How to use a Skew-T Log-P diagram

Whether you’re a high or low altitude pilot, you can see how the temperature and amount of moisture in the air changes as you rise and descend through the atmosphere. How can we better understand these vertical changes to improve weather safety and awareness?

Let’s get acquainted with a meteorological diagram called a Skew-T Log-P. This diagram’s name stems from the fact that lines of equal temperature are skewed at a 45 degree angle along the horizontal axis and pressure in millibars is plotted on a logarithmic scale along the vertical axis. As we will soon see, meteorologists overlay temperatures, dewpoints, and wind barbs on this diagram to make thermodynamic calculations easier. Temperature, dewpoint, and wind information on a Skew-T Log-P diagram can either be observed (as sampled by a weather balloon) or forecast (raw computer forecast model output).

When looking at a sounding, another name for a Skew-T Log-P diagram, we can see several things immediately. We can easily identify levels of the atmosphere that are saturated, or where the temperature and dewpoint are equal to each other and where clouds are likely to be or form. This is particularly helpful for higher altitude pilots, as a sounding shows the depth of saturated and drier layers and thus layers where clouds are more or less likely to be.

Next, we can trace the temperature line with altitude to find where the freezing level is and how saturated air is above this level. This is quite helpful for pilots concerned with icing and the likelihood of encountering supercooled water droplets.

Wind barbs on the Skew-T Log-P diagram not only show you winds aloft but also allow you to easily see wind shear. Higher directional and speed shear suggests convection is more likely to be organized.

Lastly, we can easily see inversions, or where temperature increases with height, on a sounding. Inversions are important for the formation of fog and can inhibit thunderstorm development. While we can learn a lot about the atmosphere in this snapshot, a deeper analysis can help us predict future weather changes.

Before beginning our analysis, we need to understand a key meteorology term: adiabatic. Adiabatic temperature changes involve the expansion or compression of air without the addition or subtraction of heat. A closed parcel of air will expand if it rises, and the temperature of the air inside that parcel will fall.

Alternatively, a closed bubble of air will compress if it sinks, and the temperature of the air inside that bubble will rise. This rate of cooling or warming is a function of how saturated that bubble of air is. Unsaturated air warms at the dry adibatic lapse rate (which is about 3°C per 1,000 feet) if sinking and cools at the same rate if rising. Saturated air warms at the moist adiabtic lapse rate (which is 1.2 to 1.8°C per 1,000 feet depending on altitude) if sinking and cools at the same rate when rising. The saturation mixing ratio is the ratio that a parcel must have at a given pressure and temperature to be considered saturated. Moist adiabats, dry adiabats, and mixing ratios can be found on Skew-T Log-P diagrams and are important in our analysis of how air moves vertically in the atmosphere.

Suppose you have a flight scheduled two days from now during the summer. You’re seeing forecasts for clouds developing during the afternoon, but at what altitude or pressure level will cloud bases be? A TAF will not help you as are looking 48 hours not 12 or 24 hours out. Thankfully, a forecast Skew-T Log-P diagram can help you determine the approximate cloud base or, as meteorologists call it, the lifted condensation level or LCL.

To determine the LCL based on air at the surface, we’ll take the dewpoint at the ground up the mixing ratio line and the temperature up the dry adiabat simultaneously until they intersect. This is not only the approximate point where the lowest clouds will be found, it is also the location where the parcel stops rising at the dry adiabatic lapse rate and switches to the moist adiabatic lapse rate. This slower rate of cooling often allows the parcel to remain warmer than the air around it and thus continue to rise. In severe weather environments, that parcel is likely to keep rising until it hits the top of the troposphere and bottom of the stratosphere where the temperature of the parcel finally reaches the temperature of the air around it. The equilibrium level indicates the maximum cloud top height.

Our Skew-T Log-P analysis doesn’t have to center on lifting air from the ground. Air a couple thousand feet above the ground can be thermodynamically lifted to create elevated thunderstorms.

Computers often calculate parameters like the LCL or equilibrium level – and severe weather indices, such as the lifted index, Total Totals, or SWEAT index – automatically based on observed or forecast sounding data. This enables a meteorologist to quickly compare parameters and indices among computer models and observations to make forecasts. Also, remember that soundings show part of the weather story, and fronts or lifting mechanisms must be analyzed when creating a forecast.

The analysis above reveals the reasons behind why warm, humid air at the ground and cold air aloft often supports thunderstorms. We have also seen how a meteorologist can forecast cloud bases and determine if parcels will be free to rise vertically. You now have a new appreciation and tool for investigating the atmosphere and its impacts on your flight.

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2 Comments

  • “The saturation mixing ratio is the ratio that a parcel must have at a given pressure and temperature to be considered saturated. ”

    Scott, ratio of what to what?

    • The math on this is quite complex, but the saturation mixing ratio is the value that a parcel must have at a given pressure and temperature to be considered saturated.

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