As you may have gleaned from the previous posts on this site, terrestrial propagation of microwaves relies on interactions with the lower atmosphere.
Propagation above 3GHz is very much affected by temperature, humidity, pressure and wind speed and a mechanism is required that pulls this information together for easy display and analysis.
The raw data is collected by radiosondes and via satellite based sensors and some of it is readily retrievable across the internet. Very quickly one can drown in the sea of data available.
In the late 1940s a way to present this data was adopted (Skew-T chart) that allowed relatively quick plotting of data to provide a view of lower troposphere activity for a particular site. Producing the same chart type for each radiosonde site allows a picture to be created for a particular region.
I rely on computers to pick up the raw data and create the data charts.
Meteorological satellite observations collect similar data in discrete grids (cubes) with a huge increase in data volumes.
The value of a Skew-T chart is in being able to very quickly identify height of temperature inversions, cloud base, cloud tops, probable precipitation type and freezing points.
All are necessary to evaluate the possibilty of ducting and scattering on propagation paths.
I found the following information on the internet that describes Skew-T charts and it derserves to be reproduced here, with a few small editing revisions.
I will add some notes outlining how I use such charts (and data) to make regular assessments. At this point I must point out that Skew-T charts are just one tool in a growing toolbox I use to assess atmospheric weather and how it affects propagation.
With the increased popularity of amateur radio digimodes with automated reporting gives access to huge amounts of indicative data on propagation paths on VHF/UHF and lower microwave bands. Band activity and digital modes on 9cm and upwards is much reduced. In either case I am looking at how to use this data to provide data on multi-plane refraction intensity.
We as amateurs also need to move our use of GPS data modes from location to timing so that better and more reliable measurement of propagation paths at VHF and above is possible.
Skew-T charts
Before I start the discussion on Skew-T charts it essential you have some idea of how pressure varies with height. I have produced a simple table listing pressure with height in the average atmosphere along with a comment on key events. These events will be expounded in further posts discussing propagation.
Key points to bear in mind:
Surface Ducts have a ceiling of around 1000m, Frequent seurface ducts will be observed at a height of around 500m.
Elevated ducts and weather based scattering in the western part of Europe up to about 4km.
Rain scatter can happen up to about 8km.
Thunderstorms can affect up to about 14km.
| hPA | km | Comments |
| 100 | 15.750 | |
| 200 | 11.750 | |
| 250 | 10.000 | |
| 300 | 9.250 | |
| 350 | 8.000 | Temperature inversion |
| 400 | 7.250 | |
| 450 | 6.250 | |
| 500 | 5.500 | |
| 550 | 4.750 | |
| 600 | 4.250 | Temperature Inversion |
| 650 | 3.500 | Dew Point |
| 700 | 3.000 | |
| 750 | 2.500 | |
| 800 | 2.000 | |
| 850 | 1.450 | |
| 900 | 1.000 | Anomalous Region |
| 950 | 0.475 | |
| 1000 | 0.100 |
How To Read Skew-T Charts
Skew-T charts are useful for quickly and accurately viewing the structure of the atmosphere all the way from the surface to 100,000 feet.
Skew-T charts are most commonly used to plot parameters measured by radiosondes as they rise throughout the atmosphere. They only plot three measurements: temperature, dew point, and wind velocity (the speed AND direction of the wind). Additionally, there are 5 lines on a Skew-T: isotherms, isobars, dry adiabats, moist adiabats, and saturation mixing ratio lines.
Besides simply acting as a template to plot the temperature, dewpoint, and wind, Skew-T charts are useful for easily finding the locations and values of important levels and parameters of the atmosphere. CAPE, the LCL, and the LFC are just a few things that can easily be found on a Skew-T chart.
Description of each line grouping on a Skew-T chart.
Isotherms
Isotherms are lines of constant temperature. They are the namesake of the Skew-T chart because they are skewed 45 degrees to the right. Skewing the Ts may seem a little unintuitive, but a Skew-T allows us to easily calculate important atmospheric levels and parameters like the Lifting Condensation Level (LCL), Level of Free Convection (LFC), the Equilibrium Level, and CAPE. A Stüve is like a Skew-T but without the skewed temperature lines. It is not as useful for most meteorological applications because the adiabats on it are not curved, meaning we can’t accurately calculate the things listed above.
Isobars
Isobars are defined as “lines of constant pressure.” On a Skew-T chart, pressure, NOT height, is plotted on the y-axis, so isobars are simply parallel to the x-axis. Because pressure decreases more slowly with height the higher you go, pressure is plotted in a logarithmic fashion on Skew-T charts. For this reason, Skew-T charts are also commonly called Skew-T/Log-P charts. If we didn’t plot pressure in logarithms, the Skew-T charts would be as high as the weather balloons they plot traveled – approximately 100,000 feet high!
Dry Adiabats
Adiabatic processes are processes in which no heat is exchanged with the outside system (in our case, the atmosphere), and dry adiabats show how much an unsaturated parcel cools when lifted through the atmosphere. You are probably thinking “how can a parcel cool and maintain the same heat content?” Well, keep in mind that as an air parcel rises, it expands due to the surrounding atmosphere exerting less pressure on it, so the total heat content remains the same.
Adiabatic processes are a consequence of the First Law of Thermodynamics, which states that the heat added to a certain mass of a gas is equal to its change in internal energy + the work done BY the gas ON the environment. My doing some nifty mathematical maneuvering and applying the ideal gas law, we find that the first law states that changes in temperature are positively correlated with changes in pressure. I’ll discuss this and more in a tutorial in the future, but the important thing to know is that when an unsaturated air parcel rises and ANY air parcel sinks, it will travel parallel to these adiabats.
These adiabats follow the “Dry Adiabatic Lapse Rate,” which is approximately 10 degrees Celsius per kilometer.
Moist Adiabats
When saturated air rises, it follows the “saturation” or “moist adiabats.” When air reaches saturation, gaseous water vapor condenses into liquid water droplets, and this phase change releases “latent heat” into the atmosphere. Because of this, the moist adiabatic lapse rate is ALWAYS less than the dry adiabatic lapse rate, but as you can see above, moist adiabats are NOT parallel and vary quite a bit with both temperature AND altitude.
The most important thing to remember about moist adiabats is that a saturated air parcel will ONLY follow them if it is rising. If the parcel is sinking, it is warming away from saturation and will follow the dry adiabats.
Saturation Mixing Ratio Lines
The saturation mixing ratio is the ratio, in grams of water vapor per kilogram of air, that an air parcel must have at a given pressure and temperature to be considered “saturated.” Once an air parcel is saturated, it generally cannot hold any more water vapor.
Now that you know the lines – let’s find out how we can use them to calculate some particularly important levels of the atmosphere. We’ll learn how to calculate the lifting condensation level (LCL), the convective condensation level (CCL), the level of free convection (LFC), and the equilibrium level (EL), as well as convective available potential energy (CAPE) and convective inhibition (CIN).
Lifting Condensation Level (LCL)
Credit: UCAR MetEd COMET Program
The LCL is the pressure level an air parcel would need to be raised (dry adiabatically) to to become saturated. To find the LCL, follow a dry adiabat from your surface environmental temperature and a saturation mixing ratio line from your surface dewpoint temperature. The intersection of these marks the location of the LCL. The LCL is important because it marks location where the air parcel stops rising at the dry adiabatic lapse rate and switches to the moist adiabatic lapse rate.
Convective Condensation Level (CCL)
The Convective Temperature (Tc) can be found by taking a dry adiabat down from the CCL to the surface.
A closely related level is the Convective Condensation Level, or CCL. The CCL is the pressure level that a parcel, if heated to the “convective temperature,” would freely rise and form a cumulus cloud. The convective temperature is the temperature the surface must reach so that air can freely rise, and the CCL is at the intersection of the environmental temperature (NOT a dry adiabat from the surface… that’s the LCL) and the saturation mixing ratio line from the surface dewpoint temperature.
Notes: The LCL and CCL are useful for determining the height of cloud bases. For non-convective clouds that are forced to rise, the LCL is a good approximation. On the other hand, the CCL is a better estimate for clouds formed by convection, like cumulus clouds. In reality, cloud bases are generally somewhere between the LCL and CCL.
The reason why thunderstorms in the desert often have high bases is because surface dewpoints are low there, causing the LCL and CCL to be high in the atmosphere. Conversely, thunderstorms in humid locations generally have lower bases because the LCL is lower.
Level of Free Convection (LFC)
It is calculated by taking a moist adiabat from the LCL until you intersect the environmental temperature.
The LFC is the pressure level an air parcel would need to be raised so that its temperature is equal with the environmental temperature. It is found by taking the moist adiabat from the LCL until it intersects the environmental temperature. After this, the air parcel is warmer than its environment and can freely rise (hence the name – level of free convection).
There are a few isolated situations where this approach won’t work – for example, if the surface has reached the “convective temperature” mentioned above, the LFC is at the surface. But for the vast majority of situations, this method works beautifully.
Not all soundings have an LFC. If the moist adiabat never intersects the environmental temperature because the atmosphere is relatively stable and does not exhibit a sharp decrease in temperature with height, there is no LFC. Additionally, many places that have an LFC during the day may not have one at night, when the surface is cooler and the atmosphere is more stable.
Equilibrium Level (EL)
The slanted red lines are lines of constant temperature, the dotted purple lines are lines of constant mixing ratio, the solid curved green lines are dry adiabats, and the curved green lines are moist adiabats
The Lifting Condensation Level (LCL), Level of Free Convection (LFC), and Equilibrium Level (EL) are labeled. The CAPE is bounded on the bottom by the LFC and the top by the EL and is the total area between the black line (path of the air parcel) and red line (environmental temperature).
The equilibrium level only exists if there is an LFC, and it is defined as the level at which the moist adiabat denoting the parcel’s path recrosses the environmental temperature. At the EL, the air parcel is the same temperature as its environment, and above it, it is cooler and more dense. The EL can be found by looking at the “anvils” on thunderstorms, as these mark the location where a rising air parcel is no longer positively buoyant. The “overshooting top” of a thunderstorm exceeds the equilibrium level, but this is only because the momentum of the storm’s uber-powerful updraft is allowing it to reach a higher altitude, NOT because the air above the equilibrium level is positively buoyant.
Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN)
Credit: UCAR
CAPE is the area bounded by the environmental temperature and the temperature of a parcel as it rises along the moist adiabatic lapse rate. By definition, the lower bound of the CAPE is the LFC, and the upper bound is the EL. Because CAPE measures how buoyant an air parcel is relative to its environment, it can be used to estimate the maximum strength of updrafts in a storm, and by association, how severe a storm can become. If you want big storms, you need big CAPE. Period.
CIN is CAPE’s antithesis: while CAPE measures positive buoyancy and the strength of convection possible, CIN measures negative buoyancy and the resistance to convection. CIN is bounded by the environmental temperature on the right and the temperature of the rising parcel on the right, and is measured from the LFC down to wherever the temperature of the environment and temperature of the parcel are the same, which is almost always the surface. In this area, the temperature of the parcel is less than the environment, thus rendering the parcel more dense and causing it to sink in the absence of any external forcing. CIN generally peaks during the early morning and decreases during the day as the sun heats the surface.
CIN is actually a necessary ingredient for severe storms because it allows CAPE to build to tremendous levels by preventing convection and mixing of the atmosphere during the morning hours. When heating from the surface finally erodes the CIN, CAPE values have grown astronomically large and any storm development is explosive, leading to powerful supercells with large hail, damaging winds, and tornadoes.
Here’s a classic severe weather sounding from Oklahoma City that was taken 3 hours before the devastating 2013 Moore, OK EF-5 tornado. See if you can find the LCL, CCL, LFC, EL, CAPE, and CIN on this sounding!
A classic severe weather sounding, with a pronounced “capping inversion” (CIN) that keeps convection from gradually occurring throughout the day, allowing it to explode all at once in the late afternoon/evening hours when the cap breaks. There is also a ton of CAPE and strong wind shear throughout the atmosphere. The 2013 Moore EF-5 tornado touched down 3 hours after this sounding was taken.
Using Skew-T Plots
As the radiosonde balloons ascends, it records the temperature and relative humidity at certain prescribed pressure levels (called the mandatory levels) and anytime a significant change occurs in the temperature, humidity, or wind.
Typically, a radiosonde observation is complete when the balloon, carrying the radiosonde, bursts and begins to descend. At that time the data is compiled into a series of five-digit groupings containing temperature, dew point depression and wind speed/direction for mandatory and significant levels. This data is plotted onto a skew-T.
The five-digit coded radiosonde observation is complicated to decode and plot onto a Skew-T diagram. As such, there are several private weather vendors and universities who have written programs to decode and plot (or redisplay the info in a tabular format) these observations. A simple Internet search for “atmospheric soundings” will provide you with several choices.
There are two basic lines plotted on a Skew-T from which we can derived much information. These represent the dew point which is calculated from the relative humidity (in blue, left line) and air temperature (in red, right line).
While it is generally true that the air temperature decreases with height, it is readily seen that this decrease is not uniform nor is it consistent. There may be several places where the air temperature remains the same or increases with height. These particular places are called ‘temperature inversions’ where the normal temperature decrease is ‘inverted’ and the temperature will increase with height.
Another common characteristic of radiosonde soundings is the location of the tropopause. The tropopause is the boundary between the troposphere and stratosphere and is also indicated by a large temperature inversion.
The dew point line will be the most ‘wiggly’ as the radiosonde ascends through intervening pockets of moist and dry air. At each level on the Skew-T, the closer the dew point is to the temperature, the higher the relative humidity is at that level. The dew point will occasionally equal the air temperature and will be seen by the intersection of both lines.
The other piece of information plotted on a Skew-T is the wind speed and direction. This info obtained as the radiosonde is tracked using GPS during its ascent. The wind speed and direction is reported for the same mandatory pressure levels with additional required elevations above sea level and for any significant changes in speed or direction.
Skew-T Examples
Radiosonde observations provide the condition of the atmosphere above the launch site (typically within 25 miles/40 km) at the time of launch. While they do not provide any direct forecast information, they do help explain why we experience different types of weather. Following sample soundings are typical for different weather conditions.
Snow
The atmosphere is very moist as indicated by the small amount of separation between the air temperature (red line) and the dew point (blue line). Even though the air temperature increases a few hundred feet above the ground (a temperature inversion) the air temperature, throughout the entire atmosphere, remains below freezing.
So, when precipitation begins, it will be in the form of snow and will remain frozen as snowflakes reaching the ground.
Ice pellets (Sleet)
As with the previous sounding, the atmosphere is very moist. So much so, the air temperature and dew point are the same from near 900 millibars (3,000 ft. / 1,000 m) to a little above 700 millibars (10,000 ft. / 3,000 m).
At the surface, an arctic cold front had moved south of the observation station with an air temperature well below freezing. The air temperature begins to decrease with height (which is normal) dropping from 23°F to 12°F (-5°C to -11°C).
However, the density of the arctic air is such that it lays close to the ground with its vertical extent fairly small, in this case only about 3,000 feet (1,000 meters) deep. Above 900 millibars (3,000 ft. / 1,000 m) the air becomes considerably warmer. This area is called an inversion, where temperature change with height is ‘inverted’ as it increases with height instead of typically decreasing with height. This inversion is often also referred as a ‘warm nose’.
Eventually, the temperature of the atmosphere will return to the typical decrease with height (near 800 mb) and will continue to cool until it falls to below freezing again (about 720 mb).
While there may be some precipitation forming as rain in the warm ‘nose’ region where the air temperature is above freezing, the vast majority of precipitation will form as snow in the colder below freezing air above the inversion.
As snow falls into the ‘warm nose’, it melts into a liquid drop/rain. Then the liquid drops fall back into the arctic air mass (near the ground) that is cold enough and deep enough for the liquid to freeze into ice pellets before reaching the ground.
Freezing Rain
The basic pattern for freezing rain is similar to ice pellets. The main difference is the sub-freezing air near the surface is very shallow and/or ‘warm nose’ is large. In the end, the melted snow does not have sufficient time to freeze into ice pellets before it reaches the ground.
Therefore, precipitation falls as rain but freezes upon contact with an elevated surface such as a tree, power line, automobile or bridge.
These elevated surfaces may be capable of accumulating ice as soon as the air temperature falls below 32°F (0°C). For road surfaces in contact with the ground, they usually begin to ice when the air temperature falls to 28°F (-2°C).
Of all winter weather situations, freezing rain causes the most havoc. There are more car accidents, injuries and deaths from freezing rain than in any other type of winter weather.
Hurricane
Inside hurricanes, the velocity of the air helps keep the air mixed. Therefore, other than the normal decrease with height, variations in temperature (and dew point) are fairly minimal.
Thanks to Charlie Phillips at Weather 101 – charlie.weathertogether.net.
References:
- National Weather Service (n.d.). Skew-T Log-P Diagrams. Retrieved May 10, 2017, from http://www.srh.noaa.gov/jetstream/upperair/skewt.html
- University Corporation for Atmospheric Research (n.d.). Skew-T Mastery. Retrieved May 17, 2017, from http://www.meted.ucar.edu/mesoprim/skewt/
- Ladd, R. (2014, April 25). The Basics of a severe weather sounding. Retrieved May 17, 2017, from http://wx4cast.blogspot.com/2014/04/the-basics-of-severe-weather-sounding.html
Illustrations of real weather in Europe
A couple of Skew-T charts to show the atmosphere on 4th October 2020.
The first is from Tromsoe, the most northerly sounding I retrieve. Usually I use it as the basis for my baseline atmosphere as it usually varies little.
Today though a surface duct has formed so makes the plot newsworthy
The second chart is from the same time for Luxembourg.
It shows cloud from about 500 to 8km asl.
This is the heart of the very active Low pressure system that spanned western Europe from 3rd-5th October 2020
For reference here is the Enhanced RGB satellite image a little later
