We’ve had a lot of people ask us about how to read Skew-Ts and what they mean, so we decided to create a little lesson for those trying to learn how to interpret them. If you have any additional questions that aren’t answered here, let us know!

When a weather balloon is launched, it records data that is displayed on what is called a Skew-T/Log-P diagram. Two images of a blank Skew-T are shown here, with labels for what each line means:

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

• Isobars – Lines of constant pressure. Pressure, in millibars, is on the Y-axis as the blue horizontal lines on this Skew-T. It is shown on a logarithmic scale, hence the “Log-P” in the name. As height increases in the atmosphere, pressure decreases, which is why the numbers decrease as you go up the axis.
• Isotherms – Lines of constant temperature. The red diagonal lines represent temperature in degrees Celsius. They are skewed to the side (giving the name of Skew-T) because temperature decreases so rapidly with height that if they were straight up and down on the X-axis, temperature would drop off so quickly that most of the atmosphere would be off to the left of the chart.
• Constant Mixing Ratio – I’m not sure what the actual name for these is, but if anyone is then let us know! Mixing ratio is the amount of water in the atmosphere in grams of water vapor per kilogram of dry air. This is shown as the pink/purple dashed lines.
• Dry Adiabats – Lines that show the temperature an unsaturated, or “dry” parcel of air would have if lifted adiabatically (essentially, no heat energy gained or lost). In Earth’s atmosphere, a parcel cools at about 9.8 degrees Celsius for each kilometer it is lifted adiabatically.
• Moist Adiabats – Lines that show the temperature a saturated, or “moist” parcel of air would have if lifted adiabatically.

There you go! Hopefully that wasn’t too confusing. We’ll go into a bit more detail about adiabatic processes and the role that they play in severe thunderstorm development in this page. When a sounding is displayed on these charts, what you will see plotted will be temperature and dewpoint temperature. A sample I tossed together can be seen here:

The red line represents the temperature throughout the atmosphere, and the green line represents the dewpoint temperature. The dewpoint is the temperature to which you would have to cool a parcel of air with a given mixing ratio in order to reach saturation. For example, let’s say you the temperature outside is 74F, and the dewpoint is 68F. That night, if the temperature falls 6 degrees to reach 68F (assuming the dewpoint remains unchanged), the air will become saturated, and you will see fog. Now just to explain a little more and make sure you can understand how to read this sounding, find the temperature at 500 millibars. 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 red line), and follow the isotherms down and to the left to see which one it intersects with (note: it usually won’t be right on a drawn isotherm, so you have 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 how rapidly the atmosphere cools with height. 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.

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! When parcels of air are lifted from a level of the atmosphere, they expand. Due to some physics that we won’t go into right now, air cools when it expands if no heat is added to it, and warms when it shrinks given the same condition. This is very important to understand!

Note: The lessons below are based off of a parcel lifted from the surface. Realistically, the parcel you want to lift will typically be “mixed” through the lowest 100 millibars or so, but using a surface parcel will make things easier.

Now we’ll discuss how this affects severe weather. In order for thunderstorms to form, we all know we want warm moist air below cool, dry air. But there’s a catch. The air above the surface can’t just be cooler than the air at the surface. It usually is, so we would have thunderstorms all the time if this was the case! For surface based storms, 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 on its own! In this situation, you are said to have an unstable atmosphere. 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 of cloud bases for parcels rising from the surface. 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, until the two meet each other. This level is the LCL. Once a parcel reaches its LCL, it is saturated and will from that point cool at the moist adiabatic lapse rate. Parcels usually are cooler than the atmosphere around them when they reach the LCL, so you need something to lift them. 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 whole process is shown below:

Next I’ll discuss how to identify instability from a sounding once you’ve found these. On most severe weather days, a cap will exist for at least part of the day. The cap is a region where warm air above the surface prevents thunderstorms from forming, even with very cold temperatures aloft. Until a parcel breaks the cap, it cannot freely convect. The strength of a cap can be approximated by the amount of CINH, or Convective Inhibition. Once the parcel 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. These areas are both shaded below. More CAPE means more instability and more severe thunderstorms, and more CINH means more difficulty in initiating thunderstorms. CINH is helpful in preventing storms from forming early, so that the surface can heat up and create more CAPE. Too much CINH, however, can prevent storms from forming now matter how unstable the environment is. Another measure of instability that can be easily calculated from a Skew-T is the Lifted Index, or LI. Lifted Index is the environmental temperature and 500 millibars minus the parcel temperature at the same level. In this sounding, the environmental temperature is about -22C, and 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.

There you have it! If you’ve made it this far, you should have a pretty good idea of how to read a Skew-T. It takes a lot of practice to learn to interpret them for use in forecasting, but you’ll get the hang of it soon, so just keep on practicing!