Intimidating as they are when you first see them, tephigrams can tell you more about the atmosphere than any other single tool. This site visualises observational data from radiosonde balloons, launched twice a day from sites throughout the world.
The solid black line shows the temperature profile from the surface to the top of the atmosphere, while the dashed line shows the dew-point temperature. The closer together these lines are, the more moisture at that level; this implies cloudier skies. Tephigrams are also indispensable for diagnosing thunderstorms, so try moving the purple dot at the surface to change the instability of a parcel of air at the surface. The further the red line is from the temperature profile, the more energy (see the "CAPE" field) a potential thunderstorm would have.
Move around the map (courtesy of windy.com) and select the balloon icons to display the tephigram or skewT for that location. SkewTs display the same data as tephigrams, the difference being that the dry adiabats are straight lines in a tephigram, while it is the isobars that are straight and horizontal in a skewT-logP diagram.
You can also view the observed wind at any height through the atmosphere, simply hover your mouse over the wind barbs. A visualisation of the likeliest clouds is also present, though this is meant as an aid to visualisation and should not be taken too literally.
This site was developed and is maintained by John C. Kealy, a graduate student at the University of Exeter. Email me with any comments or requests for additions. More radiosonde sites will be added soon, as well as model data from the GFS.