Convective Addiction

Skew-T

Posted on 16. Jan, 2010 by Ryan in Education, Skew-T

A Skew-t is a graphical representation of the atmosphere in one single column of air. It allows forecasters to gauge nearly everything about the environment: moisture, instability, shear, and environmental temperatures just to sample a few. A seasoned forecaster can quickly glance at a sounding and understand the basic stability of the atmosphere. A Skew-t is not the only graph used to show a sounding, Stuve diagrams are also used frequently. But in severe weather convective situations, forecasters tend to rely on the Skew-t because it is easier to visually see stability.

Skew-T

Stuve Diagram

Other than a real-time sounding, which is launched at 00z and 12z from weather stations across the globe, weather models also produce forecast soundings. Forecasters can use this model information to try and predict severe potential in the summer or precipitation type in the colder months.

Here are a few places to find real-time soundings:

Storm Prediction Center

Unisys

College of Dupage

University of Wyoming -Stuve Diagram

The first time you saw a Skew-t, it probably just looked like a bunch of chaotic lines. But each one has a meaning and is shown graphically below.

Here is a break down of what each line is:

1) ISOBARS: Vertical lines of equal pressure and are put 50mb apart. Spacing decreases as pressure decreases, similar to the actual atmosphere.

2)ISOTHERMS: Lines of equal temperature in Celsius that start in the bottom left corner and run to the top right.

3)DRY ADIABAT: These represent an unsaturated parcels accent in the atmosphere of 10 degrees Celsius per kilometer.

4)MOIST ADIABAT: Once a parcel of air has been saturated, it will follow this line. The moist adiabat moves to the left as the parcel decreases temperature since colder air cannot hold as much moisture.

5)DEWPOINT: The left of the two roughly vertical lines on the Skew-t is the dewpoint of the air at that level.

6) TEMPERATURE: The right line is the environmental temperature, the dew point and temperature are derived from the actual sounding.

7) On the right side of nearly every sounding are wind barbs which show the wind speed and direction at the different levels of the atmosphere.

A parcel of air will follow this line until it becomes saturated and then follows the moist adiabat. This allows a forecaster (or computers now) to calculated CAPE and CINH along with a host of other thermodynamic indices. To learn more about how to properly draw a parcel line click here.

The Skew-t can be very important in predicting severe weather, from deciphering storm modes to the amount of instability and possibility of storm initiation. There are a number of indices which are generated as a result of a forecast and real-time sounding which will be discussed in length during future sections.

There two big downsides to using soundings which do not outweigh the positive effects, but are important to remember when using a sounding. The first being that a sounding is a snap-shot of the atmosphere in one specific location. While typically the atmosphere does not change all that fast, it is important to keep in mind that fronts and incoming/departing air masses can radically alter what a sounding looked like just a few moments prior. Secondly, weather balloons (RAOB) are only released at 00z and 12z, which means there is 12 hours between soundings and A LOT can change during that time. Monitor upstream soundings to predict changes that may be coming to your location.

During the colder months, soundings can be used to predict what type of precipitation will fall. When the environment temperature stays below freezing through the whole column, then snow will be the precip type. If the temperature creeps above freezing for a rather shallow depth (50-100mb), but falls back below freezing at the surface you will likely experience sleet. When the temperature is above freezing for an extended period of time aloft but below freezing at the surface, freezing rain is likely.

Sleet Sounding

Freezing Rain Sounding-NWS SGF

Here are a few places to find forecast soundings:

Earl Barker

Northern Illinois University

TwisterData

NOAA