and dew point curves will start at that level on the chart.)

If we are talking about summertime, we are probably interested in finding severe weather parameters like the LCL, LFC, and CAPE.

The Lifted Condensation Level, or LCL, is the lowest place in the atmosphere that clouds will form. It is the spot where parcels of air rising dry adiabatically from the surface will saturate and condense. You can find the LCL by tracing a line up from the surface temperature parallel to a DALR line, and then finding the point where it intersects with a line drawn up the surface dew point parallel to a Mixing Ratio Line.

If you’re confused, don’t worry. Seeing it will help.

This is what the LCL would be on the example sounding I plotted:

On a normal sounding, there would also be heights, in meters, written on the left side of the chart, so that you could ascertain what the LCL height above ground level, AGL, is. You can subtract the surface height from the LCL height to find how high off the ground the cloud base will be in that particular atmosphere. (Note that the surface is not 0 meters in this case, because the heights plotted are geopotential height, not simply height above ground level.)

Now, for just a moment, stop thinking about lines on a chart and think about the real atmosphere. The LCL, as we said, is the point where a parcel of air becomes saturated. If it is saturated, is it going to continue to rise and cool at the dry adiabatic lapse rate? No! It will now rise and cool with a moist adiabatic lapse rate.

Switching back to Skew-T world, this means that our air parcel now rises parallel to a MALR line above the LCL.

When the parcel crosses the temperature curve, it is now warmer than the environmental, and is therefore free to rise on its own. Hence, we call this place the Level of Free Convection, or the LFC.

After the parcel crosses the temperature curve, it continues to rise parallel to a MALR line. If the atmosphere is “conditionally unstable,” (most will refer to it simply as “unstable") that parcel will rise all the way to above the tropopause, where it once again crosses the temperature curve, this time becoming cooler than the environment and hence sinking instead of rising.

Above, the yellow-shaded area is our CAPE, or Convective Available Potential Energy.
Now let’s assume that that first sounding was from the morning, and that by afternoon, the surface temperature has warmed to about 31 C (~88 F). Along with this, the dew point has dropped slightly, as some moisture has mixed out.

Notice that the temperature curve now parallels a DALR line from the surface to about 850 mb, which means that Low-Level Lapse Rates must be around 9.8 C/km. When this happens, the LCL and LFC will essentially be the same place (as you would expect). Notice, also, how the CAPE profile is a little bit thicker with that additional surface warming, meaning that the atmosphere is more unstable.

However, along with this surface warming, the LCL height increases. LCLs that are too high are unfavorable for severe thunderstorm development. Usually LCLs above 2000 meters would be considered “too high,” but that is rare around here. High LCLs like this will also negate some of the gains made by warming the surface, which is why the CAPE area only looks slightly bigger.

Another detriment to severe weather can be a cap, which is a temperature inversion, as depicted below.

Notice the large “nose” where there temperature is either isothermal or even warming slightly with height between roughly 875 mb and 750 mb. If we try to draw our parcel path, we’ll see that in this area the parcel is colder than the environment. The area shaded in blue is CIN, which is Convective Inhibition. If CAPE causes things to rise, then CIN pushes things down.

In the example above, there is plenty of CAPE, but we will never get to use unless we either have enough forcing to mechanically lift a parcel through the CIN (like from a strong cold front or dryline) or we have that cap erode throughout the day by “mixing out.”

This fake sounding would probably be from the morning, since the surface temperature is only about 21 C (~70 F), but there is much warmer air above the surface.

If we wanted to “erode” that cap, we would need the environmental lapse rate to be at least moist adiabatic. In Skew-T terms, that means we would need the temperature curve to be parallel to a MALR line from the tip of the “nose” all the way down to the surface. Trace this with your finger, and you will find a surface temperature of about 25 C (77 F) should theoretically negate the effect of the cap. However, if lapse rates are only moist adiabatic in the lower levels of the atmosphere, the LFC will still be very high, which would still preclude severe weather.

Ideally, you would want lapse rates of at least 7 C/km. This is roughly half way between moist adiabatic and dry adiabatic, so to do a rough estimate with your fingers, draw one line from the nose to the surface parallel to a DALR and a second parallel to the MALR. Then draw a line half way between those lines. That would get us a desired surface temperature of somewhere around 30-33 C (86-91 F).

This means that, in the given atmosphere, we would need to warm roughly 16-21 F from morning to afternoon in order to even have a fighting chance at seeing severe weather. Now you know why severe weather enthusiasts hate strong caps.

I will be back Wednesday with real soundings (instead of my fakes ones) from real events in the Philadelphia area, like severe thunderstorm days and ice storms. (Yes, Skew-T diagrams are useful in winter, too.)