The COMET modules on the MetEd site are quite good. Of course there is no substitute for interactive instruction and in-depth reading. But the modules present concepts in a very visual and clear fashion, and they're well-organized with ratings to indicate beginner through advanced difficulties. Many can be downloaded, they can be continued at later times, and a quiz at the end can be taken and results submitted to employers/supervisors, etc. I have found the Skew-T Mastery module especially helpful and have returned to that one several times. I would definitely recommend the COMET modules. You do have to register, but it cost nothing and email notices can be sent to you announcing new modules.