What is CAPE.

One term you will hear when listening to NOAA Weather Radio, Myself, and other weather outlets during severe storm season is CAPE. Here is a little write up about what CAPE is and how it pertains to severe weather.

This is a very intricate subject, and involves a lot of math and science. I will try to stay away for most of the math and science, as much as I can.  This will be quick lesson in plain speak. When you're done with it, you should have a good understanding of what CAPE is.

CAPE is an acronym that stands for Convective Available Potential Energy.  The amount of CAPE is a measure of just how much energy is available convection.

In meteorology, convection is the vertical movement of heat and moisture in an unstable atmosphere.   Clouds are a visible signal that convection is taking place. There is a process called dry convection, but that is beyond the scope of this post. For our purpose in this write up, convection has to do the updrafts needed for thunderstorm development. Another term for this type of convection is called  moist convection.

CAPE represents the amount of buoyant energy available to speed up a parcel vertically in an updraft. The potential energy available for convection is expressed mathematically using a standard measurement of energy represented as Joules Per Kilogram (J/Kg).  

one more thing I think I need to make clear.

One Joules is the amount of energy expended in moving an object through one meter when it is opposed by a force of one  Newton ( a Joules is equal to one Newton-meter).  If you remember your high school science , you will recall that energy is stored work, Work being the application of a force through a distance. Power is the rate of flow of energy, or the rate at which work is done. Stored energy becomes working energy when it is used.

CAPE is a measure of instability in the atmosphere. Therefore, the higher the value the greater the potential for severe weather.

CAPE Values:
0001-1000 J/kg: Marginally (Weakly) Unstable
1000-2500 J/kg: Moderately Unstable
2500-3500 J/kg: Very Unstable
3500 J/kg and higher: Extremely Unstable
CAPE values of 5000 -6000 J/Kg  show staggering instability.

There are no threshold values above which severe weather becomes imminent. So, you can still have values of CAPE and have no thunderstorms.

Without diving into the laws of thermodynamics, this will be basic, but you will still have a good understanding.

An air parcel is a small volume of air that has the same general temperature, air pressure, and amount of moisture. There is no exact definition of the size of the parcel, but for our purpose   a cubic foot of air will work.

A few more acronyms:

Lifting Condensation Level (LCL) is altitude at which clouds begin in a rising parcel of air. LCL can be said to be the level to which an unsaturated air parcel can be lifted adiabatically before it becomes saturated.
 
Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder that its environment, and therefore becomes negatively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder than its environment, and therefore negatively buoyant.

Convective inhibition (CIN) is a numerical measure That shows an area in the atmosphere that hinders the updraft necessary to produce convective weather.  Most of you have heard it more often called the Cap on the evening weather segment. The stronger the Cap the more difficult it is to get convective thunderstorms.

CAPE can be visualized on a thermodynamic diagram by lifting a parcel dry adiabatically until it becomes saturated this is the LCL. From there, lifted until the parcel temperature crosses the environmental temperature the LFC. The area enclosed by the two curves between the LCL and the LFC is the CIN, or negative area. From the LFC up to where the parcel temperature again crosses the environmental temperature and becomes cooler than the environment This is the EL. This area often is called positive area, but normally just called CAPE.

A Skew-T

Here is a link to a post I did, that went into a little detail on the Skew-T.

How to read a skew-t-log-p

 

Types of CAPE:

MLCAPE: Mean Layer CAPE  this is sometimes referred to as Mixed Layer CAPE. This type of
CAPE is calculated using a parcel consisting of mean layer values of temperature and moisture from the lowest 100 mb above the ground level.

MUCAPE: Most Unstable CAPE:
This type of CAPE is calculated using a parcel from a pressure level in the lowest 300 mb that will give you the most unstable CAPE there is.

SBCAPE: Surface-Based CAPE
This type of CAPE is calculated using a surface based parcel.

Other Types of CAPE

DCAPE: Downdraft CAPE used to estimate the potential strength of rain-cooled downdrafts

NCAPE: Normalized CAPE is CAPE that is divided by the depth of the buoyancy layer.

NCAPE is very important.   NCAPE is found by taking CAPE and dividing it by the distance between the LFC and EL in meters.

There are two aspects of CAPE that are important in trying to forecast severe weather. The size and distribution of the area of CAPE, describes the potential strength of the updrafts.

The width of the area of CAPE shown on the sounding, is important as it describes the potential strength of the speed of the updraft needed for thunderstorm development. 

Fat CAPE

 Skinny CAPE

A larger number indicates what we call Fat CAPE.. Fat CAPE indicates the possibility of stronger updrafts as compared to Skinny CAPE.

Well  I most likely went in a bit too deep, but I wanted to give you a thorough understanding of what CAPE is and how we use it.    As always I will answer any questions you may have.

I will add other segments to this series that will talk more about instability, and other severe weather indices, such as:  Helicity, LI, BRN, EHI.

Other post you might like.

The thunderstorm lifecycle

Types of thunderstorms

Types of thunderstorms part 2

Severe thunderstorm structure

The Tornado

Hope you enjoyed this........and maybe came away with a better understanding of the subject.

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, dewpoint, and atmospheric pressure 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. I got these images from google images, I don't know the actual source, since several sites use these images .

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

Temperature Curve (B):

This plot is taking from the rawinsonde (weather balloon) as it was increasing in height. This is shown on the chart as the red meandering line (but it can be different colors on other charts) The way to know which you're looking at, is it is always found on the left side of the dew-point wavy line.


Dewpoint Curve (A):

This plot it shows the dew-point measurement as the weather balloon is increasing in height. This is shown on the chart as the bright green meandering line (but it can be different colors on other charts) The way to know which you're looking at, is it is always found on the right side of the temperature twisty line.


Isobars (D): 

Lines of constant pressure in millibars. Millibars are a measure of pressure.They run horizontally from left to right and are labeled on the left side of the diagram. For example 1000mb is much closer to the surface than 500 mb  which in itself is closer to the surface than 200mb.  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 constant (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). Increments are per degree and are typically labeled for every five degrees of Celsius. . They are labeled at the bottom of the diagram.

Saturation mixing ratio (C) (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 Adiabats (E): 

In this diagram this is the solid curved green lines; that show the rate of cooling of a piece of  rising unsaturated (dry) air.  On this diagram they are labeled every ten degrees Celsius. The changing rate is about 10 degrees Celsius per kilometer. This shows the rate of temperature change in an air parcel of  rising or sinking dry air.  These solid brown lines slightly curve from the bottom right to the upper left. They are solid lines gradually arc to the top of the chart with height. 

Saturation Adiabats (F): 

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 right toward the upper left. On this diagram they are labeled ever two degrees Celsius.  This is used to show rate of change of temperature of a saturated parcel of air as it rises. The Moist adiabatic lapse rate increases with attitude in view of the fact that cold air has less moisture content than warm air. They become parallel to the Dry Adiabats, due to the very low moisture content at high altitude and stop at 200mb (roughly 26,600 feet above sea level). 

Other useful terms to know:

Dry Adiabatic lapse rate:

This is the rate of decrease of temperature with height as an unsaturated parcel of air rises, with no gain or loss of heat. It is an estimate of the lapse rate of the dry adiabat I mentioned above. 

Moist  Adiabatic (Pseudo-Saturation) lapse rate:

The rate of decrease of temperature with height of a saturated parcel of rises where all the condensed water or sublimated ice fall out of the parcel of air as soon as it is produced. It is an estimate of the lapse rate of the saturation adiabat I mentioned above.

Environmental lapse rate:

This is the rate of decrease of temperature with height of the air surrounding the air parcel. It is an estimate of the lapse rate  of the temperature curve I mentioned above. 

Super adiabatic:

This is when the environmental lapse rate is steeper then the dry adiabats. This conditions is most commonly found just above the ground on a sunny day. 

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.  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 say...man 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