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).

Skew-T plot of the 7:00 pm radiosonde sounding from Birmingham, Ala. on the evening of April 27, 2011, the day of the Tuscaloosa-Birmingham EF-4 Tornado.

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.

Across the country and around the world, radiosondes sample the atmosphere twice daily providing answers concerning the state of the upper air.

Upper air observations from a few of the more than 90 observations sites in the United States. These select upper air observations are all taken at the same time. © 2015 Google Inc, used with permission. Google and the Google logo are registered trademarks of Google Inc.