Monday, October 24, 2011

How to read a SKEW-T / Log P

Hi, it's Rebecca back with weather forecasting part 2. Sorry it took so long but I've been busy. Today's post will be on how to read a SKEW-T / Log P. What the heck is a SKEW-T / Log P anyway?  I will cover the basics of interpreting it. Also, I will go into a little bit of detail on instability and thunderstorms. A word of warning, this post builds on some of the past posts that I've done.  The subject matter is a little more advanced than what I've covered before. I will try to keep the math, science, and meteorological jargon to a minimum. however, because of the subject matter the use of some of it is required.

 What is a Skew-T sounding?
In order to forecast the day's weather, we need to know the temperature of the air above us. At  0 Zulu (Z) and 12 Z balloons are sent up from thousands of stations around the world. The  instruments on the balloons gather data on temperature, humidity, and wind velocity above the ground from a variety of altitudes and radio this data back . These points are called mandatory levels and significant levels. The mandatory levels are used  for every sounding. The significant levels are places where the temperature and dewpoint  difference is greater than 1C. The mandatory points are listed below  The data that is radioed back is put into a chart called the Skew-T / Log P. It shows an instantaneous snapshot of the atmosphere from close to the surface to about the 100 mb level. From here on, I will refer to the Skew-T /Log P as just a Skew-T. 

Sea level .......0 ft
1000 mb ......300 ft
850 mb ........5000 ft
700 mb .......10000 ft
500 mb ........18000 ft
300 mb .......30000 ft
200 mb ......40000 ft
100 mb .......53000 ft

List of mandatory levels. When I get into weather models later-on you will need to refer to this as well. The attitudes listed are approximate.

Before I get stated you will need to know a few basic things. Below are two images of  blank Skew-T, with labels for what each line means.

Here’s an overview of each of the labeled terms:

Isobars:  Lines of equal pressure  in millibars. They run horizontally from left to right and are labeled on the left side of the diagram. Pressure is given in increments of 100 mb and ranges from 1050 to 100 mb. The blue horizontal lines (sometimes the colors are not the same on other charts)  The numbers on the left hand side decrease with height because pressure decreases with height. You will notice that the spacing between isobars becomes larger the higher up the chart you go. This is because of the logarithmic scale, hence the name Log P.

Isotherms:  Lines of equal temperature. The red solid lines, they run from the bottom left to the upper right. As you can see they are skewed to the side (giving the name of Skew-T). Increment are given for every 10 degrees in units of Celsius. They are labeled at the bottom of the diagram.

Saturation mixing ratio  (Constant Mixing Ratio) : Lines of equal mixing ratio, the  Mixing ratio is the amount of water in the atmosphere in grams of water vapor divided per kilogram of dry air. These also run from the bottom left  to the upper right. In this chart the pink/purple colored dashed lines. They are labeled on the bottom of the diagram.

Dry adiabatic lapse rate (dry adiabats) : Lines that show the rate of cooling of a piece of  rising unsaturated (dry) air. The cooling rate is about 10 degrees Celsius per kilometer.  These solid green lines slope from the bottom right to the upper left. They are solid lines gradually arc to the top of the chart with height.
Moist adiabatic lapse rate (moist adiabats) : Lines that show the rate of cooling (depends on moisture content of air) of a rising saturated parcel of air.  These dashed green lines slope from the bottom toward the upper left. The Moist adiabatic lapse rate increases with attitude in view of the fact that cold air has less moisture content than warm air.
What is a Skew-T used for?
 That explains what all the lines are on a blank Skew-T.   I hope it wasn't too bewildering. Because it is very Important that you understand what the lines mean as we move ahead.  Now I will go into more detail about adiabatic processes. This part of the lesson will focus on the role these processes play in severe thunderstorm development.   The next Skew-T will show the temperature and dewpoint temperature plotted on the chart. I will also go into a bit of detail how these two variables are used to make a forecast. Again, on severe thunderstorm development. However, an atmospheric sounding can be used to forecast all sorts of weather. Atmospheric analyses involves a good understanding of thermodynamics and higher math. However, I will try and keep the use of science and  math at a minimum. After all, the purpose of this blog post isn't  to make you a meteorologist; instead it's meant  to give you a basic overview of what a Skew-T is used for.

In the chart above the red line represents the temperature throughout the atmosphere, and the green line represents the dewpoint temperature. The dewpoint temperature, or dewpoint, is the temperature at which the liquid water, or dew, evaporates at the same rate at which it condenses.. For example, let’s say the temperature outside is 70F, and the dewpoint is 66F. That night, if the temperature falls 4 degrees to reach 66F (Of course that's assuming the dewpoint will remain unchanged), the air will become saturated and you will have ground fog develop. This is because the dewpoint temperature depends on the air temperature, since hotter air can hold more water vapor per unit volume than can colder air.
 Now I will give you a little assignment. You're going to practice using this Skew-T sounding,  find the temperature of the atmosphere at 500 millibars (mb). To do this, you find the isobar that represents 500 mb, follow it to the right to where it intersects with the environmental temperature line (the squiggly red line), and follow the isotherms down and to the left to see which one it intersects with (most of the time, it won’t be right on a drawn isotherm; so you will have to follow an invisible line to estimate how far it is between two that you can see). In this case, the 500 mb temperature would be about -22C.  
It will also be important to understand what a lapse rate is. The lapse rate is the rate of decrease of atmospheric temperature with increase in altitude. lapse rate. This is expressed in degrees Celsius per kilometer. A lapse rate of 4 C/km means that if the temperature at the surface is 30C, the temperature one kilometer above the surface will be 26C. Of course, something called an inversion layer can cause a temperature rise. But we won't go into that.
You will also want to know what an adiabatic process means and what it has to do with this! An adiabatic process is one that does not involve a change in internal energy, or heat energy. This does not mean the temperature doesn’t change. In other words, no heat has been added or removed from the system yet the expanding air cools (called Adiabatic Cooling).  When parcels of air are lifted from a level of the atmosphere, they expand. There's a fair amount of physics involved in this. But we will stay away from that.  The important thing to understand is that air cools when it expands if no heat is added to it, and warms when it shrinks given the same conditions.
Skew-T and thunderstorm development.
The next two charts are based off of a parcel lifted from the surface.  But speaking pragmatically, this wouldn't happen most of the time. normally lift will be “mixed” through the lowest 100 millibars. but doing it this way will  make things easier to understand.

Now with that said, lets dive into how this lifting affects thunderstorm development. As I've stated in my blog on thunderstorms, in order for thunderstorms to form, we all know we want warm moist air below cool, dry air. But it's not as easy as that, like I said above air cools as it  rises. Therefore, If you want to see surface based storms form; the air at the surface must be warm and moist enough that when lifted adiabatically, it remains warmer than the air around it and keeps rising. When this occurs the atmosphere is said to be unstable.  This is difficult to do, since air cools fast enough when lifted that it typically will become cooler than the air around it and sink back to its original position. In this situation, the atmosphere is considered to be stable. 

When an unsaturated parcel is lifted from the surface, it will cool at the dry adiabatic lapse rate until it reaches saturation. This elevation is called the lifted condensation level, or LCL, and represents the level in the sky where you see the cloud base. To find the LCL on a Skew-T, you follow the dry adiabat from the temperature at the surface, and follow the mixing ratio line up from the dewpoint at the surface level, until the two intersect. This level is where the LCL is located. Once a parcel reaches its LCL, it is said to be saturated, then it will start to cool at the moist adiabatic lapse rate. 

These parcels of air are usually cooler than the air around them; when they reach the LCL something else needed to keep lifting. Once they are lifted to the point where the parcel temperature is greater than the environmental temperature, the air is unstable and will continue to rise on its own. This is called the Level of Free Convection, or LFC. It then rises freely, without needing a source of lift, cooling adiabatically until it cools below the environmental temperature and stops rising. The level at which it reaches the environmental temperature and becomes stable again is called the Equilibrium Level or EL. This line is often called  Parcel lapse rate. This line shows the temperature path a parcel would take if raised from the Planetary Boundary Layer. This line is used to calculate the LI, CAPE, and CINH (which I will get into in the next section), and  several other thermodynamic indices that are beyond the scope of what I want to cover in this post. The Skew-t below shows this.

Now that we've have those three points plotted;  we can get into identifying instability on the Skew-T. Normally on a day that's going to have severe weather something called a cap is in place. The cap is normally caused by an inversion layer; where warm air above the surface prevents thunderstorms from forming, even with very cold temperatures aloft.  if the air below the cap continues to warm and become more moist, the lower atmosphere becomes extremely unstable because it can't rise and thus stabilize itself. If the cap then begins to weaken and/or the lifting mechanism (usually provided by a cold or warm front) is strong enough, the lower atmosphere will finally break through the inversion layer. Until a parcel breaks the cap, it cannot freely convect. The cap's  strength can be approximated by the amount of CINH, or Convective Inhibition in the atmosphere. Once our parcel of air breaks the cap and is above the LFC, the amount of CAPE, or Convective Available Potential Energy, determines how much instability is shown in a sounding. The CINH in a sounding can be calculated from the area between the parcel temperature and the environmental temperature below the LFC. The CAPE is calculated similarly, except that it is the area between the environmental temperature and the parcel temperature when the parcel is above the LFC and below the EL, so the environment is cooler than the parcel and the parcel is unstable. The CIN and CAPE are shown in the blue and yellow shaded areas of the Skew-T below. The higher the CAPE the higher the instability and a greater likelihood that the thunderstorms will be severe, On the other hand,  more CINH means thunderstorms will have a harder time  initiating. CINH is helpful in preventing storms from forming early, so that the surface can heat up and create more CAPE. However, if the CINH is too high it can prevent thunderstorms from forming no matter how unstable the air is.  LI is just another measure of instability; it can be easily calculated from a Skew-T.  The  LI is the environmental temperature at 500 mb (the squiggly red line) minus the 500 mb parcel temperature (the black curved line).  As I said above, the environmental temperature on this sounding is about -22C, you can see that the parcel temperature at 500 mb is about -7C, so the LI would be -15. This is considered to be extremely unstable. A positive lifted index is stable, a negative LI is unstable, and the more negative the LI gets, the more unstable the atmosphere is.

When forecasting for tornadoes, you want lower LCLs and LFCs so that you can have lower cloud bases and more CAPE near the surface. Hail and wind can occur with higher cloud bases, but tornadoes will rarely occur with LCLs above 1500 meters, and even that is very high.

As you can see the Skew-t shown above looks a little different; but not that different. This is the current Skew-T for Albany, NY. See if you can figure out what it's saying. The reason I'm showing you this is that on the far right of the graph are wind arrows pointing at the true wind direction with barbs showing the wind speed in knots. The illustration below the skew-T to the right shows examples of wind symbols and what they mean. The barbs can be used in combination to signify the wind speed at different levels.


There you have it, I will close with a little narrative. The temperature outside your door is 95 degrees, the dewpoint is 70 degrees. There are strong southeast winds at 25 gusting to 35 mph in your area. There is a dryline  approaching with strong west winds behind it. All the upper level jets are in place; the lifted index is -14C.  When you're eating lunch a  tornado watch is issued. This gets your attention right away.  For the rest of the afternoon  you listen for any weather updates  you can find.  During dinner you watch the evening news and hear that severe thunderstorms are still possible. After dinner you're watching the sky in dread or if you're crazy like me in anticipation. Then just before the sun sets, you see towering cumulus along the approaching dryline. you watch as foam and froth; then they suddenly dissipate. At dusk, you hear there were no  storms within 250 miles of you.  The NWS cancels the  tornado watch. You sit there and say to yourself "silly weather person the forecast was a bust".  If you followed my lesson you know what happened. You got it, blame it on the cap.  The point I'm trying to make is, when you're dealing with the a cap, there is little difference between thunderstorm BOOM or BUST. If the cap is too weak, the atmosphere can overturn early in the day and you are left with a squall line at 11 am. If the cap is too strong, you get a brilliant blue sky and maybe a sunburn. But if the cap burns off at just the right time of day (say 2 to 5 pm, the time of maximum heating) you may be left with isolated severe storms that can be possibly tornadic. Forecasting is hard work. Meteorologist make their forecast based on the information on hand and judgment. There are so many things that have to come together at exactly the right place at the right time. If one thing quickly changes it can and does throw the entire forecast out the window. So all I ask is the next time a forecast bust...don't blame the Meteorologist....instead I bet their upset all that work and mother nature turns around and says sorry I changed my mind. 
 I hope you made it through my Skew-T lesson. If you did, you should have a pretty good idea of how to read a Skew-T. However, It takes a lot of practice to learn to interpret them when you make a forecast. The more you practice the better you will get, so just keep on practicing, before long you will be forecasting like a pro.
I tried to keep things easy to understand but at the same time be thorough..I hope you enjoyed reading and learning about Skew-T diagrams...If you have any additional questions that aren’t answered here, Let Andy or I know, we would be happy to answer them.
Rebecca Ladd

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