Tephigram

 

Introduction

A tephigram is a plot of the information gained from a radiosonde, or weather balloon. At first sight, it isn't very pleasant viewing.

This is the plot from 24th Sept 2009, which developed into a bit of an epic between 2pm and 3pm. I was whisked up to 3500ft and after about half an hour began to feel quite sick. Apparently this is quite common on a paraglider - I have never felt sick on my hangie. The trouble was, by then I was in cloud suck rising at about 1000ft/min, and I couldn't go down. It took another half an hour of speed bar, big ears and hoping for sink in blue areas before I reached the ground. I can't say I have ever actually wanted to come down before. 

What it all means

 

First, the axes: Down the left is the atmospheric pressure, and the corresponding height is down the right. Temperature is along the bottom from the left. The red numbers bottom right are the wind speed.

From the right, the first red line plot is the wind speed. Next left are the purple wind direction lines. The matched red and blue lines up the middle are the dry and wet thermometer readings, and the yellow lines are the dry adiabatic lapse rate (dalr) and dewpoint lapse rates.

The two bars to the left are the relative humidity (cloud forms where the relative humidity reaches 100%, corresponding to where the blue and red thermometer lines meet), and the thermal index. The thermal index is calculated from the difference between the dalr and the dry thermometer reading. I have added a line at a thermal index of -3, which is around the value required to lift a hang glider or paraglider. I say around -3, as it depends how many pies you have eaten recently, along with a few other factors.

The faded cyan/magenta line corresponds to the freezing point which is quite important on a hang glider or paraglider.

 

Is it complete?

 

No:

  • The thermal index calculation is inaccurate above the point where the dalr and dewpoint lapse rate meet. It should then be calculated from the salr, which I have yet to do.
  • I have made no attempt to add the salr line as I need to figure out the rate at which it changes with height, and where it really begins: is it where dewpoint lapse rate and dalr meet (estimation), or at the point where the relative humidity reaches 100% (measured)?
  • I have made no attempt to calculate the change in thermal index caused by the water vapour freezing. I have no idea how to do this at present.

Can I plot my own?

 

An online version for newer browser is available here.

Download the software from here. You will need Microsoft Windows and the .Net Framework version 3.5 with service pack 1.

The data is loaded from the University of Wyoming. For more info, and station numbers, go here: http://weather.uwyo.edu/upperair/sounding.html. To load today's data, select File | Load Data from the menu, then type in the relevant station number and date/time. It defaults to Nottingham in the UK, and also to the current date.